Ornithological Monographs 19

BIRD

POPULATIONS

ASPEN

OF

FORESTS

WESTERN

NORTH

IN

AMERICA

BY

J. A. DOUGLAS

FLACK

Department of Zoology
University of Wisconsin

ORNITHOLOGICAL

MONOGRAPHS

PUBLISHED

THE

AMERICAN

BY

ORNITHOLOGISTS'
1976

NO.

UNION

19


BIRD

POPULATIONS

IN

WESTERN

OF

NORTH

ASPEN

FORESTS

AMERICA


ORNITHOLOGICAL

MONOGRAPHS

This series,publishedby the American Ornithologists'Union, has been
establishedfor major paperstoo long for inclusionin the Union's journal,
The Auk. Publicationhas been made possiblethrough the generosityof
Mrs. Carll Tucker and the Marcia Brady Tucker Foundation,Inc.
Correspondence
concerningmanuscriptsfor publicationin the seriesshould
be addressed
to the Editor, Dr. John William Hardy, Departmentof Natural
Science,The Florida State Museum, University of Florida, Gainesville,
Florida

32611.

Copiesof OrnithologicalMonographsmay be orderedfrom the Assistant
to the Treasurerof the AOU, Glen E. Woolfenden,Departmentof Biology,
Universityof SouthFlorida, Tampa, Florida 33620. (See price list on back
and inside back cover.)

OrnithologicalMonographs,No. 19, viii + 97 pp.

Editor of A.O.U. Monographs,John William Hardy
SpecialAssociateEditor of this issue,Victor Lewin, Department
of Zoology,Universityof Alberta, Edmonton,Alberta, Canada
Author, J. A. Douglas Flack, Wildlife Service, 14 Kotuku Rd.,
Kaikoura, New Zealand

First received,July 1970; accepted,September1971; final revision
completed,July 1975
Issued January 30, 1976

Price$7.50 prepaid($6.00 to AOU Members)
Library of CongressCatalogueCard Number 75-46439
Printed by the Allen Press,Inc., Lawrence, Kansas 66044
Copyright ¸

by American Ornithologists'Union, 1976

ii


BIRD

POPULATIONS

ASPEN

WESTERN

OF

FORESTS

NORTH

IN

AMERICA

BY

J. A. DOUGLAS

FLACK

Department of Zoology
University of Wisconsin

ORNITHOLOGICAL

MONOGRAPHS
PUBLISHED

THE

AMERICAN

BY

ORNITHOLOGISTS'
1976

NO.

UNION

19


DRURY 1953

RAND 1944

MUNRO
COWAN
1947
/

SOPER
/
1949
/
•

•

I
e34 f

33- ß 35-36
•

e32

•/

•, e31

• ,

'•.•.•

"•, •
/

-.

• HICKEY
1956

•

IKENDEIGH
1956

•

•e 20-27

'

'

40

]

ß 16J18

•

.......
-t

• ß 13-15•

ß

1 - 41

AUTHOR'S STUDY AREAS, 1966--1969
DRURY 1953, AND OTHER STUDIES
FROM LITERATURE

CYPRESS HILLS
o

I

0

200

I,

200

I'

400

p ,I

600

600

}

I,

1000 km

Aspen forest study areas mentioned in text,

Tropicof Cancer


TABLE
INTRODUCTION
METHODS

OF

CONTENTS

.................................................

..........................................................

SELECTION

WITHIN

THE

HABITAT

THE

CANOPY

3

........................

COMPARISONS BETWEEN REGIONS

1

.....................

...................................

7
15
23

CAVITY NESTING BIRDS ..........................

33

FLYCATCHING BIRDS ......................................

44

SHRUB AND GROUND NESTING

47

DIVERSITY

PATTERNS

AVIFAUNAL
SPECIES

BIRDS

.........................

AFFINITIES

ACCOUNTS

SUMMARY

............................

..................

...........................

.................................................

ACKNOWLEDGMENTS
LITERATURE

CITED

.........................
....................

57
70
79
86
87
88

APPENDIX

1 ........................................................

96

APPENDIX

2 ................................................

96

LIST

OF

FIGURES

Frontispiece ...............................................

iv

Figure 1. Bird speciesdiversity (H' Shannon-Weiner) in relation to tree density.
Dots:
stands 1-27; open circles: stands 28-41; X : stand 34. This
system pertains to all figures with exceptions noted
.................
16
2. Bird speciesdiversity (H') in relation to average tree diameter at breast
height

..........................................

16

3. The total number of bird species in the summations in relation to tree
density .................................
17

4. The total number of birds in the summationsin relation to tree density _ 18

5. The total number of bird speciesin the summationsin relation to average
tree diameter at breast height (dbh) ............................
19
6. The total number of birds in the summations in relation to average tree
diameter at breast height .................................................
20

7. Nesting guild diversity in relation to tree density. See text for explanation
of guilds ....................................................

22

8. The number of birds nesting in the canopy (guilds 1 and 2) in relation
to tree density in stands 1-19 .......................................................
23


9. The number of birds nesting on large limbs (guild 2) in relation to tree
density in stands 1-18 ...........................................................................
24

10. The number of speciesnestingin the canopy (guilds 1 and 2) in relation
to tree density ...................................................................................
24
11. The number of birds nestingin the canopy (guilds 1 and 2) in relation to
average thicknessof the canopy in stands 28-41 ...................................... 25
12. The number of birds feeding in the canopy by gleaning (guild la) in relation to averageheight to the top of the canopy ....................................... 25
13. The number of Western Wood Pewees in relation to tree density .............

26

14. The number of Western Wood Pewees in relation to average tree diameter

at breast height (dbh) ....................................................................

26

15. The number of American Robins in relation to average tree diameter at

breast height (dbh) ........................................................................

27

16. The averageheight to the top of the canopy in relation to the average
thicknessof the canopy ...................................................................
27
17. The number of birds nestingin the canopy (guilds 1 and 2) in relation
to the average height of the canopy .............................................
28
18. The number of birds feeding in the canopy by gleaning (guild la) in

relation to the average thicknessof the canopy ......................................
28
19. The number of Baltimore Orioles in relation to the average height to the
top of the canopy ..............................................................
29
20. The number of Baltimore Orioles in relation to the average thickness of
the canopy ..........................................................................................
29

21. The number of speciesfeeding in the canopy (guild la) and canopy and
shrubs (guild 2a) in relation to tree density .............................................. 30
22. The number of birds feeding in the canopy (guild la) and canopy and
shrubs (guild 2a) in relation to tree density .......................................
30
23. The number of hole nestingbirds in relation to tree density ....................... 31

24. The number of hole nestingspeciesin relation to tree density.................... 31
25. The number of woodpeckersin relation to tree density .......................... 32
26. The number of woodpeckersin relation to average tree diameter at breast
height (dbh) ........................................................................................
32
27. The number of birds that excavateholes in relation to tree density .........

33

28. Averagetree diameterat breastheight (dbh) in relation to tree density.... 34
29. The number of sapsuckers
and Downy Woodpeckersin relation to average
tree diameter at breast height (dbh) ......................................................... 35
30. The number of hole nestingbirds excludingHouse Wrens in relation to

average tree diameter at breast height (dbh) .................................
36
31. The number of House Wrens in relation to averagetree diameter at breast
height (dbh) .......................................................................................
37

32. The number of cavity dependentbirds in relation to the number of birds
that excavate

cavities ....................................................................................

37

33. The number of cavity dependent speciesin relation to the number of
species that excavate cavities .................................................................
38


34. The number of cavity dependentbirds in relation to the number of species
that

excavate

cavities

................................................

38

35. The number of birds that feed by flycatching in relation to the average

height to the bottom of the canopy ......................................
43
36. The number of Least Flycatchers in relation to the average height to the
bottom of the canopy ..........................................................
44
37. The averageheightto the bottom of the canopyin relation to tree density.
Stand28 was locatednear the upper altitudinallimit of parkland aspenand
the trees were stunted in height only ............................................
45
38. The number of flycatching birds in relation to tree density. Stand 29 is
discussed in the text .....................................................

46

39. The number of speciesof flycatchingbirds in relation to tree density .... 46
40. The number of birds nestingin shrubsin relation to the percentcover of
shrubsabove 35 inches (88.9 cm) ....................................................
47
41. The number of birds nestingon the ground and in shrubsin relation to the
total shrub cover above 5 inches (12.7 cm) ...................................
48
42. The number of birds that feed in both shrubs and canopy (guild 2a) in
relation to total shrub cover above 5 inches (12.7 cm) ..................
49
43. The number of birds nesting in shrubs in relation to the understory
cover value

...........................................................................

50

44. The numberof birds nestingin the shrubsand on the groundin relation
to the understory cover value. Stands 16, 17 and 30 are discussedin the
text

..................................................................................

51

45. The numberof grosbeaks(PheucticusIudovicianus
and P. meIanocephaIus)
in relation to total shrub cover. Stand 19 lacked much shrub cover, but
the trees were extremely stunted in height. The grosbeaksnested in the
trees

.........................................................................................

51

46. The number of grosbeaksin relation to the understorycover value ........ 52
47. The number of Yellow Warblers in relation to total shrubcover. *Only
standswith pools of surface water nearby are included, except stand 41 __52
48. The number of birds nesting on the ground in relation to percent cover
of litter

..............................................................................

53

49. The numberof speciesfeedingon the groundin relationto tree density.... 54

50. The number of birds feeding on the ground in relation to the percent
cover of litter.

Stand 31 is discussed in the text

..........................

54

51. The numberof birds feedingon the groundin relation to the percentcover
of forbs ..............................................................................

55

52. The number of juncos in relation to percent cover of grassesand forbs 55
53. The number of Brown-headed Cowbirds in relation to the total number
of birds in the summations .......................................................
56

54. The number of Ruffed Grouse in relation to the total shrub cover. Males

(M) and females (F) are distinguished.........................................

57

55. Bird speciesdiversity in relation to total bird speciesin parkland and
mountain aspen stands .............................................................

60

56. Bird speciesdiversity in relation to total abundanceof birds in parkland
and mountain aspen stands ...........................................................
61


TABLES

1. BreedingBirds of Aspen Forests ................................................................................ 9
2. Frequency of Occurrence of Hole Excavating Species ...........................................

33

3. Height and Tree dbh of Nests in Holes ..................................................................

35

4. Relative Abundance and Bird Species Diversity in Stands with < 600 Trees/
Acre

.............................................................................................................................

59

5. Geographical Replacement of Morphologically Similar Species............................ 62
6. Uncommon Birds (f < 35%) Arranged by the Strata in Which They Nest .... 66
7. Common Birds (f > 35%) Arranged by the Strata in Which They Nest ............ 68
8. Regional Affinities and Frequency of Occurrence (f) of the Breeding Birds of
Aspen Forests ............................................................................................................. 69
9. Summary of Regional Affinities and Sources of Species ........................................ 71
10. Affinities Within Regions .........................................................................................

viii

72


INTRODUCTION

The patterningof populations
of plantsand animalsis probablythe result
of selectionovertime of organisms
accordingto their individualphysiological
tolerances,behavioraladaptationsto an environmentalcomplex, and geographicalavailability(Gleason 1926). In certain situationsa singlefactor,
oftenfloral, exhibitsa controllinginfluenceon the aggregation
of organisms
(Curtis 1959). Dominantorganismssuchas aspentreesinfluenceimmediate
environmental
conditions
in a waywhichtendsto sortthe species
aggregations
accordingto the characteristics
of the dominant. The relationsbetweenthe
aggregations,however, are generally continuous(Bray and Curtis 1957;
Bond 1957; Beals 1960), but the limitation of somemajor feature of the environment,or of the aggregation
of plant speciessuchas by physiognomy,
or
speciescomposition
enablesthe ecologistto definea unit for analysis.
Aspen (Populus tremuloides)often occursin essentiallypure standsat
mid-altitudes

throughout
a largeregionof thewesternmountains
of theUnited
States,and at lower elevationsin the Canadianparkland and mixed boreal
forest. The physiognomy
of theseforestsis everywheresimilarbut differs
from the coniferousforests,meadow,and prairie with which aspenforests
are closelyassociated.I have arbitrarily chosenthe sharp physiognomic
boundarycreatedby this dominantdeciduousspeciesto definethe limits of
a communitythat on the basisof understoryplant speciescomposition,is not
alwaysso discrete(Lynch 1955; Langenheim1962).
Practicallyall of the quantitativeanalysesof bird populationsin western
North America

have been restricted to local areas and the habitats studied

havenot oftenbeendescribedquantitatively.Regionalstudieshavebeendescriptivein nature. As a result,the ecological
relationships
of bird populations
of the major foresttypesthroughouttheir entirerangeare not well known.
Therefore,the immediateobjectiveof thisstudyis to analyzequantitatively
and to comparesystematically
the speciescomposition
and densityof breeding bird populationsas they are relatedto the distributionand structureof
monotypicaspenforestsin the two areas: a) from Arizona to northern
Wyoming,and b) from southwestern
Alberta to Manitoba.
Quantitativedata of this natureare relevantto five problemsto which this
study is specificallyapplied: 1. the nature of communityorganization;2.
factors in forest structurewhich determinehabitat selection; 3. ecological

specializationand equivalencein animal communities;4. relationsof forest
structureand latitude to diversityin bird populations;5. the historicaland
geographical
originof thebirdsof aspenforestsin relationto the establishment
of Populustremuloidesin the westernmountainsand northern prairies.


2

ORNITHOLOGICAL

MONOGRAPHS

NO.

19

THE NATURE AND CHOICE OF STUDY AREAS

Throughoutmostof its extensive
range,aspenis a pioneerspecies
that is replacedrather quicklyby longerlived, more shadetolerant species(Baker
1925; Foweils 1965). The clonal root systemsare more permanentthan
abovegroundstems(Barnes 1966). Yet in the centralRocky Mountain region the speciesapparentlymaintains itself in extensivepure standsfor
centuries (Fetherolf 1917; Baker 1918, 1921, 1925; Reed 1952; Marr 1961;
Langenheim 1962; Ream 1963). The distribution of these forests is dis-

junct, the populationsbeing separatedby great distancesand topographic
barriersof considerablesignificance.The largestpure standsare found in
Utah and westernColoradowhere they are often the most importantforest

type. Large standsof aspenare absentfrom most of Montana.
Justsouthof Canadaand eastof the Front Range, the parklandaspenof
the prairie-forestecotonebeginsand extendsin a continuousarc of variable
width north into Alberta

and Saskatchewan and south into Manitoba.

Here

also the speciesmaintainsitself in extensivepure standsfor centuries(Bird
1930, 1961; Moss 1955; Lynch 1955). In this regionthe poplarsPopulus
trichocarpaand P. balsamiferaare sometimes
a minor component.North of
the parkland region and extendingwest into Alaska, the dominantspecies
over millionsof acresof the mixedborealforestare aspenand balsampoplar.
Aspen is clonal in nature (Gifford 1966; Barnes 1966), and the extensive
root systemsgive rise to dense,even-agedforestsfollowing removal of the
canopyby somesuddendisturbance.As a stand maturesthe densityof the
stems drops, while height, size of crown, and diameter of bole increases.
Beyondmaturity, the canopybeginsto open up, making it possiblefor suppressedsaplingsto fill in the gaps. I have found that at this stagetwo lines
of developmentare possible.The standcan either developinto a many-aged
forest,or in the Rocky Mountainsit can continueto openup, with little or no
successful
reproduction,until there are essentiallyno mature trees standing.
The former trend seemsto be the more usual;the factorscausingthe second
are not understood,but heavy use in winter by elk in someregionsand by
cattle and sheepin summerin others, microclimaticchangesdue to loss of
canopyand regionalclimatic changesare probably important influences.
A typical mature aspenstandis composedof straighttall treesprunedof
dead branches to within at least ten feet of the bottom of the crown.

Tree

crownsfrequentlyare not touchingand are not densewith foliage but are
similar in height. Understorytrees are sometimespresent. Shrub cover in
the Rocky Mountainsvaries from being moderateto absent,while ground
cover is composedof varying amountsof grasses,sedges,and forbs. The
generalappearanceis one of openness.In Canadasomeforestshave an exceedinglydenseshrub cover.
The importantplantcommunities
foundin association
with aspenin Arizona


1976

FLACK:

BIRDS

OF ASPEN

FORESTS

3

and New Mexico are those dominatedby EngelmannSpruce (Picea engelmannii) and firs (Abies spp.), PonderosaPine (Pinus ponderosa) and
grassesand forbs. In westernColorado and Utah, spruce-firforests,sagebrush(Artemisiaspp.) andmeadowsare in closeassociation,
while in western
Wyoming and southeasternIdaho, forestsof Lodgepolepine (Pinus contorta), EngelmannSpruce, and cottonwood(PopuIus angusti/oliaand P.
balsami[era);and willow (Salix spp.) and sagebrush(Artemisiaspp.) flats;

and meadowsof sedges,grassesand forbs are importantsurroundingvegetation types. In California,sagebrush
and meadowwere associated
with aspen
standsstudied.In Canada,grasslands
interdigitatewith aspenon one sideand
mixedborealforeston the other. More completediscussions
may be found
in Reed (1952), Beetle (1961), Langenheim (1962), Ream (1963) and
Fowells(1965) for the westernmountains,and Lynch (1955), Moss (1955)
and Bird (1961) and Rowe (1959) for the parkland.

Studyareaswerelocatedin theSanFrancisco
Mountains(1-4) andWhite
Mountains (5) of Arizona at elevationsof 8,000 to 10,000 feet, and the
Sangrede Cristo Range (6) and San Antone Peak (7) of New Mexico at
9,800 and 10,000 feet; the Telluride Mountains of southwesternColorado
(8-10) at 9,000 feet, the areaof Gothic, and KebblerPassColorado (11-12)
at 9,500 and 9,000 feet; at 9,000 feet on Websterflat (13-15) in southern
Utah, at 8,000 feet on the Wasatch Plateau of central Utah (16-18); at 8,000
feet on Monitor Pass (19) in east central California, and at 6,000 to 6,800

feet in JacksonHole, Wyoming(20-27).

In Canadastudyareaswere lo-

cated in southwesternAlberta at 4,200 feet (28-29), in central Alberta at
3,800 to 2,000 feet (30-36), in Saskatchewanat 2,800 to 1,600 feet (37-

40) and in Manitoba at 1,500 feet (41). Stands28-30 were locatedin the
narrowband of aspenparklandin the foothills(see Lynch 1955), while

stands31-33 and 35-36 were locatedin the ill-definedregionbetweenparkland and aspenpoplar woodland(see Bird 1961, Rowe 1959 and the Atlas
of Alberta 1969). Stands37-41 were within the more easternparts of the
parkland. Stand 34 was locatedwell within the mixed borealforest region
and is thereforetreatedseparately.The frontispiece
showstheselocationsin
relationto someother descriptions
of bird populationsin aspenforests.
METHODS

Data were collectedfrom relativelyhomogeneous
standsfor tree size and
distribution.They measured900 x 900 feet and were locatedwithin pure
(or almostpure) aspenforestsrangingfrom 30 to many thousandsof acres
in extent. In someof the standswidelyscatteredconiferswere found in the
understoryand occasionally
in the overstory.Most standsoutsideof Arizona
were isolatedfrom coniferousforest by no less than severalhundred yards
of aspen,meadow, or sagebrush.Forestswith an extremelydense shrub


4

ORNITHOLOGICAL

MONOGRAPHS

NO.

19

cover,typicallyof hazel (Corylus cornuta) in Canada, were avoidedbecause
of the great difficulty in censusingtheir birds.
In orderthat variabilitybetweenstandsand betweenregionscouldbe more
thoroughlysampled,asmanystandsaspossiblewerestudied.Therefore,rapid
techniquesfor samplingbird populationsand vegetationwere used.
Four parallel transects300 yardslong and spaced225 feet apart were
markedout with coloredplasticflaggingin eachstandduringthe springof
1968 and 1969. The useof a compasswas necessary
in order to accomplish
this accuratelyand rapidly. The lines servedto guide strip surveysof the
birds.

Stands20-27 were sampledin the spring and summerof 1966 using
slightlydifferentmethods.The 1200 yardstripsurveywasbentaroundwithin
a stand. Stand 19, in California,was atypicalsincedata from two blocks
of ratherirregularshape,separatedby 50 yardsof sagebrush,
were combined
to form one stand. Stand 38, in Saskatchewan,was large enough for only
one-half (600 yards) of a transect,but was used becauseit was the only
stand of high tree densitythat could be located.

Ten setsof threecircularquadratsmarkedwith a tapearounda centerstake
wereusedto sampletrees,saplingsand shrubsduring 1966 (20-27). These
were located using a grid and table of random numbers. The diameter at
breastheight (dbh) of each tree (greater than 1") and height to top and
bottom of the crown of four trees (measuredwith an inclinometerand range
finder) were recordedin 550acre circular quadrats. Within that quadrata
fortiethacre (18'6" radius) quadratwas usedand the heightof eachsapling
was measured.A quadratof •A00acre (11'7" radius) was usedto estimate,
with a tapemeasure,heightandpercentcoverof shrubs.

In 1968 and 1969 for stands1-19 and 28-41 twentyfortieth acrequadrats
were evenlyspacedalongthe lengthof the four marked,parallellines. The
abovedata on vegetationwere recorded.
A periscopetubewith a plumb,grid, and calibrateddiaphragmwas used
to estimatepercentcoverof the canopy. Eightyreadingswere evenlyspaced
alongparallellinesin all stands.Eachreadingestimatedthe percentof the
skyobscuredby the vegetationoverhead.A similarinstrumenthasbeendescribedby Emlen (1967) for takingpoint samplesof the overstory.
Groundcoverwas estimatedusing40 quadrats,one yard square,spaced
evenlyalongparallellinesin all stands.Percentcoverof litter, bare earth,
grasses,forbs, and averageand maximumheight of herbaceous
vegetation
were estimated
in each. Speciespresentin eachquadratwere noted. Since
no morethan oneday wasspentsamplingvegetation(June-July) in any one
stand(except20-27), identification
of all understory
species
wasnotpossible.
Objectivesamplesof this naturegiverapidlyobtainedrelativelyaccurate
summarydata for many structuralparametersin a standand are usefulfor


1976

FLACK:

BIRDS

OF

ASPEN

FORESTS

5

comparisons
betweenstands. Statisticaltreatmentof within-standparameter
variationis not justifiable(except in 20-27) becausethe samplingmethod
was not random.

A strip surveymethod of estimatingrelative abundanceof bird populations was used, recordingbirds detectedwithin 75 ft of either side of a
transect

route

followed

in a standard

manner.

After

the observer

stood

silentlyfor severalminutes,he walked slowlyfor 100 yards, and stoodfor
severalmore minutes. This was repeatedtwelve times for a total coverage

of a strip 1200 yards in length and 150 feet in width (12.4 acres). The
data are usedfor comparisonof abundanceof speciesand total populations
between

stands.

It took between1.5 and 2 hoursto surveyone standand, sincethe stands
were spreadout and the censususuallycarried out only betweensunriseand
midmorning,a maximumof two standscouldbe sampleda day.
During 1966 (20-27), the strip loopedback and forth within a standin
variablepatternsthat did not crossor overlap. In all other stands,the strip
followedcarefullymarkedout parallel centerlines. Four surveyswere made
in stands20-27 and two surveysin all other stands.
The relatively easily registeredsingingmale was the unit recordedfor
passerines.Swallowswere recordedwhen seenflying or perchedin or near
a hole. Hummingbirdswere recordedas they flew as were specieswith large
territories such as owls, hawks and ravens. Recorded single woodpeckers
were countedas pairs if they appearedin two counts,the pair was seen,or
the nestwas found. The discoveryof nestsconfirmedthe breedingstatusof
most species.
The maximumpopulationwas obtainedby summationof the maximum
number of pairs (singingmale = pair) of each passerine(Enemar 1959),
and woodpecker
species.When therewasa largediscrepancy
betweencounts,
averagingor countingnestsdeterminedthe count. Not included in the summationwere Pine Siskins(Spinuspinus), Brown-headedCowbirds (Molothrus
ater) andhummingbirds
becauseof their unusualbehavior,Goldfinches(Spinus
tristis) and Cedar Waxwings(Bombycillacedrorum)becauseof late breeding,
and specieswith largeterritories.High countsmadeduringmigrationwere excluded.

Surveyswere conductedbetween5 May and 12 July, and each stand,with
the exceptionof No. 19, had one survey during June near the peak of the
breedingseason. Calm, fair days were considerednecessaryfor the counts
to be comparable.
The use of a bird censusmade during the breedingseasonmust be approachedwith caution. Enemar (1959) has pointedout that censusmethods
are directedtowardsfour typesof populations.The breedingpopulationcan
be determinedonly by findingall nests,while the stationarypopulationis de-


6

ORNITHOLOGICAL

MONOGRAPHS

NO.

19

terminedby mappingterritories.Stripsurveysdeterminethe total population,
but summationof the resultsof severalsurveysintroducesa distortionin the
proportionsof the speciesrepresented.
Sincethisstudydemanded
that a largeareabe surveyed,I couldmakeonly
two countsin any onestand. Enemar(1959) foundin his analysisof a long
narrow studyarea (13 hectares)with a densepopulation,that on the average
60 percentof the stationarypopulationis registeredat eachsurvey,and after
two surveys84 percentof the populationhas been registeredonce. Bond
(1957) checkedthe samplecountmethodagainstthe moreintensivespotmap

methodof Kendeigh(1944) on threeplots,and found his own samplecounts
22 to 30 percentlower than spotmap counts. Emlen (1971) reviewsthe
problemsin variouscensusmethodsandpresentsa new methodfor obtaining
absolutecountsefficiently.
There are a number of sources of error in the method used. Differences

in conspicuousness
betweenspecies
shouldremainconstantfrom areato area,
exceptasit is affectedby densityof vegetation.However,changes
in the behaviorof speciesas the seasonprogresses,
as with juncos,probablyhad an
effect on results. The chanceof recountinga bird was reducedthrough
cautionand by the 75-footseparation
betweenstrips. Both aural and visual
detectionof speciesassuredlesserror in separationof singingmalesfrom
females.The numberof unmatednonbreeding
malesin a populationcanbe
large (Lack 1937), but shouldbe comparablein the stands.Transientbirds
could not often be detectedbut their numbersshouldalso be comparable
from standto stand. In a few caseswhena specieswasrecordedearly in the
season,
but not at the peak, and not detecteduponlater observation,
it was
not includedin the summation.Speedof movementalongthe transectwas
relativelyconstant,aswasweatherandtime of day of surveys.Somestands
weresurveyed
in earlyandmiddleJune,whileothersweresurveyed
in middle
Juneandlate Juneor earlyJuly. The potentialerror heredueto changes

in
the populations
is not systematic.A singleobservercarriedout all surveys,
andhisfamiliaritywith eachstandand the species
presentprior to surveywas
enhancedduringvisitsto chooseand mark the standand analyzethe vegetation. Thisexperience
demonstrated
to the authorthatfew species
weremissed
in the counts. Familiarity with each stand and the well-markedtransects
assuredgreatereffectiveness
of the surveys.
In stands20-27, in whichthe arrangement
of the stripvaried,errorsmade
will probablydiffer somewhat
from the othersurveys.Also, possibleyearly
fluctuationsin bird populationsintroducesseasonto seasonvariation.
No censusmethodis without sourcesof error and disadvantages,
or without worth as long as it is completelydescribedand consistentlyapplied
(Enemar, 1959). With thesereservations
in mind, the data are used for
comparisonof abundanceof speciesand total populationsbetweenstands.


1976

FLACK:

BIRDS

OF ASPEN

FORESTS

7

The questionof the absolutesize of populationscannotbe safely considered
without knowingthe reliability of the method.
SELECTION

WITHIN

THE

HABITAT

During late April and May the birds arrive in the standsand begin pair
formation, exhibit territoriality, and searchfor suitablenest sites. Arrival
datesdiffer amongspeciesby as much as a month, but standsin the western
mountainsare well settledbefore leating-outof the trees occurs,and before
the shrubsand herbshaveshownmuchgrowth. In Canadaleating-outoften
occursduring or just prior to the arrival of many species.Whether an individual exhibits philopatry (Welty 1962: 225; Mayr 1963: 670) or,
alternatively,arrivesat a standfor the first time, the habitat in springoften
differs greatly in appearancefrom that in which it fledged or from which it
migratedthe previoussummer.In the caseof speciesthat gleaninsectsfrom
leaves,a mostimportantfeatureof their environment
is completelyabsent.
Surrounding forests (coniferous) are physiognomicallydifferent and
branchesare coveredwith needles. It is possible,then, that individualsreact, either by geneticallydeterminedprocesses
or by thosemodifiedby experience,to prominentdifferencesin physiognomy(leaflessdeciduousforest)

whenlocatingan appropriatebreedinghabitat. It doesnot seemlikely that
sorting of speciesinto different habitats at this time occursthrough interspecificinteractions.Parnell(1969) hasshownthat manymigratorywarblers
prefer habitatssimilarto the breedinghabitat. It is alsopossiblethat species
make a choiceon the basisof more specificstructural(or other) features
of a habitat that are importantto protection,territorial behavior,nest construction,or feeding. This processcouldoccurin conjunctionwith habitatselectionon the basisof physiognomy.Finally, it is evidentthat under certain
conditionsenvironmentalfactorsotherthan habitatstructureandphysiognomy
influencehabitat selection(Root, 1967).
Throughoutthe following discussionstand 34, located near Rochester,
Alberta (-• 54øN), in the mixed boreal forest, is singledout becauseit so
often did not fit patternscommonto most other stands. The unique geographicalpositionof this standappearsto have givenrise to the differences.
It may be arguedthat differencesin this singlestand were due to chance.
However, every guild (Root 1967) showedstriking changesin densityor
compositionand the other forestsin the area that the author walked through
gave a similar impression.Then too, the accountsof Soper (1949) in this
region and similar areasto the north and west in Alberta, the descriptions
by Munro and Cowan (1947) for the PeaceRiver parklands,and thoseof
Rand (1944) along the Alaskan Highway confirm, as well as descriptive
studiesare able,the gradualreductionin the numberof bird speciesin aspen
forestsasonemovesnorthwest
from theparklandsof Alberta. Somespecies


8

ORNITHOLOGICAL

MONOGRAPHS

NO.

19

appear to be replaced geographicallyby a similar form that itself may
eventuallybe lost. Some of these changesare treated in the last section
of this monograph.The forestsare structurallyvery similar to thoseof the
parklandand southernpoplar forests. Drury (1953) has describedthe bird
speciesand their abundancein aspengrovesin the southwesternYukon Territory, and althoughhe remarks on the comparativerichnessin speciesof
this habitat, the total number of speciesbreeding, and their abundances,
are low in comparisonto parkland. Compositionis very different and certain guilds are almost absent. Williamson (1957) describesa similar reduction in numberof bird speciesin Alaskanspruceforestsalong a generally
east-westgradient.
The stand (34) may characterizegeneraldifferencesbetweenthe southern
mixed boreal forestand the parkland,althougha singlestandcannotrepresent that region. Structurallythe stand was similar to other mature aspen
forests with moderate shrub cover. The rich litter, however, seemed less
subjectto drying out. Populusbalsamiferawas more numerousthan in most
but not all parkland stands. Frosts may continueinto mid-June and the
growingseason(those weeks above forty-two degreesFahrenheit mean) is
shorterin this region (Longley 1967). Rainfall may be somewhatlessthan
in the parkland,but moisturestressis considerablyless. Variability in precipitation is less (Currie 1953). The presenceof a few paper birch (Betula
papyrifera) and spruce (Picea glauca) in the stand in part reflectedthese
differences.

A total of 14 speciesand 79 individualswere recordedin stand34 (10 and
70 in summation)in contrastto the 20 to 28 speciesand 104 to 147 individualsrecordedin standsof similartreedensityand dbh in the parkland(Table
1). In the canopythe BaltimoreOriole (Icterusgalbula) wasuncommonand
the American Goldfinch (Spinus tristis) and the Western Wood Pewee
(Contopussordidulus) were not recorded. The latter two speciesare uncommonor rare in the region (Soper 1949). Orioles are absentfurther west

(Munro and Cowan 1947). The only abundantspeciesrecordedwas the
LeastFlycatcher( Empidonaxminimus
).

Among shrub nestingspecies,the Rose-breasted
Grosbeak(Pheucticus
ludovicianus)was conspicuously
absent. The Yellow Warbler (Dendroica
petechia) nestedin a nearbyarea. HouseWrens (Troglodytesaedon) were
alsoconspicuous
by their absence,and no cavity dependentspecieswere present in spite of the abundanceof woodpeckercavities. The Pileated Woodpecker(Dryocopuspileatus)is morecommonin thispart of the mixedboreal
forestthan in the parkland,and its workingswereseenthroughoutthe stand.
Ground nestingbirds were abundant,a differencepartly substantiated
by
Soper (1949) and Weatherilland Keith (1970). Ovenbirds(Seiurusaurocapillus) were more than two times as numerousas in the parkland, and the


1976

FLACK:

BIRDS

OF ASPEN

TABLE

FORESTS

1

BREEDING BIRDS OF ASPEN FORESTS

SUMMARYDATA FOR ALL STANDSSAMPLED,1966--1969

Relative

abundance

in each stand

Ar•ona

Birds by nesting guild'•

Colorado

1

2

3

4

Warbler

8
8
16
12

8
8
10

6

14
4
14
10

4
6
10
2

Western Tanager
Evening Grosbeak

2

5

8

9

10

8
10
14
8

4
10
14
8

8
8
14
8

4

4

Canopy
Western Wood Pewee
American Robin

Warbling Vireo
Audubon's

8

12

2

6

12

4

12
8

2

4

6

4
2

4
2
2

6
2

4

2

2

Cassin's Finch
Shrubs

Empidonax flycatcher
Black-headed

2

Grosbeak

Chipping Sparrow
Song Sparrow

4

2

Holes

Red-shafted

Flicker

Yellow-bellied Sapsucker
Hairy Woodpecker
Downy Woodpecker
Violet-green Swallow
Black-cappedChickadee

2
2
6

White-breasted

2

Nuthatch

2
2
12

Pigmy Nuthatch
House Wren
Western Bluebird
Mountain Bluebird

2

2
2
4

6
2

2
8
4
4

6

2
2

14
4
4

Hermit Thrush
Townsend's Solitaire

2

2

2

2

4

4

Gray-headed Junco

4

4

4

2

4

11
17
48 102

16
58

2

4

10

6

4

2

Ground

Summation

2

2

2

6

4
2
6

4

6

6

3
20

5
28

13
62

10
62

9
56

14
76

2

1

2
4

2

1

1

2

2

4

Totals

No. species
No. individuals

15
84

Others

American Kestrel
Great Horned Owl

P
2

Long-eared Owl

Broad-tailed Hummingbird

6

2

2

2

Brown-headed Cowbird
Pine Siskin

4
2

2

1

2

2


ORNITHOLOGICAL

10

NO.

MONOGRAPHS

19

TABLE 1 (continued)
Relative

abundance

Birds by nesting guild-•

in each stand
Cal.

Utah

Colo.
11

12

13

14

15

16

17

18

19

4
6
14
6
4

2
8
8
10

8
6
14

8
2

6
8
12
8
2

10
8
14
6
4

8
12
10
6
6

4
8
6
4
6

4
8
12

6
4

8
14

2

2

4

2

6

12

Canopy
Western Wood Pewee
American
Robin

Warbling Vireo
Audubon's
Warbler
Cassin's Finch

8

Shrubs

Empidonax flycatcher

4

Yellow

2

Warbler

8

MacGillivray's Warbler
Black-headed

2

Grosbeak

2

Green-tailed

4

2
2

Lazuli Bunting
Towhee

6

2
6

Chipping Sparrow
Song Sparrow

4

6

4
8

2

2

Holes

Red-shafted

Tree

2

Flicker

Yellow-bellied Sapsucker
Red-bellied Sapsucker
Hairy Woodpecker
Downy Woodpecker
Violet-green Swallow

House Wren
Mountain
Bluebird

2

2

6

3

2
2
8
10

2
6

2

8
6

4

Swallow

Purple Martin
Black-capped Chickadee

4

2

2
4

2
6

Starling

4
2

4

2
2

2
2
8
46
8

8
8
2

4
10

2
4

2
6
2

4
14
8

2

2
2
10

Ground
Hermit Thrush
Townsend's Solitaire

Oregon Junco
Gray-headed Junco
White-crowned Sparrow
Lincoln's Sparrow
Summation

2

8

4

4

10

16

12

4

13
62

11

72

14
78

12
58

6

6

5

2
7

2
5

5

14

3

10

6

14

6

8

12

4

2

10

10
20
2

6
30

8
30

Totals

No. species
No. individuals

16

19
104 174

15
17
88 108

Others

Great Horned

Owl

Broad-tailed Hummingbird
Brown-headed
Pine Siskin

Cowbird

16
106


1976

FLACK:

BIRDS

OF

ASPEN

FORESTS

11

TABLE 1 (continued)
Relative

abundance

in each stand

Wyoming

Birds by nesting guild'[.

20

21

22

2
2
12
2

8

2
12
2

23

24

25

26

27

8

4
6
6
2
2
4

4

4
12
14
2

8

12
6

12
20

2

2
2

4

4

4

2
6
2
8
2

8
2
8
2

8

4

6

Canopy
Western Wood Pewee
American Robin

Warbling Vireo
Audubon's

12

Warbler

4
2
16
2

8
4

Western Tanager
Cassin's Finch
American Goldfinch

6

2
2

Shrubs

Empidonax flycatcher
Yellow

Warbler

8
4

8
6

MacGillivray's Warbler
Black-headed

Grosbeak

12
4
2

10
4

14

6
2

2
2

Chipping Sparrow

4

Holes
Red-shafted

Flicker

4

Yellow-bellied Sapsucker
Hairy Woodpecker
Downy Woodpecker
Tree Swallow

Black-capped Chickadee

2

House Wren
Mountain
Bluebird

2
4

4
2
2

2

4
2

10
4
4
4

8
4
12
6

2
6
4
4
4

6

6

Ground

Pink-sided

Junco

12

8

White-crowned Sparrow
Summation

6

16

8

4

2

8

14
78

16
96

14
80

Totals

No. species
No. individuals

6
40

9
46

14
66

12 16
58 102

Others

Red-tailed Hawk
American
Kestrel
Ruffed Grouse
Great Horned Owl

Common Raven
Brown-headed
Cowbird
Pine Siskin

P

P

P

2
2

1

P

1
P

2

2

1

2
2
1

P

12

2

9


ORNITHOLOGICAL

12

NO.

MONOGRAPHS

TABLE 1 (continued)
Relative

abundance

Foothills

Birds by nesting guild-•

28

in each stand

Parkland

29

30

6

2
24

6
2

4
2
4

31

Boreal

32

33

34

24

2
36

34

28

4
8

2
4

4
12

2
4
4

4
2
6

4
2
14

6

4
4
12

4
8

4

16
2

P
2

4

2

2
P

Canopy

Eastern Kingbird
Least Flycatcher
Western Wood Pewee
American Robin

Red-eyed Vireo

Warbling Vireo
Baltimore

22
4
6
4

Oriole

Western Tanager

2

2

Shrubs

Black-billed

Cuckoo

Empidonax flycatcher
Gray Catbird
Yellow Warbler
Rose-breasted Grosbeak
Black-headed Grosbeak

Chipping Sparrow
Clay-colored Sparrow

2
2
2

2

12

4

2

6
8

2

Holes

Yellow-shafted Flicker
Red-shafted Flicker

Pileated Woodpecker
Yellow-bellied Sapsucker
Hairy Woodpecker
Downy Woodpecker
Black-capped Chickadee
Red-breasted
House

Nuthatch

Wren

Mountain

Bluebird

2
2
2
2

2
2
2
4

2
18
P

2
14

12

8

2
2
2
10
P

4
2

10

2
4

20
P

10

Ground

Hermit

2
6

Thrush

Veery

14

Ovenbird
Connecticut Warbler

White-crowned Sparrow
White-throated
Summation

4

P

P
6

2
4

4

4

15
14
76 126

12
96

10
70

4

Sparrow

totals

No. species
No. individuals

19
110

17
74

15
76

19


1976

FLACK:

BIRDS

OF

ASPEN

FORESTS

13

TABLE 1 (continued)
Relative

abundance

Foothills

Birds by nesting guild'•

28

29

in each stand

Parkland

30

Boreal

31

32

33

34

3

1

1

3
5

6
9

2
5

2

10

Others

Red-tailed Hawk

P

P
P

Broad-wingedHawk
Ruffed Grouse
Great Horned Owl

Long-eared Owl
Cedar Waxwing
Brown-headed Cowbird
Pine Siskin
American Goldfinch

2

2
2

1

1

4
10
9
3

7

5
11
3

6
5
5
2

1
2
15
2

2

Relative

abundance

in each

stand

Parkland

Birds by nesting guild-•

Canopy
Lesser Yellowlegs

Eastern Kingbird
Least Flycatcher
Western Wood Pewee
American Robin

Red-eyed Vireo
Warbling Vireo
Baltimore
Common

Oriole
Grackle

35

36

37

38*

32

14

6

6
4
10

6

39

40

41

4

28
10
10
6
14
4

12

2
30
2
2
8
8
10

34

36

10
6
8

4
4
2

Purple Finch
Shrubs
Black-billed

Cuckoo

Gray Catbird
Yellow

Warbler

American Redstart
Rose-breasted Grosbeak
Rufous-sided Towhee

Chipping Sparrow
Clay-colored Sparrow
Song Sparrow

14

4

4

12

2

6

2

6

2
2

2
20
8

6
2
2
2

4

2

2

2
2
4
4
6
2


14

ORNITHOLOGICAL

MONOGRAPHS

NO.

19

TABLE 1 (continued)
Relative

abundance

in each

stand

Parkland

Birds by nesting guild-•

35

36

37

38*

39

40

41

Holes

Yellow-shafted

Flicker

2

Pileated Woodpecker
Yellow-bellied Sapsucker
Hairy Woodpecker
Downy Woodpecker
Great Crested Flycatcher

P
6

Tree Swallow

4

2
4
2
2
4

2
2

2
2

2
2

2

Black-capped Chickadee

2

Red-breasted Nuthatch

House Wren

P
2

12

P

2

2

4

2

2

10

P
8

4

2

10

Ground

Hermit

Thrush

2

Veery
Ovenbird
Connecticut

4
6

6

Warbler

P

Mourning Warbler
Summation

6
2

2

2

22

14

16

28 122 118

84

Totals

No. species
No. individuals

16

14

10

134

80

42

5

Others

Red-tailed

Hawk

2

Broad-winged Hawk

P

P

Ruffed Grouse
Great Horned Owl

1

1

1

19

10

Cedar Waxwing
Brown-headed Cowbird
American Goldfinch

P

2

2

6
P

2

1

2

5

1

3

3
1

4
10

6
2

2
5

11
3

4

Abbreviations: N.M. = New Mexico, Colo. = Colorado, Cal. = California, Foothills = Foothills
of parkland in Alberta, Boreal = southern boreal region of Alberta, P : present in similar adjacent

habitat. •Guilds are composedof speciesshowingsimilaritiesin environmentalexploitation(Root,
1967); ott•ers, the last grouping of birds for each locality is not a guild. It records birds that could
not be included in .the summation, as described in the text under methods. * Stand 38 was large
enough for only one-half of a transect; recorded abundances were doubled except for the House Wren

whose high relative abundancewas disproportionatelyinfluenced by favorable habitat on edges.

Hermit Thrush (Catharusguttatus) and White-throatedSparrow (Zonotrichia albicollis) were common. ConnecticutWarblers (Oporornis agilis)
also nested in the area.

The compositionand densityof stand 34, with the exceptionof ground
nesters,suggested
a depauperateparklandavifauna. This standhad the lowest
diversityindex of any matureforestfrom Arizona to Manitoba (Fig. 1 and
2). It mayproveto be of somerelevancethatthereis a similarabruptchange
in bird diversityat 20-25 ø latitudein centralAmerica, a phenomenathat

1976

FLACK:

BIRDS

OF

ASPEN

FORESTS

15

may also be associatedwith certain climatic thresholds(Klopfer and MacArthur 1961; Tramer 1968).
COMPARISONS

BETWEEN

REGIONS

The higher total number of breedingspeciesrecordedin the parkland
standsthan in the westernmountainsappearsto be a result of severalhistorical, geographicaland ecologicalfactors some of which are discussed
later. The greaternumberof speciesand individualsfound in a given stand
in Canadain contrastto the westernmountains(Fig. 3 and 4, Table 1) receives more attention here.

The first point to examineis whetherstandsin the two regionsare actually
structurallysimilar, becausea structurallymore diversestand might be expectedto attract more birds. Standsin both regionsare dominatedby essentiallypureaggregations
of Populustremuloides,
althoughin bothregionsseveral speciesof conifermay occurin low numbers.In Canadaanotherpoplar,P.

trichocarpa,occurredin stand 28 in southwesternAlberta and P. balsamifera
occurredin eightother stands,and in the two mosteasternstands(40 and 41)
an oak, Quercus macrocarpa, and an ash Fraxinus pennsylvanicawere found
in low numbers.This wouldsuggest
that the Canadianforestsare morediverse
structurallybecauseof the presenceof theseadditionaltypesof trees. However, most standsinclude few or none of these additional trees, and with the
exceptionof P. balsamiferanone were representedby more than a few individuals.Of the nine standshavingan additionalpoplar, in only four (Nos.
34, 35, 36, 39) did the total number of treesexceedfive percent, most individualshavingbeen in clumpsin one or two places. A greaterrange of
forest densitieswere presentin the mountains. There, tree dbh and height
alsooften reachedgreaterproportionsthan in Canada,and standsin Canada
showedmuchgreaterhomogeneity
for tree dbh. The tallestforestsaveraged
59 feet in Canada and 70 feet in the mountains, while the maximum thicknessof the canopyin both regionswas 28 feet.
There was a greatdeal of overlapin the two areaswith regardsto amount
of shrubcoverand the speciesof shrubspresent.Coverin the westernmountainsrangedfrom nonein the southto 19% in Wyomingwhile that in Canada
rangedfrom 8 to 46%. Dominant shrubspeciesin the two areaswere variable but typically different. Standsin Canada with very denseshrub cover
were not studiedbecauseof the great difficultyin movingthroughthem.
The groundcover in the two regionsis alwaysa mixture of grassesand
forbswith varyingamountsof litter coveredbare earth. Standsin the western
mountainscould have as much as 68% grassor 94% forbs, comparedto a
maximumof 30% grassor 75% forbs in Canada. Ground coveredonly by
a thick layer of litter compriseda greaterpercentof the coverin Canada.
In summary,the differencesbetweenthe two regionsare mainly floristic


16

ORNITHOLOGICAL

ee

)-

MONOGRAPHS

NO. 19

0

e¸

ß

n•
hi

0

6) ß
ß ß0

ß

0
0

0

ßß

0

hi

hi

ß

0

n•

200

400
TREES

600
PER

ACRE

IN

800

"2000

7000

OVERSTORY

FIGURE

1. Bird s•cies diversity (H' Shannon-Weiner)in relation to tree density.
Dots ---- stands1-27; open circles---- stands28-41; X : stand 34. This system

pertains to all figures with exceptionsnoted.

ß ..

C• O0
O0

ß

ß

0 0 ß

(/)
ILl

x
ILl

(/)

r•

IN.O

2
5.1

CM.O

4
10.2
AVE.

FIGURE

2.

height.

6
15.2
dbh

8
20.3

OF OVERSTORY

I0
25.4

12

30.5

14
35.6

TREES

Bird speciesdiversity (H') in relation to averagetree diameter at breast