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- LITERATURE CITED ALATALO,R. V. 1982. 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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