INTERSEASONAL
AND
INTERSEXUAL
PARTITIONING
IN HAIRY
WHITE-HEADED
MICHAEL
RESOURCE
AND
WOODPECKERS
L. MORRISON
AND KIMBERLY A. WITH •
Department
of ForestryandResource
Management,
Universityof California,
Berkeley,California94720USA
ABSTRACT.--Foraging
behavior in the closely related Hairy (Picoides
villosus)and Whiteheaded(P. albolarvatus)
woodpeckerswas examinedin an area of sympatryto evaluateinterseasonaland intersexualresourceuse. Similar foraging heights were observedfor each
species-sexsample during summer, but significant differenceswere evident during winter.
Male and female White-headsmaintained similar relative foraging heights between seasons,
whereasmale and female Halties foragedrelatively higher during winter. Use of tree species
differed significantlyfor each classbetween seasons,exceptfor male Halties. Differencesin
foraging substratesand tree health also were noted interseasonally.All foraged at similar
timesof day during summer,but negativerelationshipsoccurredbetweentimesof foraging
during winter; foraging times were significantlydifferent between male Halties and Whiteheads.
A general trend toward decreasedoverlap in foraging behaviors during winter was a
reflectionof concentrationof foraging activitieson live incensecedar(Calocedrus
decurrens),
especiallyby female White-heads. This change apparently was due to the presenceof an
abundant and accessibleprey (incense cedar scale,Xylococculus
macrocarpae)
on cedar. Behavioral shifts in foraging activitiesmay be related to the differential ability to extractprey
as a function of bill morphology; such an idea can be extended to intersexual as well as
interseasonalconsiderations.Thus, segregationof foraging activities in these two woodpecker speciesmay be attributed to morphologicaldifferencesand habitat complexityrather
than to competitive interactionsdictated by resourcelimititions. Received16 May 1986,accepted13 October1986.
THEutilization and partitioning of resources patric speciesof woodpeckersdiffer greatly in
by closely related, syrupattic speciesof birds size and in foraging habits (Short 1971). The
has attractedmuch study (e.g. see reviews by two speciesconsideredhere were describedby
Schoener 1974, Eckhardt 1979, Rotenberry Short (1971) as "ecologicallyseparate"when
1985). As a group, co-occurring species of sympatric,with albolarvatus
occuringin "pine"
woodpeckers (family Picidae) have received and villosus in "other coniferous forest and
frequent attention (e.g. Kisiel 1972; Williams mixed woods." We evaluated modes of re1975;Conner 1980,1981),possiblyowing to the source partitioning by these two similar-size
ease of observation and the occurrence of sex(by body mass;Dunning 1984) and closely reual dimorphisrn.
Thus,woodpeckers
makegood lated (Short 1971, 1982) speciesin an area typsubjects
for evaluationof resourcepartitioning ified by a ubiquitousmixture of five coniferous
on bothinterspecific
and intersexuallevels(e.g. and one deciduoustree species.Thesedatawere
Kilham 1965,Ligon 1968,Austin 1976,Jenkins analyzed on an intersexual and interseasonal
1979, Williams 1980, Peters and Grubb 1983).
During preliminary studies of bird communities in the western Sierra Nevada, California,
we
noted
the
co-occurrence
of two
basis to determine
of resource
use
Picoides
speciesin a mixed-coniferforest:the Hairy (P.
villosus) and White-headed
if methods
and the magnitude of resourcepartitioning
varied by sex,season,or both.
STUDY AREA AND METHODS
(P. albolarvatus)
woodpeckers(Morrisonet al. 1985,1986).Sym•Present address:Department of BiologicalSciences,Northern Arizona University, Flagstaff,Ari-
The study area was the Blodgett Forest Research
Station(administeredby the Department of Forestry
and ResourceManagement,University of California,
Berkeley), El Dorado Co., California. This 1,200-ha
zona 86011 USA.
forest is located in the mixed-conifer zone (see Grif-
225
The Auk 104: 225-233. April 1987
226
MORRISON
ANDWITH
fin and Critchfield 1972) at about 1,350 m elevation
in the west-centralSierra Nevada. The forest is predominatedby incensecedar(Calocedrus
decurrens;
25%
of total basalarea, unpubl. data), white fir (Abiesconcolor;21%), ponderosapine (Pinusponderosa;
19%),
Douglasfir (Pseudotsuga
rnenziesii;
15%),sugarpine (P.
larnbertiana;10%), and California black oak (Quercus
kelloggii;
8%).The foresthasbeen divided into 5-40ha compartments
managedunder varioussilvicultural systems.The forest is now mostly mature (>70 yr
[Auk,Vol. 104
vations).We modified our methodsbecauseanalysis
of sequentialobservationshascertainstatisticalproblems. Results of both techniques are comparable,
however, given adequatesample sizes;our data exceeded this requirement (Morrison 1984). The presenceof observerslikely affectedforaging activities.
Becausethe initial sighting of a bird probably was
biasedtoward conspicuousindividuals, using only
the first observation
of an individual
would not solve
the problemassociated
with observereffectson forold) conifer.
aging activities.We assumedthat theseproblemsinDuring summer(early May to late July) 1983 and fluencedthe data in a similar manner for all species
1984, 24 compartmentstotaling about 420 ha were and sexes.Becauseof problemsconcerningindepenselectedfor study.During summer1985we selected denceof data recordedsequentially,we were espe~
9 compartments,7 of which differed from thoseused cially carefulwhen interpreting resultsat P near 0.05.
We calculated resourceoverlap,
during 1983-1984;the 1985 study area totaled about
210 ha. Because access to much of the forest was lim-
ited during winter, a subsetof 4 large compartments
usedduring summertotalingabout 100 ha was selected for study during winter (early November to
mid-March) 1982-1983 and 1983-1984. Summer and
winter siteswere of similar ageand tree-speciescomposition.
The abundanceof Hairy and White-headedwoodpeckersis similarduring winter (0.19and 0.17birds/
count,respectively)and summer(0.13and 0.08birds/
count, respectively)(Morrison et al. 1986). Other
woodpeckerspeciesthat occurredat Blodgettwere
Northern Flickers (Colaptesauratus), Red-breasted
Sapsuckers(Sphyrapicus
tuber),and Downy (P. pubescens)and Pileated (Dryocopus
pileatus)woodpeckers
(Morrisonet al. 1986).Adequatedatawere not available, however, to analyze these speciesinterseasonally or intersexually.
About 1,250person-hoursduring summerand 700
person-hoursduring winter were spent observing
foragingbehavior.An observerwalked systematically througha compartment
recordingdataon birdsas
encountered; I0 different observersspent varying
amountsof time recordingdataduring the study.We
divided our activitiesso that roughly equal time was
spent in each compartment during each season.Data
where Px,and py,are the proportionaluse of the ith
resourcestate by the xth and yth species.C ranges
from 0 (totaldissimilarity)to 1 (total similarity)(Colwell and Futuyma 1971).A two-dimensional measure
was calculatedfrom a foraging tree-foragingsubstratematrix. Calculatingmultidimensionalresource
overlapin this manner is preferableto usingadditive
or multiplicativevalues (Alatalo 1982). We did not
calculate three-dimensional
values
would
become
matrices because cellular
too small
for
calculation
of
meaningful results (Alatalo 1982). These indices
presentsimplesummariesof data,but do not indicate
statisticalsignificance(althoughthey may be biologically important). Therefore, we compared(for independence)the distribution of certain foraging activities for the variousresourcestatesbetweenspecies
and sexesusing a Chi-squareanalysis(Fisher'sexact
test was used for small samples;see Norusis 1983).
Further, through computersimulations,Ricklefsand
Lau (1980) found that a difference of 0.1-0.2 was required to rejectthe null hypothesisthat two overlap
indices were drawn from the same sampling distribution. Therefore, we adopted the general rule that
two indices must differ by at least 0.2 to merit dis-
on only one individual of a specieswere recordedat
a particularplaceand time regardlessof the number
of birds present.Data on only one or two species
were recorded when observing flocking birds to cussion.
Resourcepartitioning may occurif speciesforage
minimize the potential problem of correlatedactiviat different times (Schoener1974). For this study we
ties of co-occurringindividuals.
Except for winter 1982-1983, foraging activities useda simple index of foraging time: assumingobwere recorded
for a minimum
of 10 s to a maximum
of about 30 s. For each individual
we recorded date,
time of day, species,sex,speciesof plant, foraging
substrate(e.g. limb, trunk), time spenton eachsubstrate,perch height, plant height, and type of foraging motion (e.g. glean, peck).This sameinformation was recorded during winter 1982-1983, but
observationswere made on an individual every 30 s
to a maximumof about 5 rain (i.e. sequentialobser-
servers encountered birds at random, we calculated
species-and sex-specifictimes of activity by determining the frequencyat which birds were observed
in 1-h periods. We then used Spearman'srank correlation (rs)to determineif timesof foragingactivities were similar. Observerssystematicallysurveyed
birds throughoutthe day, which reducedpotential
biasesin foraging activity times.
The external morphology of Hairy and White-
April1987]
Woodpecker
Resource
Partitioning
SUMMER
227
WINTER
FORAGING
MHAWO
HEIGHT
FHAWO
MWHWO
5-10
15-20
FWHWO
10-15
>20
TREE SPECIES
USE
pine
I......I-'r--r///I//J--•.....I
white fir
Douglas-
sugar
black
oak[] other
pme
cJ I...............
•
I
SUBSTRATE
TREE
MHAWO
FHAWO
USE
CONDITION
• healthy
declimng
MWHWO
dead
FWHWO
100
0
PERCENT
100
USE
Fig. 1. Use of foraging heights, tree species,foraging substrates,and tree condition for male (M) and
female (F) Hairy (HAWO) and White-headed (WHWO) woodpeckersduring summerand winter at Blodgett
Forest, California.
headedwoodpeckers
variesboth interspecifically
and
intersexually (Grinnell 1902, Ridgway 1914, Ligon
1973, Dunning 1984).We supplementedthese data
by measuringthe length of the exposedculmen of
10 individuals for each sex of the two woodpeckers
using museumspecimensfrom within the range of
the subspeciesoccurringat Blodgett(A.O.U. 1957).
Analyseswere conductedusingSPSSx (SPSS1983),
exceptfor the Chi-squaretests,which were executed
on the Number Cruncher Statistical System version
4.2 (J. L. Hintze 1985, Kaysville, Utah).
RESULTS
Foragingheight.--The distributions of foraging activitiesby height categories(Fig. 1) were
significantly (X2 test, P < 0.01) different between seasonsfor all but female Hairies (P >
0.05). Male White-heads had a more even dis-
tribution of foraging heights during winter
Mean foraging heights were similar for all
samplesduring summer,but significantdifferenceswere evident during winter, when male
Hairies foragedhigher and female White-heads
lower than was evident at other seasons(Table
2). Except for female White-heads, all others
increasedsignificantlytheir absoluteforaging
height during winter. A trend toward increasedabsoluteheight of the foragingtree also
was observedduring winter, except for female
White-heads,which used significantly shorter
treesduring winter. The relative positions(foraging height/tree height ratio) of male and female White-heads
sons, however.
were
identical
between
sea-
In contrast, male and female
Hairies foragedrelatively higher in treesduring winter (Table 2). Although all except female Hairies used smaller (diameter at breast
height)treesduring winter, only femaleWhiteheadsshoweda significantchange.Becausewe
centrated between 5 and 10 m (Fig. 1). Males report data for all tree speciescombined, no
of both speciesexhibited high overlap (>0.7) consistentrelationshipbetween the height and
during winter and summer (Table 1). Females dbh of foraging trees can or should be taken
of the two speciesshowed a decreasedoverlap from Table 2.
(>0.3 change)during winter, possiblybecause
Tree-species
use.--Male Hairies showed a simof differential concentration of activities, with
ilar use of tree speciesduring summer(except
Hairies occurring primarily (60%) at greater for sugarpine, which was relatively low in all
heights(>10 m) than White-heads(Fig. 1).
species-sexsamples;Fig. 1). The other samples
relative to summer, when activities were con-
228
MORRISON
ANDWITH
[Auk,Vol. 104
TABLE1. Foragingresourceoverlap for male and female Hairy and White-headedwoodpeckers.Sample
sizesare given in the Appendix.
Comparison
Male
Resource
Female
Male
White-
Hairy,
male
Hairy,
female
Hairy,
male
Hairy,
female
head,
female
female
White-
White-
White-
White-
White-
Hairy
S
0.65
W
0.78
Tree speciesuse
Female
Male
Hairy,
Season a
Foragingheight
Male
head
head
head
head
head
0.87
0.73
0.63
0.56
0.69
0.85
0.88
0.55
0.67
0.66
S
0.74
W
0.61
0.70
0.56
0.59
0.34
0.66
0.65
0.64
0.55
0.70
0.44
Substrate
use
S
W
0.93
0.87
0.89
0.93
0.88
0.82
0.89
0.93
0.95
0.71
0.91
0.75
Substrate
condition
S
W
0.98
0.87
0.76
0.91
0.65
0.66
0.75
0.97
0.63
0.79
0.80
0.76
S
0.68
W
0.61
0.55
0.50
0.56
0.35
0.50
0.60
0.63
0.53
0.61
0.41
Tree speciesby substrate
summer, W = winter.
exhibited
a concentration
on one or two tree
all species-sex
samples,exceptbetween female
species:female Hairies on white fir and ponderosapine, male White-headson incensecedar, and femaleWhite-headson ponderosapine
(Fig. 1). The distributionsof foraging activities
among tree species(Fig. 1) were significantly
different between seasonsfor each species-sex
sample (X• test, P < 0.05), except for male Hairies (P > 0.05). During winter, male Hairies increaseduse of white fir and ponderosapine,
Hairies and male White-heads,
which
had a
similar index between seasons(Table 1).
Foraging-substrate
use.--All species-sexsam-
female Hairies increased use of cedar and oak,
ples exhibited a concentrationof foraging activities on trunks during summer and winter
(Fig. 1). Except for female White-heads, all
species-sex
samplesshowedan increaseduseof
limbsduring winter; twigs were seldom(<5%)
usedduring any season(Fig. 1). The distributions of foraging activities on substrates(Fig.
male White-headsincreaseduse of Douglasfir,
and femaleWhite-headsshoweda pronounced
use of incensecedar(Fig. 1). Resourceoverlap
exhibiteda decreasingtrend during winter for
sons,exceptfor femaleWhite-heads(P < 0.05).
Resourceoverlap was high for all samples,but
droppedduring winter betweenmale and fe-
1) were similar (X2 test, P > 0.1) between sea-
TABLE
2. Foragingheight,foraging-tree
heightanddiameterat breastheight(dbh),andforaging/treeheight
ratio.a Valuesare meansñ SD; samplesizesare given in the Appendix.
White-headedWoodpecker
Hairy Woodpecker
Season •
Foragingheight (m)
Tree height (m)
S
11.4A ñ 7.33
W
22.1 *c ñ 10.44
S
W
Foraging/treeheight
ratio c
Tree dbh (cm)
Male
21.1Bñ 10.70
31.8 *c ñ 10.02
Female
11.6A ñ 5.59
17.0 *B ñ 7.80
20.2B_+8.44
26.4 *B _+ 9.17
Male
Female
10.1Añ 6.95
11.8A _+6.65
13.9 *B _+ 6.66
20.7B-+ 10.43
28.1 *B,c --+ 10.20
S
0.54
0.57
0.49
W
0.69
0.64
0.49
S
50.5A,•ñ 27.10
W
43.0 A _+ 27.78
43.3• ñ 19.97
47.4 A,e + 26.70
9.7 A ñ 8.05
25.8A ñ 10.77
20.9 *A ñ 10.61
0.46
0.46
61.1A_+31.52
54.2A,•ñ 25.84
56.8 • + 27.37
39.4 *A ñ 28.01
• Within a season,valueswith sameletter (A, B, C) did not differ significantly(P > 0.05)as determinedby ANOVA and Duncan'smultiple
rangetest.Betweenseasons
(summervs.winter),an asteriskdenotesa significantdifference(P -< 0.05)asdeterminedby a t-test.
b S = summer, W = winter.
' Values were obtained from the means presentedin this table.
April 1987]
229
Woodpecker
Resource
Partitioning
TABLE3. Daily distribution of foraging activity (percentageof total observations).
Hairy Woodpecker
Hours
after
sunrise
1
2
3
4
5
6
7
8
9
10
11
Male
Summer
Winter
11.1
12.4
15.3
11.5
14.7
9.7
9.7
7.4
4.4
1.2
2.6
White-headed Woodpecker
Female
13.5
1.9
0.0
0.0
30.8
7.7
3.8
7.7
34.6
--•
--
Summer
Winter
2.5
15.7
14.2
21.1
19.9
6.7
4.6
2.3
6.1
3.1
3.8
4.9
4.9
0.0
0.0
17.1
0.0
26.8
36.6
9.8
---
Male
Summer
Female
Winter
7.2
13.6
11.7
14.2
15.5
9.3
6.3
3.8
5.1
6.8
6.6
0.0
12.9
40.3
21.0
3.2
3.2
1.6
17.7
0.0
---
Summer
Winter
3.1
0.0
7.1
12.0
16.4
22.8
6.4
6.2
6.8
6.0
5.7
7.5
0.0
67.1
7.1
0.0
4.3
1.4
20.0
0.0
---
Dusk to sunset; no observations made.
male White-headsand especiallybetween female Hairies and female White-heads (Table 1).
Foraging-substrate
condition.--During summer, the two speciesused substrateswith contrasting conditions. Hairies concentrated on
of foraging activitiesby tree species-substrate
categories(Appendix) were similar (X2test,P >
0.1) for male and female Hairies, but significant
for male (P < 0.05) and female (P < 0.001)
dead,and White-headson live, substrates
(Fig.
White-heads.Resourceoverlap decreasedduring winter for all samplesexcept between fe-
1). During winter, a more even distribution of
male Hairies and male White-heads, which
activities on living and dead substrateswas ev-
showed a slight (0.10) increase(Table 1). The
decrease in overlap was rather substantial
ident for all exceptfemale White-heads,which
concentratedactivitieson living trees.The dis-
(about 0.20) between male Hairies and female
tributionsof foragingactivitiesby tree condi-
White-heads
and between
male
and
female
tion (Fig. 1) were similar between seasons(X2
White-heads,primarily a result of the concentest, P > 0.1), except for female White-heads trated use of cedar trunks by female White(P < 0.05). Resourceoverlap showed moderate headsduring winter.
(>0.15) increases in winter between male HairTime of foraging activities.--All species-sex
ies and male White-heads, female Hairies and
samplesconcentratedforaging activities durmale White-heads, and female Hairies and feing the first 5 h after sunrise during summer
male White-heads (Table 1).
(Table 3); foraging times were correlated sigTree species-foragingsubstrateuse.--Each nificantly exceptbetween male Hairies and fespecies-sexsample showed a concentrationof
activitieson two or three tree species-foraging
4. Correlations(rs)of time of day of foraging
substrate combinations during both seasons TABLE
activities during summer (below diagonal) and
(Appendix).FemaleHalties increaseduseof oak
winter (abovediagonal).an = 11 daily time periods
limbs
and cedar trunks
and decreased
use of
white-fir and ponderosa-pinetrunks during
winter.
Male
White-heads
increased
use
for summer,9 for winter. Original data are given
in Table
3.
of
Douglas-fir limbs and maintained use of cedar
trunks during winter. FemaleWhite-headsdra-
maticallyincreaseduse of cedartrunks during
winter (74% winter use vs. 13% summer use);
Hairy
Woodpecker
Male
Female
The distributions
Male
Female
-0.78*
-0.36
-0.64
-0.25
Hairy Woodpecker
Male
-0.49
no other tree species-substrate
combinationwas
Female
0.67*
-usedfor >7% of foragingactivitiesduring winter (Appendix). Male Hairies increased use of White-headedWoodpecker
Male
0.76**
0.81'*
white-fir limbs and trunks, as well as ponderFemale
0.58
0.71'
osa-pinetrunks,and decreaseduseof Douglas-
fir and oak trunks in winter.
White-headed
Woodpecker
• * = P < 0.05, ** = P < 0.01.
-0.61'
0.77*
--
230
MORRISON
^NDW•H
[Auk,Vol.104
male White-heads (Table 4). During winter,
however, foraging activities of male and fe-
faceof any tree speciesduring winter at Blod-
male
Differentialuseof cedarby woodpeckers
may
be related to the ability to extract prey as a
function of bill morphology.Smaller-billed fe-
Halties
were
concentrated
5-9
h after
sunrise. Foraging times for male and female
White-heads occurred primarily 3-4 h after
sunrise,although a minor increasein activity
occurred about 8 h after sunrise (Table 3). Cor-
relation analysisshowed that winter foraging
times
of male
Hairies
and male
White-heads
were significantly negative, and three of the
remaining relationshipswere negative but not
significant.Activity periods of male and female
White-heads
and
of male
and
female
Hairies
gett (unpubl. data).
male Hairies may be unable, relative to the
larger-billedmale Hairies, to obtain prey by
drilling deep into wood for overwintering insects.Kilham (1970: table 4) also noted that fe-
male Hairies foraged superficiallyby scaling
bark in contrastto the deep excavationsinto
wood performed by male Halties. Male Whiteheads,possessingroughly the samebill size as
were positively related, although only the former was significant (Table 4).
Morphologicalrelationshi?s.--All species-sex
samples overlapped in body mass, although
male Halties (œ= 70 g) are about 10%heavier
time on cedar. In southern California,
than
female White-headsseparatedin foraging lo-
female
Hairies
and
male
White-heads
(both about 63 g), and about 15%heavier than
female White-heads (about 59 g; data from
Dunning 1984). Bill length followed a similar
pattern: male Hairies (œ= 33.3 mm, SD = 1.06)
had the longestbills, followed by female Hair-
female Halties, also used cedar during both
winter and summer. Female White-heads,
the
smallestamong the four species-sexsamples
examined, spent about 75% of their foraging
male and
cations on Coulter pine (Pinuscoulteri),with
males concentratingon the trunks and large
cones(Hilkevitch 1974). Males were able to feed
on the conesof Coulter pine becauseof their
larger bill relative to females. Otvos and Stark
ies (30.2, 1.19), male White-heads (28.7, 1.03), (1985) analyzed the stomachcontentsof woodand femaleWhite-heads(27.1, 0.86).Bill length peckers at several locations in California, indiffered significantly among all species-sex cluding BlodgettForest,during the 1960's.They
samplesas determined by analysis of variance found that femalesof both speciesfed on bee(P < 0.001) and Duncan'smultiple range test tles that inhabited the upper stems of trees,
(P < 0.05) (see also Short 1982: 319, 328 for size where bark was thin. Stomach analyses recomparisons).
vealed that X. macrocarpae
comprisedthe second-largest portion of the diet in both sexesof
the White-headed Woodpecker: 15% in males
DISCUSSION
and 19%in females.For Hairies, X. macrocarpae
comprisedonly 2% of the diet in males and 7%
A general trend toward decreasedoverlap in in females. Thus, the occurrence of scale insects
use of foraging sitesby Hairy and White-head- in the diet of these woodpeckersagreeswith
ed woodpeckersduring winter was primarily a the trend expectedfrom a considerationof bill
reflection of the concentration
of activities on
size (e.g. female White-heads,with the smallest
the trunks of live incense cedar by female bill, had the greatestpercentageof scaleinsects
White-heads.Exceptfor male Hairies, the oth- in their diet).
In the eastern United States, smaller-billed
ers also increaseduse of cedar during winter.
Morrison et al. (1985) documented the inwoodpeckersforage more intensively on thincreased use of live cedar in winter by many barked trees in winter than do larger-billed
other bird speciesthat winter in BlodgettFor- woodpeckers (Kilham 1970, Travis 1977, Conest. Analysis of food availability revealed the ner 1981; see also Morrison et al. 1985). The
presence of a scale insect, Xylococculusmacro- availability of insectsthat overwinter under the
carpae,under the loose,flaky bark of cedar,the
most abundant tree speciesat Blodgett. Even
small-billed species,such as kinglets (Regulus
spp.) and chickadees(Parusspp.), were able to
use this seemingly abundant food source.Few
insectsare availableon the bark or foliage sur-
bark of thin-barkedtreeswas given as the reason smaller
birds use this resource.
This does
not explain why larger-billed birds did not exploit this abundant food resource,especially
given the small difference in bill size for the
speciesanalyzed in our study. Prey, other than
April1987]
Woodpecker
Resource
Partitioning
231
that available on cedar, may be readily available to these woodpeckers(such as prey occurring in tree cambium).But theseprey were not
sampledat Blodgett.A decreasein niche over-
serveddifferencesin tree species-foragingsub-
diverse
Pacific Northwest
strate use, and that male Hairies
and male
White-heads foraged at different heights, it is
unlikely that the differencesin foraging times
lap during winter would be predictedif re- were related to interspecificinteractions (here,
sourceswere limiting and specieswere segre- competition for resources).We do not know if
gating foraging activities in responseto this physiologicaldifferencesbetween the species
limitation. Although not measured,resources or sexesaffectedthe activityperiodsof the birds.
apparentlywere not limiting during our study. Although winter data are not sufficient for
Changes in foraging behavior during winter analysis,count data indicated that no negative
appeared to be in responseto readily available relationship existed between numbers (index
prey on cedar and not becauseof interspecific of abundance) of Hairy and White-headed
competition for resources.Resourceavailabiliwoodpeckers (competition for space) in sumty-including difficult-to-measureprey found mer 1983-1985 (unpubl. data).
beneath
bark and in the cambium--would
be
Although no negativerelationshipswere oba useful addition
to further studies of these
servedamongany of the woodpeckersin terms
birds.
of abundanceor time of foraging activitiesat
Northern and southernpopulationsof White- Blodgett,the potential for suchinteractionsexheaded Woodpeckers have been classified as istsin times of resourcelimitations (e.g. Wiens
separate subspecies (A.O.U. 1957) based on 1977). Further, some or most of the woodpeckmorphologicaldifferences(Grinnell 1902). In ers overwintering at Blodgett may not be the
the southernpart of its range(i.e. southernCal- same individuals that breed there, as some
ifornia), White-heads apparently concentrate downslope movement during winter is possiactivitiesin pines,oftenfeedingheavilyon pine ble (Grinnell and Miller 1944). Therefore, while
seeds (Bent 1939, Koch et al. 1970, Short 1971,
our analysesdo not assigna strong role to inHilkevitch 1974, K. L. Garrett pers. comm.). Li- terspecificinteractions in shaping patterns of
gon (1973) noted similar feeding by "north- resourceuse between Hairy and White-headed
ern" White-heads in a ponderosapine forest in woodpeckers,such a role cannot be ignored.
Idaho. We did not find a concentration
of forHairy and White-headed woodpeckersmay inaging activitieson pines by White-heads.Like- teract when foraging on pine cones (Ligon
wise, Otvos and Stark (1985) reported that pine 1973). Based on limited observations,Ligon
seedsformed a small part of this species'diet, suggestedthat male White-heads were domiwith most use confined to the fall. Thus, there
nant over female Hairies. The potential for inis apparently a difference in habitat use terspecificinteractionsat a specificfood source
throughout the range of White-headed Wood- thus exists,especiallyduring winter if prey are
peckers.Sparseuseof pine at Blodgettmay re- scarce (Ligon 1973). In contrast, Hilkevitch
flect the mix of tree speciesthere comparedwith
(1974) noted virtually no interspecific interacand other woodthe more monotypic standsof conifer species tions between White-heads
outside the mixed-conifer zone. Ligon (1973) peckers,including Hairies. Further,Short (1982:
attributed the lack of differencein foraging be- 323) observed interspecific territory overlap
havior between male and female White-heads
during breeding. Short (1971) postulated that
in Idaho to habitat homogeneityand a lack of the White-headed Woodpecker evolved in the
food
resources.
from
an ancestor
similar
to
Times of foraging activities were similar for
all species-sexsamplesin summer. The similar-
that from which the Hairy Woodpecker
evolved. He suggestedthat the White-headed
ity of foragingtimesduring breedingprobably Woodpeckerbecamerestrictedin range as the
reflects the similarity in requirements both
sexeshavebecauseof nestingactivities.During
winter, however, foraging timesof male Hairies and male White-headsshoweda significant
negative relationship. The foraging times of
male and female White-heads remained positively related during winter. Given the ob-
Hairy Woodpecker became sympatric with it.
Our resultsindicate that although within-hab-
itat adjustmentsin behaviormaybe evidentbetween thesetwo species,the grossdistribution
of the two is relatedprimarily to factorsother
than interspecific interactions (e.g. habitat
complexity). Evaluation of resource use be-
232
MORRISON
ANDWITH
[Auk,Vol.104
peckers.M.S. thesis,LongBeach,CaliforniaState
tween these speciesin areas of allopatry, and
Univ.
especiallyexperimental manipulations of habJ. M. 1979. Foragingbehavior of male and
itat and prey, would help clarify this question. JENKINS,
female Nuttall Woodpeckers.Auk 96: 418-420.
KILHAM,L. 1965. Differencesin feeding behavior of
male and female Hairy Woodpeckers.Wilson
ACKNOWLEDGMENTS
Bull. 77: 134-145.
We are extremelygrateful for field assistance
provided by K. A. Milneß P. N. Manley, W. M. Block,S.
1970. Feeding behavior of Downy Woodpeckers.I. Preferencefor paper birchesand sex-
A. Laymon, R. Etemad, and J. M. Anderson. I. C. Ti-
ual differences.
Auk 87: 544-556.
mossiassistedwith fieldwork and data analysis.Reviews of earlier drafts by R. P. Balda, R. N. Conner, KISIEL,D.S. 1972. Foragingbehavior of Dendrocopos
villosus
and D. pubescens
in easternNew York state.
D.E. Capen,K. L. GarrettßJ. D. LigonßJ. M. Marzluff,
Condor 74: 393-398.
and B. L. Winternitz were informative and very helpKOCH, g. F., A.E. COURCHESNE, & C. T. COLLINS.
ful. Although our conclusionsmay differßcomments
1970. Sexualdifferencesin foragingbehaviorof
by L. L. Short greatly improved evaluation of our
White-headedWoodpeckers.Bull. SouthernCalresults.R. C. Fieaid and the staff of BlodgettForest
ifornia Acad. Sci. 69: 60-64.
helped at the study site. We thank N. K. Johnsonfor
LIGONß
J. D. 1968. Sexual differencesin foraging
access
to the Museumof VertebrateZoology;hisstaff
behavior in two speciesof Dendrocopos
wood-
provided assistancewith the collection.
peckers.Auk 85: 203-215.
1973. Foragingbehaviorof the White-head-
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APPENDIX.Percentageuseof 15 substrate-tree
speciescategoriesusedto forage.
Hairy Woodpecker
Substrate
Male
and
tree species
White-headedWoodpecker
Female
Male
Female
Summer
Winter
Summer
Winter
Summer
Winter
Summer
Winter
Incense cedar
White fir
5.0
2.5
0.0
11.5
6.3
4.2
7.3
4.9
5.5
0.0
1.6
0.0
6.7
3.3
7.1
0.0
Sugarpine
Ponderosapine
0.0
2.5
0.0
4.2
0.0
0.0
3.6
7.3
1.6
1.6
5.0
5.0
0.0
0.0
Douglas fir
0.0
Black oak
Other
Limb
5.0
1.9
1.9
0.0
4.2
0.0
24.2
6.7
0.0
2.5
0.0
9.6
0.0
6.3
0.0
19.5
0.0
3.6
0.0
3.2
0.0
1.7
0.0
0.0
0.0
Incense cedar
White fir
5.0
17.5
5.8
23.1
8.3
22.9
22.0
12.2
27.3
10.9
22.6
3.2
13.3
1.7
74.3
4.3
Sugarpine
Ponderosapine
Douglasfir
5.0
17.5
17.5
7.7
26.9
3.8
2.1
27.1
0.0
4.9
12.2
2.4
9.1
10.9
3.6
8.1
14.5
9.7
15.0
31.7
3.3
5.7
0.0
2.9
Black oak
Other
12.5
5.0
0.0
7.7
4.2
10.4
4.9
7.3
7.3
0.0
8.1
1.6
5.0
0.0
0.0
5.7
Other
n of individuals
2.5
40
0.0
60
0.0
48
2.4
48
10.9
55
0.0
83
!.7
60
0.0
81
Trunk