Blackwell Science, Ltd Group breeding dramatically increases reproductive success of yearling but not older female scrubwrens: a model for cooperatively breeding birds? ppt

Journal of Animal
Ecology

2001

70

, 370–385

© 2001 British
Ecological Society

Blackwell Science, Ltd

Group breeding dramatically increases reproductive
success of yearling but not older female scrubwrens:
a model for cooperatively breeding birds?

ROBERT D. MAGRATH

Division of Botany and Zoology, Australian National University, Canberra 0200, Australia

Summary
1.

Many studies of cooperatively breeding birds have found no effect of group size on
reproductive success, contrary to predictions of most adaptive hypotheses. A model is
proposed for variation in group-size effects: group size has a reduced effect on success
when conditions for breeding are good, such as in good environmental conditions or in
groups with older breeders. This hypothesis is tested with a case study of white-browed

scrubwrens

Sericornis frontalis

and a review of the literature.

2.

The scrubwren is a cooperatively breeding passerine with male helpers. Previous analyses
revealed no effect of group size on reproductive success, but those analyses were restricted
to groups with older females (Magrath & Yezerinac 1997). Here 7 years’ data are used
to contrast the effect of group size on reproductive success for yearling and older females.

3.

Yearling females breeding in groups had more than double the seasonal reproductive
success than those breeding in pairs, even after controlling for territory quality. How-
ever, group size still had no effect on the reproductive success of older females. Yearling
females tended to survive better in groups, but older females tended to survive better in
pairs, emphasizing this pattern.

4.

Yearlings breeding in pairs were more likely to be found on poor-quality territories
than those breeding in groups, exaggerating the already-strong effect of group size on
yearling success. Older females were not affected significantly by territory quality.

5.

Group size, territory quality and female age affected different components of seasonal

reproductive success. Group size increased the success of individual nesting attempts,
while both territory quality and female age affected the length of the breeding season,
and thus the number of breeding attempts.

6.

A sample of the literature on cooperative breeders shows that group size has a larger
effect on reproductive success in poorer conditions, caused either by younger, inexperienced
breeders or poorer environmental conditions. Scrubwrens therefore illustrate a widespread
pattern, which provides an explanation for much of the variation in group-size effects
among and within species. Clearly single estimates of group-size effects for species can
be inadequate to test ideas about the evolution of cooperative breeding.

Key-words

: breeder age, cooperative breeding,

Sericornis frontalis

, territory quality,
white-browed scrubwren.

Journal of Animal Ecology

(2001)

70

, 370–385

Introduction

In cooperatively breeding birds, more than a simple
pair provide care to the young in a single brood (Brown
1987; Emlen 1997; Cockburn 1998). Breeding groups
are often formed through natal philopatry of young
previously reared by the group, and such groups usually
consist of a dominant pair and related subordinates
who provide alloparental care (Brown 1987; Stacey &
Koenig 1990).
There are many potential benefits to subordinate
‘helpers’ of remaining in the natal group and providing
care to young (Emlen & Wrege 1989; Emlen 1991;
Heinsohn & Legge 1999). Helpers can increase the
reproductive success or survival of relatives and so
increase their indirect fitness (e.g. Emlen 1991; Mumme

Correspondence: Robert D. Magrath, Division of Botany and
Zoology, Australian National University, Canberra 0200, Australia.
Fax: +61 2 6125 5573. E-mail:

JAE498.fm Page 370 Monday, April 16, 2001 9:57 AM

371

Group size and
reproductive
success in birds

© 2001 British

Ecological Society,

Journal of Animal
Ecology

,

70

,
370–385

1992; Komdeur 1994). Helpers may also increase their
own reproductive success in future, for example by
gaining skills at breeding (Komdeur 1996a), gaining
territories (Woolfenden & Fitzpatrick 1984) or gaining
mates (Reyer 1990). In some species subordinates share
reproduction with dominants, and so gain some current
direct fitness (e.g. Rabenold

et al

. 1990; Whittingham,
Dunn & Magrath 1997).
Although there are many potential benefits to helpers,
the most widely cited is that they increase the repro-
ductive success of relatives by increasing reproductive
productivity of groups ( Brown 1987; Emlen 1997; Mumme
1997). Mumme (1997), for example, notes that for both
birds and mammals, there is both ‘extensive correlational

evidence’ and experimental evidence that helpers increase
the reproductive success of recipients. In some cases,
larger groups can have more than double the success of
pairs (e.g. Rabenold 1990; Emlen & Wrege 1991; Hein-
sohn 1992). This finding, in combination with evidence
that helping is often preferentially directed to kin, sug-
gests that indirect benefits are often crucial to explaining
cooperative breeding (Emlen 1997; Mumme 1997).
Increasing a group’s reproductive success can also
bring direct benefits to helpers, even if kinship is un-
important. Four of the seven potential direct benefits
of helping listed by Emlen & Wrege (1989) require an
increase in reproductive success with group size. Higher
group success can be beneficial to helpers because: (1)
increasing group size can enhance survival, (2) larger
groups may allow territorial expansion and budding,
(3) helpers may form coalitions with young produced
in the group and (4) young produced in the group may
later become helpers. Overall, it is important to quantify
the relationship between group size and reproductive
success in order to assess adaptive explanations for the
evolution of cooperative breeding.
Although reproductive success is often higher in
groups, this is not always true; about one-third of studies
find no effect of group size (reviews by Cockburn 1998;
Hatchwell 1999). The true proportion might be higher
because correlations between group size and repro-
ductive success are likely to be inflated, particularly in
species in which groups form through natal philopatry
(Brown

et al

. 1982; Brown 1987). Pairs on high quality
territories may have higher reproductive success, lead-
ing to larger groups in later years, so that a correlation
between group size and success may be confounded by
territory quality, or the age or quality of breeders.
In addition to variation among species, there is also
variation in the effect of group size on reproductive
success within species. In an experimental study of
Florida scrub jays

Aphelocoma c. coerulescens

Bosc, for
example, Mumme (1992) found an effect of group size
on reproductive success in only one of two years. Sim-
ilarly, the effect of group size appears to be greater in
poor years in some species (e.g. Acorn woodpeckers,

Melanerpes formicivorus

Swainson, Koenig & Stacey
1990; Discussion). Even within a population, there can
be variation among groups in the same year. In Seychelles
warblers,

Acrocephalus sechellensis

Oustalet, the effect
of an additional helper depends on territory quality
and the original group size; although a single helper
raises reproductive success, further ‘helpers’ may depress
reproductive success, especially on poor territories
(Komdeur 1994).
One suggested cause of variation in the magnitude of
group-size effects is that helpers could have a greater
effect on reproductive success when nestling starvation
is common (Emlen 1991; Magrath & Yezerinac 1997;
Hatchwell 1999; Legge 2000). In this situation, the
provision of food by helpers is likely to have a greater effect
on reproductive success than when food is abundant.
Hatchwell (1999) and Legge (2000) provide support for
this idea, by showing that helpers increase the total rate
of provisioning specifically in those species in which
brood reduction is more common. Hatchwell also found
some comparative evidence that helpers have a greater
effect on fledging success in those species that suffer more
brood reduction when breeding in pairs.
Here a general model is proposed for variation in
group-size effects among and within species (Fig. 1a).
Fig. 1. Models of the joint effect of conditions for breeding
and group size on reproductive success of cooperatively breeding
birds. (a) Group size has a larger effect on reproductive success
in poorer conditions (except in very poor conditions), and no
effect in the best conditions for breeding. (b) Group size has
no effect on reproductive success under any conditions. (c) Group
size increases reproductive success by a fixed amount regardless
of conditions.

JAE498.fm Page 371 Monday, April 16, 2001 9:57 AM

372

R.D. Magrath

© 2001 British
Ecological Society,

Journal of Animal
Ecology

,

70

,
370–385

In good breeding conditions, helpers may have little or
no effect on the groups’ reproductive success, because
there is no major limiting factor that they can ameliorate.
‘Good breeding conditions’ includes both environmental
conditions, such as food supply and predator abundance,
and the quality, age or experience of breeders. For
example, if food is abundant, a pair may be able to pro-
vide easily for optimal growth of offspring; or when
predators are absent, there may be no benefit to extra
vigilance. In poorer conditions, however, provisioning

by helpers could reduce the risk of starvation of young
or vigilance might reduce the risk of predation, and
so increase the reproductive success of the group. In
extremely poor conditions, however, helpers may be of
little benefit, since their activities may rarely be suffi-
cient to allow successful breeding. This model suggests
that the failure to find group-size effects in many species
might be because studies have been conducted at benign
locations, or because averaging effects over individuals of
different age or quality may obscure important variation.
An alternative model for failure to find an effect of
group size on reproductive success is that there truly is
no effect, regardless of conditions (Fig. 1b). Another
possibility is that instead of having a reduced effect in
benign conditions, groups might increase reproductive
success by a constant increment, regardless of environ-
mental conditions (Fig. 1c). In this case, groups will
none the less have a proportionately smaller effect in
better conditions, which might make it more difficult to
detect an effect of group size.
One uncertainty in making predictions about group-
size effects is that helpers might affect the survival and
thus future reproductive success of breeders rather than,
or in addition to, current reproductive success. Again, the
magnitude of future benefits to breeders may depend on
the conditions for breeding. Thus it is relevant to assess
breeder survival as well as current reproductive success.
Testing these models requires examining the magni-
tude of group-size effects under different conditions for
breeding. Because the demographic models that form

the basis of cooperative breeding theory require detailed
and long-term studies of marked individuals, it is rarely
possible to conduct simultaneous studies at several
sites differing in environmental conditions (Reyer 1990
provides an important exception; see below). However,
it may be possible to examine group size effects in good
and poor years (e.g. Woolfenden & Fitzpatrick 1984;
Koenig & Stacey 1990), or on territories of different
quality (e.g. Komdeur 1994, 1996a).
I suggest that focusing on breeder age may be a par-
ticularly powerful way to test these models, because
individuals become more proficient at breeding as they
age and gain experience (Sæther 1990; Forslund & Pärt
1995). Therefore, one can test the prediction that group
size will have a greater effect on reproductive success in
poorer conditions by comparing younger, inexperienced
breeders with older, experienced individuals. Further-
more, an effect of group size on the reproductive success
of younger individuals is important because effects
early in life will have a disproportionate effect on lifetime
reproductive success.
The white-browed scrubwren,

Sericornis frontalis

Vigors & Horsfield, is one of the species in which there
appears to be no effect of group size on reproductive
success Magrath & Yezerinac (1997). However, analyses
were restricted to groups in which females were at least
2 years old, since the sample of yearlings was too small

to include. In this paper, the effect of group size on
reproductive success is examined for yearling females
compared with older females, using data gathered over
7 years. Given that group size may covary with territory
quality and breeder quality (above), these potentially
confounding variables are also examined. It is concluded
that yearlings do have greater reproductive success when
breeding in groups rather than pairs, and the lack of
benefit from breeding in groups for older females is
confirmed. Finally, a survey of the literature shows
that the pattern found in scrubwrens is of widespread
importance in cooperatively breeding birds.

Materials and methods

 

The white-browed scrubwren is a small (11–15 g)
passerine, endemic to Australia, placed either in
the family Acanthizidae (Schodde & Mason 1999),
or included in the subfamily Acanthizinae in the
Pardalotidae (Christidis & Schodde 1991; Christidis &
Boles 1994). Scrubwrens are largely sedentary as adults
and breed in diverse habitats with thick vegetation, from
coastal rain forest to alpine heath (Blakers, Davies &
Reilly 1984). They feed primarily on arthropods found
on or near the ground, often under leaf litter, but also
search under bark and in thick foliage above the ground
(Keast 1978; Ambrose 1985; Cale 1994; personal obser-
vation). Adults can be sexed by plumage.

Female scrubwrens build domed nests, usually on or
near the ground and hidden under low vegetation or
leaf litter, in which they usually lay three eggs at 2-day
intervals (Magrath

et al

. 2000). The birds are multi-
brooded, commonly laying up to four clutches and
raising up to two broods in a breeding season that
extends from July (mid-winter) to January (mid-summer;
Magrath

et al

. 2000).
The female alone incubates the eggs, for a period of
17–21 days, after which both sexes feed young. The
nestlings fledge after about 15 days, and are usually fed
by adults for a further 6–7 weeks (Magrath

et al

. 2000).

 

The study was conducted on a colour-banded popula-
tion resident in and adjacent to the Australian National
Botanic Gardens, in Canberra (35

°

16

′

S 149

°

6

′

E), over
the seven breeding seasons 1992–98. All birds were
colour-banded for these years and many were banded
in a pilot year in 1991, and so were of known age in

JAE498.fm Page 372 Monday, April 16, 2001 9:57 AM

373

Group size and
reproductive
success in birds

© 2001 British
Ecological Society,

Journal of Animal
Ecology

,

70

,
370–385

1992. Scrubwrens bred both in the Gardens and in the
adjacent Canberra Nature Park (

c

. 9 km

2

), and there
was dispersal both into and out of the Gardens. The
Gardens occupy an area of 40 ha, of which 27 ha are
planted with native Australian plants. Most of the
remainder is natural woodland, which is contiguous
with the woodland of Canberra Nature Park.
The birds are resident throughout the year, and
territories were visited at least three times a week during
the breeding season to document group composition
and reproductive attempts. A complete record was

available of fledging success throughout the breeding
season for between 37 and 47 resident females per year.
Success of a nesting attempt was measured as the
number of young fledged or the probability of fledging
any young. Number fledging was estimated as the number
banded (when 9 or 10 days old), less the number found
dead in the nest. If the nest was damaged, or found
empty before the expected fledging date, only those
seen alive during intensive searches of the territory
were counted as having fledged. Seasonal reproductive
success of females was measured as the total number of
young fledging over the whole breeding season or the
probability of fledging any young.
The date the first egg was laid was used as the measure
of the beginning of the breeding season for a female.
The ‘end of the breeding season’ was estimated as the
date of failure or fledging of the final attempt, but females
were excluded that died before they had the opportunity
to lay again that season. It was assumed that females
had had the opportunity to lay again if they survived
more than 19 days after a failed attempt or 41 days
after a successful attempt, as only 5% of females that
survived beyond these periods initiated another clutch
if they had not already done so. (Failed and successful
attempts were treated separately following analyses in
Magrath

et al

. 2000.)

    

Social groups are territorial throughout the year, and
usually consist of a single breeding pair, or a trio of a
female and two males (about 10% of groups had more
males, usually three; Magrath & Whittingham 1997).
Males in a group form a dominance hierarchy, with
older males being more dominant. Groups larger than
pairs generally form through natal philopatry of males,
although occasionally males immigrate into groups as
subordinates (Magrath & Whittingham 1997). Females
always disperse from their natal group before or at the
onset of the following breeding season, and attempt to
fill vacancies with territorial males. Thus yearling females
can breed in pairs, if they join a single male, or groups,
if they join two or more males.
In 94% of 317 cases, females bred either in pairs or
groups throughout the season. In those cases where
group size changed within a season, the mean size over
individual breeding attempts was used. The census date
for each nest was the date of hatching or failure, which-
ever came first. Females breeding in ‘pairs’ had a mean
of less than 1·5 males per attempt; those in ‘groups’ had
a mean of 1·5 or more.
Females were classified as ‘yearlings’ or ‘older’. Females
were classed as ‘yearlings’ throughout the breeding
season following that in which they were hatched. In
70% of cases (47/67) a yearling’s age was known from
banding records or because she was caught with traces

of juvenile plumage. However, it can be difficult to age
birds from plumage when more than about 3 months
old, so 30% of ‘yearlings’ were classified as such simply
because they were immigrants to the study population.
It was assumed that immigrants were yearlings because
only 9% of older females moved territories between
years once they had bred, and in 15/16 of these cases
they moved to an adjacent territory (Magrath, unpub-
lished data). Furthermore, during the breeding season,
breeding vacancies were never filled by unbanded immig-
rants until the date at which juvenile females began to
disperse, so there was no evidence of a ‘floater’ popula-
tion of older birds. Females were classified as ‘older’
birds if they were known to be older than ‘yearlings’.
Some females were of unknown age in a given year, but
were classified as ‘older’ females in later years.

 

Scrubwrens are most common in wet areas with dense
cover (Blakers

et al

. 1984; Ambrose & Davies 1989;
Christidis & Schodde 1991; personal observation), so
it was assumed that these habitats were of higher ‘quality’
and territories were ranked according to three criteria
that reflected these features. The classification was
carried out before any analyses of the effect of habitat

on reproductive success. First, each territory was clas-
sified as being in a gully (score = 1), or not in a gully (0);
gullies are wetter and have thicker vegetation. Secondly,
each territory was classified as being primarily in rain
forest (1), cultivated garden beds (0·5) or uncultivated
areas (0); the rain forest is densely vegetated and heav-
ily irrigated, cultivated beds are regularly irrigated and
uncultivated areas irregularly or not irrigated. Thirdly,
territories were classified as being in the south-east (1),
north-east (0·66), south-west (0·33) or north-west (0·0);
the ground slopes down primarily from west to east,
and secondarily from north to south, and lower areas
receive water running from upper areas and are more
heavily vegetated. Finally, a sum of scores from the three
criteria was used as an estimate of ‘territory quality’.
In practice, about one-third of territories had a sum of
exactly 1·5, one-third had higher scores and one-third
had lower scores. Given this distribution, ‘territory qual-
ity’ was classified as ‘low’ (sum < 1·5), ‘medium’ (1·5)
or ‘high’ (> 1·5) before analyses were carried out.

 

Survival was recorded until the next breeding season
(1 August of the following year) for all females from

JAE498.fm Page 373 Monday, April 16, 2001 9:57 AM

374

R.D. Magrath

© 2001 British
Ecological Society,

Journal of Animal
Ecology

,

70

,
370–385

the 1992–97 breeding seasons. It was assumed that a
female that ‘disappeared’ had died, given that most
females were site-faithful once they had found a breed-
ing vacancy (above). Breeding groups were not followed
in 1999, but a census of 2-year-old females was carried
out in August 1999, so that survival of 1998 yearlings
could be estimated. However, given the different methods
used in 1999, and the lack of data on older females,
data are also presented on survival that excludes the
1998 yearlings.

 

Analyses of seasonal reproductive success and number
of breeding attempts per season could only entail a

single season for an individual as a yearling, but could
potentially entail multiple years for an ‘older’ female.
To avoid repeated measures from older females, and to
make data directly comparable to those from yearlings,
the random number generator in SPSS 9·0 (SPSS Inc.
1999a) was used to select one season if there were data
for more than one season. Given that there was only
one season for an individual as a yearling and one as an
older bird, the frequency distributions for the two classes
of female were directly comparable. Some females were
represented in both the ‘yearling’ and ‘older’ class, which
enabled pairwise comparisons (Results), but many
appeared only once. The subsample used for analysis
included one complete breeding season from each of
62 yearlings (38 in pairs, 24 in groups) and 73 older
females (33 in pairs, 40 in groups).
In the analysis of seasonal reproductive performance,
normally distributed variables (e.g. number of breeding
attempts per season) were analysed using the general
linear modelling procedure of SPSS 9·0 (SPSS Inc.
1999b), while dichotomous dependent variables were
analysed using the logistic regression or log-linear mod-
elling procedures of SPSS (SPSS Inc. 1999c). Non-
parametric tests were used when a continuous dependent
variable had a non-normal distribution.
Analyses of the success of individual nesting attempts
within a season were carried out on those females and
seasons used in the random selection described above.
In this case, some females did poorly throughout a
season while others did well, so it was not appropriate

to use ‘nests’ as the unit of analysis. In this case the
proportion of nests that were successful was modelled,
using number of successful nests out of the number of
attempts for each female, assuming a binomial distribu-
tion (a binomial distribution and logit link function in
the generalized linear modelling procedure of Genstat
5, Release 4·1 for Windows [Genstat 5 Committee 1993,
p. 352; Baird & Hunt 1998]).
Modelling in Genstat or SPSS was begun with a full
model including all factors, their interactions and cov-
ariates, and then non-significant terms were progressively
eliminated. In logistic regression or log-linear modelling,
the significance of terms was determined by the change
in deviance, which follows a chi-square distribution,
when the term was dropped from the model. Thus
the change in ‘likelihood ratio chi-square’ values, not
Wald statistics, was used to assess effects in logistic
regression models. The values reported here refer to
the change in deviance at the stage the term was
dropped from the model for non-significant terms, or
the effect of dropping the term from the final model for
significant terms.

F

-ratios and significance values are
reported for similar analyses of normally distributed
dependent variables.

Results

 ,   


Yearlings in pairs fledged fewer young per season than
older females or yearlings in groups (Fig. 2; Kruskal–
Wallis;

χ

= 18·3, d.f. = 3,

P

< 0·001; medians and IQR
in Fig. 2 legend). Overall means

±

SE (

n

) were: yearling
in pair 0·8

±

0·3 (38), yearling in group 2·5

±

0·4 (24),
older female in pair 2·5

±

0·4 (33), older female in group
Fig. 2. The number of scrubwren fledglings produced over a
whole breeding season by (a) yearling and (b) older females,
according to group size. Medians (IQR) were: yearling in pair
0 (0–0·25), yearling in group 3 (0–3·75), older female in pair
2 (0–4), older females in group 2 (0–3). Sample sizes are
38 yearlings in pairs and 24 in groups, and 33 older females in
pairs and 40 in groups.

JAE498.fm Page 374 Monday, April 16, 2001 9:57 AM

375

Group size and
reproductive
success in birds

© 2001 British
Ecological Society,

Journal of Animal
Ecology

,

70

,
370–385

2·1

±

0·3 (40). These differences may reflect differences
in the probability of total failure, differences in the
number of fledglings if successful, or both. Restricting
analysis to females that did not fail totally revealed no
differences among females. Mean numbers fledging

±

SE (

n

) if the female did not fail totally were: yearling
in pair 3·3

±

0·6 (9), yearling in group 3·5

±

0·3 (17),
older female in pair 3·7

±

0·4 (22), older female in group
3·1

±

0·3 (27); grand mean 3·4

±

0·2 (75); Fig. 2; Kruskal–
Wallis,

χ

2

= 2·4, d.f. = 3,

P

= 0·5; median 3 for all group
types. Because the differences among females relate
to the probability of total failure, and because of the

bimodal frequency distributions, subsequent analyses
use a dichotomous dependent variable – the probability
of success at producing any fledglings over the season.
Yearlings in pairs had a much lower probability of
producing any fledglings over the season (24%), com-
pared with yearlings in groups (71%; log-linear model

χ

2

= 13·8, d.f. = 1,

P

< 0·001; Fig. 3). By contrast, older
females in pairs had the same seasonal success regard-
less of whether they bred in pairs or groups (both 67%).
Furthermore, the statistical effect of group size was
strongly dependent on the female’s age (interaction,

χ

2

= 7·1, d.f. = 1,

P

= 0·008).

Yearlings in pairs did not have a different probability
of surviving until the next breeding season than yearlings
in groups (pair 70%,

n

= 33; group 85%,

n

= 20; log-
linear model

χ

2

= 1·7, d.f. = 1,

P

= 0·2). Similarly, there
was no significant difference for older females (pair
83%,

n

= 36; group 67%,

n

= 36;

χ

2

= 2·7, d.f. = 1,

P

= 0·1). However, the differences in survival observed
were in opposite directions for yearlings and older
females, and the interaction was just significant (

χ

2

=
4·0, d.f. = 1,

P

= 0·05). If there are real differences,
yearlings appear to survive better in groups and older
females in pairs, reinforcing the relative benefit to year-
lings of breeding in groups.

   

 

Yearlings in pairs were typically found on lower-quality
territories than those in groups (log-linear model

χ

2

=
6·4, d.f. = 2,

P

= 0·04; data shown as sample sizes in
Fig. 4a). By contrast, there was no difference for older
females (log-linear model

χ

2

= 0·02, d.f. = 2,

P

= 1·0;
shown as sample sizes in Fig. 4b). Given this distribution,
territory quality might confound the relationship between
group size and reproductive success for yearling females,

so it is necessary to consider the joint effects of group
size and territory quality.
Yearlings in groups had a higher seasonal reproduc-
tive success than those in pairs, even while controlling
for territory quality (log-linear model including group
size and territory quality;

χ

2

= 13·7, d.f. = 1,

P

< 0·001;
Fig. 4a). Territory quality was also important; yearlings
breeding on higher quality territories had higher success
in both pairs and groups (

χ

2

= 10·8, d.f. = 2,

P

= 0·005),
and the effect of group size remained constant over

territories of different quality (three-way interaction,

χ

2

= 0·7, d.f. = 2,

P

= 0·7).
Fig. 3. The probability of scrubwrens successfully fledging
at least one fledgling over a whole breeding season for year-
ling and older females according to group size. Sample sizes
(number of females) are shown above bars.
Fig. 4. The joint effects on seasonal reproductive success of
group size and territory quality for (a) yearling and (b) older
female scrubwrens. Reproductive success is measured as the
probability of fledgling at least one young over the whole breeding
season. Sample sizes (number of females) are shown above bars.

JAE498.fm Page 375 Monday, April 16, 2001 9:57 AM

376

R.D. Magrath

© 2001 British
Ecological Society,

Journal of Animal
Ecology

,

70

,
370–385

In contrast to yearlings, there was no discernible effect
of group size or territory quality on seasonal reproduc-
tive success for older females (log-linear model: group
size,

χ

2

= 0·0; territory quality,

χ

2

= 1·2, d.f. = 2,

P

= 0·5;

interaction,

χ

2

= 0·9, d.f. = 2,

P

= 0·6; Fig. 4b).
It is possible that there are differences in the quality
of breeding locations unrelated to the ‘territory quality’
index. To assess this possibility, pairwise comparisons
were used of seasonal reproductive success of the same
female, breeding with the same male in the same loca-
tion for all breeding attempts as a yearling and then in
the following year as an older bird. If the low success of
yearling females in pairs is related to age and group size,
rather than location, then seasonal reproductive suc-
cess should be higher in the second year (a directional
hypothesis). By contrast, if the poor performance of
yearling females in pairs is related to location, and
the overall increase in success observed is due to most
females moving to a better location, then there should
be no change in success when breeding conditions are
the same. Yearlings that bred in groups should show no
change between years.
The mean probability of success over a season for
a female in her second breeding season was more

than double that of breeding as a yearling in a pair, as
expected if group size affected the success of yearlings
(compare with Fig. 4a). The probability of success was
0·25 as a yearling in a pair to 0·58 as an older bird,
despite all else being held constant (McNemar matched-
pairs test,

n

= 12 females, one-tailed

P

= 0·06; Siegel &
Castellan 1988; implemented in SPSS 9·0). There were
similar results for the number of fledglings produced
over the season (means 1·0 and 1·92 fledglings per
season; Wilcoxon matched-pairs test,

n

= 14, one-tailed

P

= 0·06). In contrast to yearlings breeding in pairs,
those that bred in groups did not have higher probabil-
ity of seasonal success in the subsequent year, and the
mean success was actually lower ( probability of success
0·82 as a yearling, 0·55 as an older bird; McNemar

matched-pairs test,

n

= 11,

P

= 0·4). Similarly, the
number of fledglings produced tended to be lower the
following year (means 2·8 fledglings as a yearling and
1·6 as an older bird; Wilcoxon matched-pairs test,

n

= 11,

P

= 0·08).

    


A relationship between group size and reproductive
success could be confounded by breeder quality (Brown
1987). This issue is addressed by using two indirect
measures that are likely to be correlated with female
quality, and using age as a possible correlate of ‘quality’
for dominant (or pair) males.

First, if yearling females that breed in pairs are lower
quality birds than yearlings that breed in groups, this
should be revealed in lower reproductive success in
subsequent breeding seasons. Therefore the seasonal
reproductive success of older females that had bred
in pairs as yearlings was compared with older females
that had bred in groups as yearlings. No difference
was found in the proportion of females producing any
fledglings over the season (previously bred in: pair, 0·60,

n

= 20; group, 0·63,

n

= 16; log-linear model

χ

2

= 0·02,
d.f. = 1,

P

= 0·9). Similarly, there was no difference in
the number of fledglings produced over the season
(previously in: pair, mean 1·9, median 2, IQR 0–3,

n

= 20; group, mean 1·6, median 2, IQR 0–3,

n

= 16;
Kolmogorov–Smirnov

Z

= 0·26,

P

= 1).
Secondly, the reproductive performance of yearling
females was examined according to whether they died
before the following breeding season. The previous
analysis could be biased if poorer-quality yearlings
were more likely to die, and so were excluded from the
comparison. Yearlings in groups had higher success
than those in pairs regardless of whether they sub-
sequently survived or died (log-linear model: survived,

χ

2

= 8·3, d.f. = 1,

P

= 0·004,

n

= 43; died,

χ

2

= 5·5, d.f. = 1,

P

= 0·02,

n

= 19; Table 1a). Furthermore, the strength
of the group size did not differ between the two classes
of females (interaction,

χ

2

= 0·3, d.f. = 1,

P

= 0·6). If
the data from 1998 are excluded (see Methods), there
are still strong effects for each class of female (log-
linear model: survived,

χ

2

= 8·0, d.f. = 1,

P

= 0·005,

n

= 37; died,

χ

2

= 13·5, d.f. = 1,

P

< 0·001,

n

= 12;
Table 1b). However, in this sample the effect of group
size was stronger if the female had subsequently died
than if she had survived (interaction,

χ

2

= 5·6, d.f. = 1,

P

= 0·02; Table 1b).
Table 1. Seasonal reproductive success of yearlings in pairs and groups according to whether they survived until the next breeding
season (1 August). (a) Includes yearlings breeding from 1992 to 1998; (b) excludes 1998 yearlings, because the population was not
followed closely in 1999 (see Methods)
Data used Fate of female Group size Number failed Number successful Percentage successful
(a) 1992–98 Died Pair 11 3 21%
Group 1 4 80%
Survived Pair 18 6 25%
Group 6 13 68%
(b) 1992–97 Died Pair 9 0 0%
Group 0 3 100%
Survived Pair 15 5 25%

Group 5 12 71%

JAE498.fm Page 376 Monday, April 16, 2001 9:57 AM

377

Group size and
reproductive
success in birds

© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
370–385
Yearling females in pairs bred typically with younger
males than other females and so male age might con-
found the relationship between group size and reproduct-
ive success of yearlings. The median age of the male was
3 years (range 1–10) for yearling-female pairs, 5 years
(2–11) for yearling-female groups, 4 years (2–11) for
older-female pairs and 5 years (1–11) for older-female
groups; Kruskal–Wallis, χ
2
= 13·6, d.f. = 3, P = 0·004;
male age is the minimum age if not known exactly.
To assess whether male age did confound the rela-
tionship between group size and reproductive success
for yearling females, a logistic regression of seasonal
reproductive success (failure or success) was used on

group size and territory quality (categorical variables),
and male age and age squared (continuous variables).
Age and age-squared were used in case the effect of
male age was not a monotonic increase. There was no
effect of male age ( χ
2
= 0·8, P = 1, P = 0·4) or age-squared
(χ
2
= 0·0) on the reproductive success of yearling
females. (As in previous analyses, both territory quality
and group size had significant effects.)
    ,
    
Group size, female age and territory quality affected
seasonal reproductive success in different ways, further
isolating the ‘group size’ effect.
Group size affected the success of individual nesting
attempts, not the number of attempts per season. The
proportion of nests producing any fledglings was much
lower for yearlings in pairs than for other females
( Fig. 5; binomial model of number of successful attempts
out of total attempts, interaction of female age and group
size: χ
2
= 8·6, d.f. = 1, P = 0·003, n = 126). By contrast,
group size did not affect the number of breeding attempts
per season either alone (F = 2·5, d.f. = 1, 130, P = 0·12),
in interaction with female age (F = 0·15, d.f. = 1, 129,
P = 0·7), territory quality or both (all P > 0·2).

Female age affected the number of breeding attempts,
and only affected the success of individual attempts
through the interaction with group size (above). Older
females had more attempts (F = 22·5, d.f. = 1, 131,
P < 0·001; Fig. 6).
Yearling females had fewer breeding attempts because
they started breeding over 4 weeks later than older
females. The median date that the first eggs of the
season were laid was 27 September (IQR 9 September–
7 October) for yearlings and 26 August (IQR 20 August–
4 September) for older females (Mann–Whitney U,
Z = 6·4, P < 0·001; the date within years was first
adjusted to the grand median over years). When date
of the first egg was included in a model of number of
breeding attempts, female age was no longer significant
(F = 0·3, d.f. = 1, 120, P = 0·6), while date was highly
significant (F = 43·7, d.f. = 1, 122, P < 0·001). There
was still no effect of group size alone or in interaction
with any other variable; the only other significant factor
was territory quality (F = 5·6, d.f. = 2, 122, P = 0·005).
Territory quality affected the number of breeding
attempts, and not the success of individual attempts.
Birds breeding on higher quality territories had more
breeding attempts (F = 5·4, d.f. = 2, 131, P = 0·007;
Fig. 6). By contrast, territory quality did not affect the
proportion successful, either as a main effect (log-linear
model: χ
2
= 0·1, d.f. = 2, P = 0·9) or in interactions
with female age (χ

2
= 0·3, d.f. = 2, P = 0·8), group size
(χ
2
= 0·6, d.f. = 2, P = 0·5) or both (χ
2
= 0·5, d.f. = 2,
P = 0·6).
Higher quality territories allowed more breeding
attempts because breeding continued longer. Median
dates of failure or fledgling of the last nest of the season
Fig. 5. The mean probability of fledgling at least one young
from individual nesting attempts of scrubwrens. Samples
sizes, shown above bars, are the number of females; statistical
analyses used number of successes out of number of attempts
for each female (see text). Females making no breeding attempt
during a season are excluded.
Fig. 6. The mean number of breeding attempts per season
(± SE) by scrubwrens according to female age and territory
quality. Group size did not affect the number of attempts
(text). Sample sizes are number of females.
JAE498.fm Page 377 Monday, April 16, 2001 9:57 AM
378
R.D. Magrath
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
370–385
were: low quality 20 November (30 October– 6 December,

n = 35); medium quality 5 December (20 November–
17 December, n = 31); high quality 18 December (28
November–6 January, n = 27); Kruskal–Wallis: χ
2
=
13·3, d.f. = 2, P = 0·001. Furthermore, territory quality
became non-significant in any model of number of
attempts when the date of failure or fledging of the final
attempt for the season was included in the model.
Discussion
,  ,   
 
Group size had a dramatic effect on the reproductive
success of yearling females, but no detectable effect on
the success of older females. Yearling females were
more than twice as likely to fledge young in the breed-
ing season if they bred in groups compared with pairs,
and there was a trend for females breeding in groups to
have higher annual survival. By contrast, older females
were equally likely to fledge young in pairs and groups,
supporting a previous study by Magrath & Yezerinac
(1997). Furthermore, there was a trend for survival to
be lower in groups compared with pairs.
The magnitude of variation in the effect of group size
on reproductive success in this population approaches
that among cooperatively breeding birds as a whole.
For example, in summarizing effect of helpers on repro-
ductive success in 19 species, Smith (1990) characterizes
the strength of the effect from ‘none’ to ‘extreme’, the
latter including bicolored wrens, Campylorhynchus

griseus Swainson, and pied kingfishers, Ceryle rudis L.
In those two species, a single helper is correlated with
an increase of 3·3 times and 2·1 times, respectively, the
reproductive success of pairs, similar to the magnitude
of the association in yearling female scrubwrens.
Territory quality did partially confound the relation-
ship between group size and reproductive success for
yearling females, but only accounted for a small pro-
portion of the difference. Yearling females breeding in
pairs were found disproportionately on poor-quality
territories, on which seasonal reproductive success was
lower, so exaggerating the apparent effect of group size.
Overall, yearling females were 3·0 times more likely to
fledge young in a season if they bred in groups, while if
yearlings in both pairs and groups had been uniformly
distributed across the three categories of territory quality,
the difference would have been 2·4 times greater. Thus
the confounding effect of territory quality contributed
about 20% of the observed increase in success of groups.
The ranking of ‘territory quality’ used was indirect,
so it is possible that a more direct estimate of quality
would reveal it to be quantitatively more important.
For example, the effect of territory quality would be
underestimated if yearlings in pairs were found on poorer-
quality territories within categories of territory quality.
However, there should still be a large effect of group
size, given that the probability of success is higher even
for groups on low quality territories compared with
pairs on high quality territories.
Territory quality and group size affected reproduc-

tion in different ways, again suggesting that finer-scale
measurement of quality will not ‘explain’ the group-size
effect. While group size affected the success of individual
breeding attempts, territory quality affected the number
of breeding attempts by allowing birds to continue
breeding longer. Prolonged breeding may be possible
because better territories were wetter and more densely
vegetated, so drying out more slowly with the onset of
hot weather in December (mean maximum 26 °C).
Pairwise comparisons of the same female breeding in
the same location with the same male still showed that
yearlings breeding in pairs tended to have higher success
in the following year, while those breeding in groups
did not. The ratio of increase in the probability of success
was 2·3 (0·58/0·25), which is exactly the magnitude of
increase from yearling pairs to older birds if ‘territory
quality’ is held constant statistically (0·67/0·29). Thus,
although only 12 females were available for this pairwise
test and the difference was not quite significant at P =
0·06, the result is that expected with a real effect of group
size. This result argues against the possibility that there
is unmeasured variation in territory quality that con-
founds the relationship with group size and that is
uncorrelated with the ‘territory quality’ index (e.g.
perhaps variation in the risk of predation).
Female quality did not confound the relationship
between group size and reproductive success of year-
lings. The relationship between group size and success
remained for both yearlings that survived until the next
breeding season and those that did not survive. Thus

differences in female quality, if reflected by survival, did
not explain the greater success in groups. Subsequently,
among those yearlings that did survive to breed as an
older female, there was no difference in the reproductive
success according to the group size in which the female
bred as a yearling. Thus there was no evidence that
yearling females that bred in pairs were of lower quality
than those that bred in groups.
Male age, as a potential measure of breeding com-
petence, also did not affect reproductive success. Male
age may be unimportant to reproductive success in
scrubwrens because males almost never bred as yearlings,
and when they did become breeders were likely to have
had experience as helpers. Specifically, although males
in pairs with yearling females were younger than those
in groups, none the less 95% were at least 2 years old,
and 66% at least 3 years old.
It is difficult to assess the importance of the apparent
effects of group size on survival with current sample sizes,
as is often the case in studies of cooperative breeders.
There was no significant effect of group size for either
yearlings alone or older females alone, but there was a
weak statistical interaction suggesting that yearlings
survive relatively better in groups. Regardless of the
true magnitude of effects, these data reinforce the results
of reproductive success showing that yearling females
JAE498.fm Page 378 Monday, April 16, 2001 9:57 AM
379
Group size and
reproductive

success in birds
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
370–385
benefit from breeding in groups, while older females
do not. Thus the true benefit of breeding in groups for
yearlings may be underestimated from seasonal repro-
ductive success alone.
     

A striking feature of the results on scrubwrens is that,
although yearlings are much more successful when
breeding in groups, older females do not benefit at all.
How could breeding in a group provide a major benefit
to yearlings but no benefit to older females? There are
three types of explanation relating to: (1) female com-
petence at breeding, (2) behaviour of males and (3)
female reproductive effort. Analyses of breeder quality
(Results) show that the difference is not due to differ-
ences in female quality, another general explanation of
age effects in birds (Forslund & Pärt 1995). The first
explanation is that yearlings may be less competent at
breeding, and so benefit from being in groups in which
older females would not benefit. In this case, the number
and behaviour of males may be identical, but the benefit
nonetheless differs. For example, if yearlings are poor
at detecting or identifying potential predators, then
they would benefit from being warned by others that a

predator is near. By contrast, an older female would
not benefit by being informed of a predator that she had
already identified. Secondly, males may behave differ-
ently when breeding with yearling females than with
older females. In this case it is not necessarily true that
yearlings are less competent at breeding. For example,
males might be extra-vigilant when breeding with year-
lings, or work harder at bringing food to the nest. Thirdly,
it is possible that the reduced performance of yearlings
in pairs is due to a reduced optimal reproductive effort
for these birds. These hypotheses are not mutually
exclusive, but each is considered in turn.
Yearlings do appear to be less competent than older
birds, and are compensated by breeding in groups.
Yearlings start the breeding season more than 4 weeks
after older females, regardless of group size. This is
consistent with yearlings being less competent at skills
relevant to breeding than older birds, as has been found
in many other species (Forslund & Pärt 1995). For
example, blackbird Turdus merula L. yearlings are less
good at foraging than older females, start breeding later
and therefore have lower seasonal reproductive success
(Desrochers 1992a). However, if they are provided with
a food supplement, they start breeding at the same time
as older females and have similar reproductive success
(Desrochers 1992b). Furthermore, the effect of group
size in scrubwrens arises primarily because yearling
females do exceptionally badly, not because yearlings
in groups do exceptionally well. This suggests that being
in a group compensates a yearling for her lack of skill.

Subordinate males do behave differently towards
yearling females, on average, than towards older females,
but this cannot explain why only yearlings benefit.
Subordinate males are more likely to provision nestlings
if they are unrelated to the breeding female (Magrath &
Whittingham 1997), and yearling females are all immig-
rants into breeding groups and so are unrelated to
the subordinates. By contrast, about 40% of the older
females in the sample in this paper are in groups with
sons (3/40 of unknown relatedness), who only provision
nestlings in about 50% of cases. However, whether a
subordinate ‘helped’ or not had no effect on the seasonal
reproductive success of older females (Magrath &
Yezerinac 1997), and so this does not explain why only
yearling females benefit from breeding in groups.
Finally, it is possible that yearling females in pairs
have a lower optimal reproductive effort than those in
groups, but this is at best a partial explanation. Year-
ling females in pairs might be selected to put little effort
into reproduction, perhaps starting breeding at a later
date, if high effort jeopardizes reproductive success in
future (Forslund & Pärt 1995). The trend for lower
survival of yearling females in pairs compared to groups
is inconsistent with this hypothesis, but it is possible
that lower effort emphasizes a pre-existing effect of com-
petence and group size. For example, general incom-
petence at breeding may mean that a yearling female in a
pair will have relatively poor success compared to later
years when she is more competent, so it may pay to cut
losses as a yearling to improve the probability of breeding

in future (Forslund & Pärt 1995). Higher effort might
lead to an even lower probability of survival.
    
 
The finding that yearling females gain a large benefit
from breeding in groups, while older females do not
benefit at all, has two major implications for the evolu-
tion of cooperative breeding in scrubwrens and other
species.
First, group size can have a substantial effect on the
lifetime reproductive success of females even if there is
no effect on reproductive success for most females in
most years. On average in the scrubwren population,
75% of females were older birds who do not benefit from
being in groups. To assess the importance of reproduct-
ive success in a female’s first year to lifetime success, the
following values were used: (1) a constant female annual
survival of 75% for females once they become breeders;
(2) a probability of producing any fledglings of 0·29 for
yearlings in pairs and 0·69 for yearlings in groups and
older females; and (3) a seasonal production of 3·4
fledglings if they are successful. All values are means
from the scrubwren population, with probability of
producing fledglings controlled for territory quality.
Given these assumptions, the expected lifetime repro-
ductive success is 15% less for females that bred as
yearlings in a pair (8·0 fledglings) compared with a
group (9·4 fledglings).
The effect of group size on lifetime success of females
is sensitive to estimates of annual survival, but the effect

JAE498.fm Page 379 Monday, April 16, 2001 9:57 AM
380
R.D. Magrath
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
370–385
Table 2. Effect of group size on reproductive success under ‘worse’ and ‘better’ conditions for breeding
Species Comparison
Measure of
breeding
performance
Worse conditions for breeding Better conditions for breeding
Ratio of
change
(worse/better)
3
Incremental
change (worse
− better)
4
SourcePair Group
Ratio
G/P
1
Increment
2
Pair Group
Ratio

G/P
1
Increment
2
Harris’s hawk Low vs. high
prey density
Fledglings
per year
1.76 1.94 1.1 0.18 2.29 2.00 0.9 −0.29 1.3 0.5 Faaborg & Bednarz
1990 (Table 12.2)
Pied kingfisher Poor vs. good
feeding location
Fledglings
per year
1.8 3.7 2.1 1.9 4.0 3.8 1.0 −0.2 2.1 2.1 Reyer 1990
(Fig. 17.7)
Red-cockaded
woodpecker
Female breeding
experience
Fledglings
per year
0.89 2.80 3.1 1.91 1.93 2.00 1.0 0.07 3.1 0.8 Lennartz et al. 1987
(Table 7)
Acorn
woodpecker
Bad vs. good years,
Hastings
Fledglings
per year

0.99 1.61 1.6 0.62 2.90 3.96 1.4 1.06 1.2 −0.4 Koenig & Stacey
1990 (Table 14.4)
Bad vs. good years,
Water Canyon
0.78 1.72 2.2 0.94 2.14 3.26 1.5 1.12 1.4 −0.2
Means 1.9 0.78 1.4 1.09 1.3 −0.3
White-browed
scrubwren
Female age
(yearling vs. older)
Percent any
fledglings
per year
24 71 3.0 47 67 67 1.0 0 3.0 47 This study
Low vs. medium
quality territory
11 50 4.5 39 36 67 1.9 31 2.4 8
Medium vs. high
quality territory
36 67 1.9 31 40 90 2.3 50 0.8 −19
Means 3.1 39 1.7 27 2.1 12
Splendid fairy-wren Female breeding
experience
Fledglings
per year
1.7 2.4 1.4 0.7 2.9 3.1 1.1 0.2 1.3 0.5 Russell &
Rowley 1993
(Table 3)
White-winged
chough

5
Control vs.
supplemental
feeding
Fledglings
per nest
0.7 2.0 2.9 1.3 2.3 3.0 1.3 0.7 2.2 0.6 Boland et al.
1997 (Fig. 4)
Florida scrub jay Years with below vs.
above average
reproductive success
Fledglings
per year
1.01 1.97 2.0 0.96 2.11 2.90 1.4 0.79 1.4 0.2 Woolfenden &
Fitzpatrick 1984
(Table 8.2)
Breeders inexperienced
vs. both experienced
1.24 2.20 1.8 0.96 1.80 2.38 1.3 0.58 1.4 0.4 Woolfenden &
Fitzpatrick 1984
(Table 8.4)
Means 1.9 0.96 1.3 0.69 1.4 0.3
JAE498.fm Page 380 Monday, April 16, 2001 9:57 AM
381
Group size and
reproductive
success in birds
© 2001 British
Ecological Society,
Journal of Animal

Ecology, 70,
370–385
Table 2. continued
Species Comparison
Measure of
breeding
performance
Worse conditions for breeding Better conditions for breeding
Ratio of
change
(worse/better)
3
Incremental
change (worse
− better)
4
SourcePair Group
Ratio
G/P
1
Increment
2
Pair Group
Ratio
G/P
1
Increment
2
Bicolored wren Poor & average vs.
most productive

territories
Juveniles
per year
0.48 1.35 2.8 0.87 0.86 1.58 1.8 0.72 1.6 0.2 Austad & Rabenold
1985 (Text)
Depredation vs.
predator exclusion
6
0.48 1.35 2.8 0.87 1.67 (1.35) 0.8 (−0.32) (3.5) (1.2)
Means 2.8 0.87 1.3 0.20 2.5 0.7
Galapagos
mockingbird
Subordinate vs.
dominant breeders
in dry years
Percent
hatchlings
fledging
25 71 2.8 46 42 77 1.8 35 1.5 11 Curry & Grant 1990
(Table 10.8)
Subordinate vs.
dominant breeders
in wet year
33 56 1.7 23 52 62 1.2 10 1.4 13
Dry years vs.
wet year
38 79 2.1 41 44 55 1.3 11 1.7 30
Means 2.2 37 1.4 19 1.5 18
Seychelles warbler Low vs. medium
territories

Fledglings
per year
0.7 1.1 1.6 0.4 0.9 1.8 2.0 0.8 0.8 −0.5 Komdeur 1994
(Fig. 1)
Medium vs. high
territories
0.9 1.8 2.0 0.9 1.2 1.9 1.6 0.7 1.3 0.2
Female age 0.1 0.7 7.0 0.6 0.6 1.0 1.7 0.4 4.2 0.2 Komdeur 1996b
(Fig. 4)
Male age, incl.
2-year-olds
7
0 0 (1) 0.0 0.5 0.7 1.4 0.2 (0.7) (−0.2)
Male age, excl.
2-year-olds
0.2 0.4 2.0 0.2 0.6 0.7 1.2 0.1 1.7 0.1
Means 2.7 0.4 1.6 0.5 1.7 0.0
Grand means (n = 11 species) 2.3 1.3 1.9
1
Ratio of group/pair reproductive success; values over one indicate that groups had higher reproductive success.
2
Incremental increase in reproductive success (group minus pair); values above zero indicate that
groups had higher reproductive success.
3
Ratio of ratio in poor conditions to ratio in good conditions; values over one mean greater relative success of groups compared to pairs in poorer conditions.
4
Difference
in increment in poor conditions minus increment in good conditions; values over zero mean a greater absolute increase in success of groups compared to pairs in poor conditions.
5
Chough. Minimum group size

of four compared with larger groups.
6
Bicolored wren. Predator exclusion carried out on pairs alone, therefore the comparison with groups uses overall mean for groups, for which depredation is naturally lower.
7
Seychelles warbler. Two-year old males had no success in either pairs or groups, therefore the ‘ratio’ is of questionable value and data excluding 2-year old males are also presented.
JAE498.fm Page 381 Monday, April 16, 2001 9:57 AM
382
R.D. Magrath
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
370–385
of group size on yearlings is always potentially import-
ant. To assess the importance of estimates of survival,
observed values were also used from the scrubwren
population of 70% for yearlings in pairs, 85% for year-
lings in groups, 83% for older females in pairs, and 67%
for older females in groups. In this case, females have an
expected lifetime success of 10·6 fledglings breeding
only in pairs but only 8·4 breeding in groups, reflecting
the lower survival of older females in groups. None the
less, in each case, females always do better if breeding in
groups rather than pairs as yearlings. Breeding in a pair
as a yearling, but otherwise in groups, would result in
6·0 fledglings in a lifetime (29% lower than the 8·4 for
groups alone); breeding in group as a yearling, but
otherwise in pairs, would result in 14·1 fledglings (25%
higher than the 10·6 for pairs alone).
Overall, scrubwren females benefit by breeding in

groups as yearlings assuming this has no effect on group
size in later years, but may do better if breeding exclus-
ively in pairs compared to groups and possibly best of
all breeding first in a group, and subsequently in pairs.
Precise estimates of lifetime group-size effects require
complete measures of social life-history combined with
more precise estimates of annual survival.
The second major implication for cooperative breeding
is that the sexes could differ in the benefits of producing
philopatric sons. While scrubwren yearling females
benefit by joining groups, through increased seasonal
reproductive success and possibly higher survival, they
do not benefit from philopatric sons in later years,
because by then females will be older and will not benefit
from breeding in larger groups. There might even be a
cost of retaining sons, if their presence reduces female
survival. A reduction in female survival in the presence
of sons, if confirmed with more data, could potentially
be offset by an increase in indirect fitness from later
successful reproduction of those sons. In other words,
any reduced survival might be due to the cost of ‘parental
facilitation’ of offspring success (Brown & Brown 1984).
In contrast to females, breeding males could benefit
from philopatric sons. The seasonal reproductive success
of a group is not affected by the female’s age, whereas a
pair’s success is much lower if the female is a yearling.
Thus, from the perspective of a dominant male, philo-
patric sons act as ‘insurance’ against breeding in future
with yearling females. The benefit to males of being in
groups will then depend in part on the mean number of

yearlings a male breeds with in his lifetime. Measuring
the benefit precisely, however, would also require know-
ing how much paternity is gained by sons and extra-group
males in social groups with yearlings and older females.
Based on a small sample which does not allow analysis
by female age, beta males sired about 20% of young when
in groups with their fathers and unrelated females,
but paternity by extra-group males was less frequent in
groups than pairs (Whittingham et al. 1997).
Overall, the relationship between age, group size
and reproductive success is potentially of widespread
importance to cooperatively breeding birds, but has
rarely been the focus of study. An effect of group size
on reproductive success or survival early in life may be
common and will have disproportionate importance to
lifetime reproductive success. Scrubwrens have a similar
high adult survival to many cooperative breeders, yet
the difference in reproductive success between yearling
pairs and groups could account for 15–29% of expected
lifetime reproductive success of recruits.
  -  

The stronger effect of group size on yearling compared
with older female scrubwrens supports the general
model that group size will have greater effects in poorer
conditions for breeding (Fig. 1a). In addition to this
comparison, two of three comparisons involving territ-
ory quality were supportive. The ratio of increase was
greater for yearling females on poor quality territories
(4·5) compared with medium (1·9) or high (2·3) quality

territories; the one inconsistency was that high quality
territories had a slightly higher ratio than medium
quality territories.
To test whether it is generally true that group size has
a greater effect in poor conditions, similar values were
calculated for other cooperatively breeding birds in
which comparisons could be made of ‘good’ and ‘poor’
conditions. To gain a representative sample, I attempted
to include all species represented in Stacey & Koenig
(1990). Data presented in that book, if available, were
used, otherwise publications were searched on those
species appearing before or after the book. Two other
species were also included that were studied more recently.
White-winged choughs, Corcorax melanorhamphos
Vieillot, were included because they were the subjects
of a food-supplementation experiment (Boland, Hein-
sohn & Cockburn 1997); Seychelles warblers were included
because of unusually detailed measurement of territory
quality (Komdeur 1994). Reproductive success of pairs
and groups was calculated in poor conditions and good
conditions. Depending on available data, ‘poor condi-
tions’ could mean young, inexperienced breeders,
subordinate breeders in plural-breeding species, sites
or territories known to be poor, years with low prey
density, or a series of years during which reproductive
performance was below average. ‘Good conditions’
were the opposite. In some cases, the authors provided
the required data, but in most cases values were calcu-
lated from tables, figures or other data in the text. In
some species, it was possible to make comparisons using

more than one classification into ‘poor’ and ‘good’
conditions; for example, using both female breeding
experience and, separately, poor and good territories.
In such cases the complete data are presented, but
species’ means are also calculated.
Most species showed a greater effect of group size
in poor conditions, as predicted (Table 2). On average,
groups produced 2·3 times as many young as pairs in
poor conditions, compared with 1·3 times as many in
JAE498.fm Page 382 Monday, April 16, 2001 9:57 AM
383
Group size and
reproductive
success in birds
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
370–385
good conditions. Furthermore, all 11 species showed a
greater ratio in poor compared with good conditions,
as shown by the ‘ratio of change (worse/ better)’ – values
all being greater than 1·0 in Table 2 (one-tailed binomial
probability < 0·001).
The small mean effect of group size in good condi-
tions means that studies under those conditions would
require large samples to detect a real difference. There
may even be no benefit at all: 5/11 species showed no
increase in success of groups over pairs for at least
one comparison in ‘good’ circumstances and another

(the splendid fairy-wren, Malurus splendens, Quoy &
Gaimard), revealed an effect so small (1·1 increase) it
was not statistically significant even after a long-term
study (Russell & Rowley 1993). Furthermore, given that
confounding variables are likely to overestimate group-
size effects (Introduction), the real benefit of breeding
in groups in good conditions may be even lower. It is
therefore not surprising that many studies have not
detected group-size effects.
A greater ratio of effect in poor conditions does not
necessarily mean that there is a greater absolute (incre-
mental) effect of groups in poor conditions compared
with good conditions. For example, if pairs can produce
one fledgling in poor conditions but two in good con-
ditions, and groups result in one extra fledgling in all
conditions, the ratio will be 2·0 in poor conditions and
1·5 in good conditions (also see Fig. 1c). To address
this issue, the incremental increase was also examined
in groups compared to pairs (Table 2, ‘Increment’
columns). In this case 9/11 species showed a greater
increment in poor conditions (positive values in the
‘Incremental change’ column of Table 2), one showed
no change, and one showed a decreased increment in
poor conditions (one-tailed binomial probability = 0·01).
In support of the usefulness of focusing on younger
breeders to explore the effects of breeding in ‘poorer’
conditions, Table 2 suggests that there are similar effects
regardless of whether ‘poor’ conditions are defined by
environmental conditions or breeder attributes.
It is concluded that groups usually do both relatively

and incrementally better in poorer conditions for breed-
ing within a species, supporting the generality of the
data from scrubwrens. Given that evolutionary ‘fitness’
is a relative measure, a greater ratio of increase in poor
compared to good conditions is important, even if there
is no incremental difference. The overall benefit to breed-
ing in groups would depend on how frequently birds
breed in different conditions.
The conclusion from this comparison of group-size
effects within species is consistent with that from Hatch-
well’s (1999) comparison among species. Hatchwell
found that species in which brood reduction is more
common (among individuals breeding in pairs) are more
likely to show a higher reproductive success in groups
compared to pairs. This result suggests that group size
has a greater effect on reproductive success in species in
which food is more likely to be scarce. However, Hatch-
well emphasizes that his study was designed to look at
risk of starvation and feeding rates, not at reproductive
consequences, and the effect on reproductive success
could be an artefact of using brood reduction in pairs
as an estimate of environmental conditions. It is also
unclear whether the differences in brood reduction and
the group-size effects represent values typical of species
or simply the conditions prevailing in specific studies.
Variability in group-size effects within species has
broad implications for the study of cooperative breeding
in birds. It is clearly inappropriate to accept or reject
hypotheses about the evolution of cooperative breeding
on the basis that there was or was not an effect of group

size on reproductive success in a particular study. For
example, helping to raise collateral kin might be im-
portant in the evolution or maintenance of cooperative
breeding even if a particular study found ‘no effect’ of
group size on reproductive success. The lack of effect
might be specific to the conditions under which the
study was conducted, rather than being typical of the
species. Conversely, a positive effect of group size in a
particular study does not necessarily mean that this is
typical of the species. Both points are illustrated by
Reyer’s (1990) study of pied kingfishers, which found
a strong effect of group size at one site, but none at
another. A geographically limited study would at best
have gained a limited insight into cooperative breeding
in that species. Comparisons within populations at
single sites can also give insight into variability in group-
size effects, and may be more practicable. Variation among
years and among breeders of different age reveal the
same pattern of a greater group-size effect in poorer
conditions for breeding.
Variation in group-size effects within species also
raises the issue of helper flexibility and how individuals
assess fitness benefits and costs of helping. Individuals
could estimate the benefits of helping most accurately
if they were able to assess their potential effect on the
group’s reproductive success in specific circumstances,
in addition to their relatedness to breeders. A likely
problem in interpreting the causes of helper flexibility
is that the benefits and costs of helping could covary
with conditions for breeding (Heinsohn & Legge 1999).

For example, poor environmental conditions could raise
both the costs and benefits of helping. Focusing on
breeder age may help isolate variation in the benefits
of helping, rather than the costs, since contributing a
constant effort can have larger effects for some breeders
than others.
Acknowledgements
The work presented here would not have been possible
without the help and collaboration of many people.
Beth Bobroff, Janet Gardner, Tony Giannasca, Ashley
Leedman, Anjeli Nathan, James Nicholls and Linda
Whittingham made substantial contributions to field-
work in more than one year, and Camille Crowley, Megan
MacKenzie, Helen Osmond, Amy Rogers, Derek Smith,
Lynda Sharpe, Kate Trumper and Stephen Yezerinac
JAE498.fm Page 383 Monday, April 16, 2001 9:57 AM
384
R.D. Magrath
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
370–385
also made valuable contributions. Sam Portelli and
Belinda Mitterdorfer helped with data entry. The paper
was largely written while on sabbatical with Jamie
Smith, and I thank him and all of those in the Depart-
ment of Zoology at UBC who provided a stimulating
environment in which to work. Andrew Cockburn,
Rob Heinsohn, Elsie Krebs, David Green, Jamie Smith,

Liana Zanette and two anonymous referees provided
insightful comments on earlier versions of the manu-
script. This research was supported by grants from
the Australian Research Council, and was carried out
under permits from the Australian National University
ethics committee, the Australian Bird and Bat Banding
Scheme, the Australian National Botanic Gardens and
Environment ACT.
Dedication
The paper is dedicated to the memory of Anjeli Nathan,
who worked on the scrubwrens during two field seasons
and died in a car accident in South Africa in 1999, aged
24. In addition to the measurable contribution of gath-
ering data, she contributed immeasurably to the project
through her enthusiasm and dedication.
References
Ambrose, S.J. (1985) Aspects of the physiological and beha-
vioural ecology of the White-browed Scrubwren Sericornis
frontalis (Aves – Acanthizidae) in Western Australia. PhD
Thesis, University of Western Australia.
Ambrose, S.J. & Davies, S.J.J.D. (1989) The social organization
of the White-browed Scrubwren Sericornis frontalis Gould
in arid, semi-arid and mesic environments of Western Australia.
Emu, 89, 40–46.
Austad, S.N. & Rabenold, K.N. (1985) Reproductive
enhancement by helpers and an experimental inquiry into
its mechanism in the bicolored wren. Behavioral Ecology
and Sociobiology, 17, 19–27.
Baird, D.B. & Hunt, L.M. (1998) Introduction to Genstat for
Windows, 3rd edn. AgResearch, Lincoln, New Zealand.

Blakers, M., Davies, S.J.J.F. & Reilly, P.N. (1984) The Atlas of
Australian Birds. Melbourne University Press, Melbourne.
Boland, C.R.J., Heinsohn, R. & Cockburn, A. (1997) Experi-
mental manipulation of brood reduction and parental care
in cooperatively breeding white winged choughs. Journal of
Animal Ecology, 66, 683–691.
Brown, J.L. (1987) Helping and Communal Breeding in Birds.
Princeton University Press, Princeton.
Brown, J.L. & Brown, E.R. (1984) Parental facilitation: parent –
offspring relations in communally breeding birds. Behavioral
Ecology and Sociobiology, 14, 203–209.
Brown, J.L., Brown, E.R., Brown, S.D. & Dow, D.D. (1982)
Helpers: effects of experimental removal on reproductive
success. Science, 215, 421–422.
Cale, P. (1994) Temporal changes in the foraging behaviour
of insectivorous birds in a schlerophyll forest in Tasmania.
Emu, 94, 116–126.
Christidis, L. & Boles, W.E. (1994) The Taxonomy and Species
of Birds of Australia and its Territories. Royal Australasian
Ornithologists Union, Melbourne.
Christidis, L. & Schodde, R. (1991) Genetic differentiation in
the White-browed Scrubwren (Sericornis frontalis) complex
(Aves: acanthizidae). Australian Journal of Zoology, 39,
709–718.
Cockburn, A. (1998) Evolution of helping behavior in co-
operatively breeding birds. Annual Review of Ecology and
Systematics, 29, 141–177.
Curry, R.L. & Grant, P.R. (1990) Galápagos mockingbirds:
territorial cooperative breeding in a climatically variable
environment. Cooperative Breeding in Birds: long-term studies

of ecology and behavior (eds P.B. Stacey & W.D. Koenig),
pp. 289–331. Cambridge University Press, Cambridge.
Desrochers, A. (1992a) Age and foraging success in European
Blackbirds: variation among and within individuals. Animal
Behaviour, 43, 885–894.
Desrochers, A. (1992b) Age-related differences in reproduction
by European blackbirds: restraint or constraint? Ecology,
73, 1128–1131.
Emlen, S.T. (1991) The evolution of cooperative breeding in
birds and mammals. Behavioural Ecology: an evolutionary
approach (eds J.R. Krebs & N.B. Davies), pp. 301–337.
Oxford University Press, Oxford.
Emlen, S.T. (1997) Predicting family dynamics in social
vertebrates. Behavioural Ecology: an evolutionary approach
(eds J.R. Krebs & N.B. Davies), pp. 228–253. Blackwell
Science, Oxford.
Emlen, S.T. & Wrege, P.H. (1989) A test of alternative hypotheses
for helping behavior in white-fronted bee-eaters of Kenya.
Behavioral Ecology and Sociobiology, 25, 303–319.
Emlen, S.T. & Wrege, P.H. (1991) Breeding biology of white-
fronted bee-eaters at Nakuru: the influence of helpers on
breeder fitness. Journal of Animal Ecology, 60, 309–326.
Faaborg, J. & Bednarz, J.C. (1990) Galapagos and Harris’
hawks: divergent causes of sociality in two raptors. Cooperat-
ive Breeding in Birds: long-term studies of ecology and behavior
(eds P.B. Stacey & W.D. Koenig), pp. 357–383. Cambridge
University Press, Cambridge.
Forslund, P. & Pärt, T. (1995) Age and reproduction in birds –
hypotheses and tests. Trends in Ecology and Evolution, 10,
374–378.

Genstat, 5 Committee (1993) Genstat 5, Release 3, Reference
Manual. Clarendon Press, Oxford.
Hatchwell, B.J. (1999) Investment strategies of breeders in
avian cooperative breeding systems. American Naturalist,
154, 205–219.
Heinsohn, R.G. (1992) Cooperative enhancement of reproduct-
ive success in white-winged choughs. Evolutionary Ecology,
6, 97–114.
Heinsohn, R.G. & Legge, S. (1999) The cost of helping. Trends
in Ecology and Evolution, 14, 53–57.
Keast, A. (1978) Zoogeography and evolutionary history of
the Scrubwren genus Sericornis. Emu, 78, 119–125.
Koenig, W.D. & Stacey, P.B. (1990) Acorn Woodpeckers: group-
living and food storage under contrasting ecological con-
ditions. In: Cooperative Breeding in Birds: long-term studies
of ecology and behavior (eds P.B. Stacey & W.D. Koenig),
pp. 413–453. Cambridge University Press, Cambridge.
Komdeur, J. (1994) Experimental evidence for helping and
hindering by previous offspring in the cooperative-breeding
Seychelles warbler Acrocephalus sechellensis. Behavioral
Ecology and Sociobiology, 34, 175–186.
Komdeur, J. (1996a) Influence of helping and breeding
experience on reproductive performance in the Seychelles
warbler: a translocation experiment. Behavioral Ecology, 7,
326–333.
Komdeur, J. (1996b) Influence of age on reproductive per-
formance in the Seychelles warbler. Behavioral Ecology, 7,
417–425.
Legge, S. (2000) Helper contributions in the cooperatively
breeding laughing kookaburra: feeding young is no laughing

matter. Animal Behaviour, 59, 1009–1018.
Lennartz, M.R., Hooper, R.G. & Harlow, R.F. (1987) Social-
ity and cooperative breeding of red-cockaded woodpeckers,
Picoides borealis. Behavioral Ecology and Sociobiology, 20,
77–88.
JAE498.fm Page 384 Monday, April 16, 2001 9:57 AM
385
Group size and
reproductive
success in birds
© 2001 British
Ecological Society,
Journal of Animal
Ecology, 70,
370–385
Magrath, R.D., Leedman, A.W., Gardner, J.L., Giannasca, A.,
Nathan, A.C., Yezerinac, S.M. & Nicholls, J.A. (2000) Life
in the slow lane: reproductive life history of the white-browed
scrubwren, an Australian endemic. Auk, 117, 479–489.
Magrath, R.D. & Whittingham, L.A. (1997) Subordinate
males are more likely to help if unrelated to the breeding
female in cooperatively breeding white-browed scrubwrens.
Behavioral Ecology and Sociobiology, 41, 185–192.
Magrath, R.D. & Yezerinac, S.M. (1997) Facultative helping
does not influence reproductive success or survival in
cooperatively breeding white-browed scrubwrens. Journal
of Animal Ecology, 66, 658–670.
Mumme, R.L. (1992) Do helpers increase reproductive suc-
cess? An experimental analysis in the Florida scrub jay.
Behavioral Ecology and Sociobiology, 31, 319–328.

Mumme, R.L. (1997) A bird’s-eye view of mammalian co-
operative breeding. Cooperative Breeding in Mammals (eds
N.G. Solomon & J.F. French), pp. 364–388. Cambridge
University Press, Cambridge.
Rabenold, K.N. (1990) Campylorhynchus wrens: the ecology
of delayed dispersal and cooperation in the Venezuelan
savanna. Cooperative Breeding in Birds: long-term studies
of ecology and behavior (eds P.B. Stacey & W.D. Koenig),
pp. 157–196. Cambridge University Press, Cambridge.
Rabenold, P.P., Rabenold, K.N., Piper, W.H., Haydock, J. &
Zack, S.W. (1990) Shared paternity revealed by genetic
analysis in cooperatively breeding tropical wrens. Nature,
348, 538–540.
Reyer, H U. (1990) Pied kingfishers: ecological causes and
reproductive consequences of cooperative breeding. Co-
operative Breeding in Birds: long-term studies of ecology and
behavior (eds P.B. Stacey & W.D. Koenig), pp. 527–557.
Cambridge University Press, Cambridge.
Russell, E.M. & Rowley, I. (1993) Demography of the co-
operatively breeding splendid fairy-wren, Malurus splendens
(Maluridae). Australian Journal of Zoology, 41, 475–505.
Sæther, B E. (1990) Age-specific variation in reproductive
performance of birds. Current Ornithology, Vol. 5 (ed.
D.M. Power), pp. 251–283. Plenum Press, New York.
Schodde, R. & Mason, I.J. (1999) The Directory of Australian
Birds: passerines. CSIRO Publishing, Melbourne.
Siegel, S. & Castellan, N.J. (1988) Nonparametric Statistics for
the Behavioral Sciences. McGraw-Hill, New York.
Smith, J.N.M. (1990) Summary. Cooperative Breeding in Birds:
long-term studies of ecology and behavior (eds P.B. Stacey &

W.D. Koenig), pp. 593–611. Cambridge University Press,
Cambridge.
SPSS Inc. (1999a) SPSS 9.0. Base Syntax Reference Guide.
SPSS Inc., Chicago.
SPSS Inc. (1999b) SPSS 9.0. Regression Models Reference
Guide. SPSS Inc., Chicago.
SPSS Inc. (1999c) SPSS 9.0. Advanced Models Reference
Guide. SPSS Inc., Chicago.
Stacey, P.B. & Koenig, W.D. (1990) Cooperative Breeding in
Birds: long-term studies of ecology and behavior. Cambridge
University Press, Cambridge.
Whittingham, L.A., Dunn, P.O. & Magrath, R.D. (1997)
Relatedness, polyandry and extra-group paternity in the
cooperatively-breeding white-browed scrubwren (Sericornis
frontalis). Behavioral Ecology and Sociobiology, 40, 261–
270.
Woolfenden, G.E. & Fitzpatrick, J.W. (1984) The Florida
Scrub Jay: demography of a cooperative-breeding bird.
Princeton University Press, Princeton.
Received 3 June 2000; revision received 3 October 2000
JAE498.fm Page 385 Monday, April 16, 2001 9:57 AM