NEST-SITE SELECTION IN THE ACORN WOODPECKER

TheAuk 116(1):45-54, 1999

NEST-SITE

SELECTION

IN THE

ACORN

WOODPECKER

PHILIP N. HOOGE,• MARK t. STANBACK, 2 AND WALTER D. KOENIG3 4Hastings NaturalHistoryReservation, 38601 EastCarmelValleyRoad,CarmelValley,California93924, USA;and Museumof Vertebrate Zoology,Universityof California,Berkeley, California94720, USA

ABSTRACT.--Acorn Woodpeckers(Melanerpes formicivorus)at Hastings Reservationin cen-

tral Californiaprefer to nestin dead limbs in large,dead valley oaks(Quercuslobata)and Californiasycamores (Platanus racemosa) thatarealsofrequentlyusedasacornstoragetrees. Basedon232nestcavitiesusedoveran 18-yearperiod,we testedwhetherpreferredormodal nest-sitecharacters wereassociated with increased reproductive success (the"nest-sitequality" hypothesis). Wealsoexaminedwhethermoresuccessful nestswerelikelyto experience more favorablemicroclimaticconditionsor to be lessaccessibleto terrestrialpredators.We

foundonlyequivocalsupportfor thenest-site qualityhypothesis: only1 of 5 preferredcharactersand2 of 10characters exhibitinga clearmodalitywerecorrelated with higherreproductivesuccess. All three characteristics of nestsknownor likely to be associated with a more favorablemicroclimate,and two of five characteristics likely to render nestslessaccessibleto predators,were correlatedwith higherreproductivesuccess. Theseresultssuggestthat nestcavitiesin thispopulationare built in part to takeadvantageof favorablemicroclimaticconditionsand, to a lesserextent, to reduceaccessto predators.However,despite

benefitsof particularnestcharacteristics, birdsfrequentlynestedin apparentlysuboptimal cavities.Wealsofounda significant relationship betweenmeangroupsizeand thehistory of occupancyof particularterritoriesand the probabilityof nestcavitiesbeingbuilt in microclimatically favorablelive limbs,suggestingthat largergroupsresidingon morestable territorieswerebetterableto constructnestswith optimalcharacteristics. Thisindicatesthat theremay be demographic, aswell asecological, constraints on nest-siteselection in this primary cavity nester.Received 20 June1997,accepted 1 May 1998.

NEST-SITE SELECTION in cavity-nestingbirds may affect fitnessin at leasttwo ways. First is

throughnestmicroclimate, whichmaybeinfluencedby the size of the cavity,bark type and thickness,densityof the surroundingwood, angle of the nest entrancefrom vertical,and orientation of the nest entrance (Conner 1975, Ricklefs and Hainsworth 1969, Austin 1974, In-

ouyeet al. 1981,Wachob1996).Secondis by reducingpredation(Li and Martin 1991)or the likelihood of nest-siteusurpationby species suchas EuropeanStarlings(Sturnusvulgaris; Troetschler1976).Thesesourcesof nestlossare

likelyto be influenced by nestheight,theangle of nest entrance, and the smoothness of the

bark, all of which may in turn affectthe ability of predatorsto detect and gain accessto nest • Presentaddress:USGS, BiologicalResourcesDivision, Alaska BiologicalScienceCenter,GlacierBay Field Station, Gustavus, Alaska 99826, USA.

cavities

as well

as allow

birds

time

to detect

and repel predators(Gutzwiller and Anderson 1987). The relative importanceof nest microclimateand predationin influencingthe constructionof nestholesisbasicto understanding the environmentaland ecologicalconditions resultingin both annualand individualvariation in reproductivesuccess. A secondissueconcerningnest-siteselection is whetherpreferredsitesare optimalin terms of yielding higher reproductivesuccess. This "nest-sitequality" hypothesishas been critically examined in secondarycavity-nesting specieswhereseveralstudieshavefoundthat cavitiescorrelatedwith higher reproductive successare, at leastin somecases,neitherpreferred nor used more frequently. The most commonexplanationfor this discrepancyis that interspecificcompetitionconstrainsoptimal cavityselection(Nilsson1984,Brawn 1988, Rendell and Robertson 1989, Li and Martin

1991). Whether comparable kinds of con-

2Presentaddress:Department of Biology,David-

sonCollege,Davidson,North Carolina28036,USA. straintsoccurin primary cavity-nestingspecies 3Addresscorrespondence to this author.E-mail: is an open question.

AcornWoodpeckers (Melanerpesformicivorus) are cooperative breedersthat live in groupsof

[email protected] 4Addressfor correspondence 45

46

HOOGE, STANBACK, ANDKOENIG

[Auk,Vol. 116

californica) are 2 to 15 individuals including, in many cases, sii) and California buckeyes(Aesculus

multipleco-breeders of bothsexes(MacRoberts and MacRoberts1976, Koenig et al. 1995). Regardlessof group size or composition,only a singlenestis producedat anyonetime.Groups are permanentlyterritorial and centeredon a storagetree or granaryin which acornsare

foundin the moreheavilyforestedareas.European Starlings,which are known to usurp AcornWoodpeckernestcavities(Troetschler1976),werepresent during the studybut generallynot sufficientlyabundant to conflictwith the nestingactivitiesof the woodpeckers.

stored each autumn. Nests holes are construct-

cavities representing 463 nesting attempts in 16

ed by morethan oneindividual,oftenoveran extendedperiod of time, and are reused frequently (MacRoberts and MacRoberts 1976, Stanback,Hooge,and Koenigunpubl.data).

Nest-site

characters

were measured

for 232 nest

yearsof the study.Elevenparameters of nestholes wererecorded:treespecies, diameteratbreastheight (dbh), diameterof limb at nest height (dnh), nest

height, relative height in tree (nest height/tree height),depthof nestcavity,compass orientationof used for nestingby AcornWoodpeckers in or- entrancehole,holeanglefrom horizontal(measured with a clinometer),distanceto primarygranarytree, der to examine the following questions:(1) tree viability (whethertree wasalive),and limb viWhat are the most frequentlyused nest-site ability (whethernestlimb was alive). Not all meacharacteristics of Acorn Woodpeckers,and surementswere availablefor all nests.Group and rewhatkindsof nestsitesdotheyprefer?(2)How productive successparametersmeasuredincluded is reproductivesuccess relatedto nestsitechar- clutchsize, numberof fledglings(estimatedby the acteristics? In particular,arethemostfrequent- numberof nestlingsbandedon day21 of the30-day ly usedor preferrednestcharacters associated nestlingperiod), nestingsuccess(whetherthe nest with increasedreproductive success? (3) Are fledgedyoung),territory occupancy(proportionof preferredor more successful characteristics of yearsterritory wasoccupied),and groupsize at the We measured

the characteristics

of holes

nestsitesthosethat are likely to be lessacces- time of nesting attempts. In order to confirm the presumedeffectsof nest sibleto nestpredatorsor thoselikely to have orientationon nest microclimate,temperatureflucmore favorablethermalcharacteristics? (Weas- tuationsin insulatednestboxesdesignedto approx-

sumedthat nestslessaccessible to predators imate the thermal characteristics of Acorn Woodare thosebuilt in deeperholesand occurring peckernestswere recordedin June1986. Four insuhigherin smooth-barked treesandorientedto- latednestboxesweremountedona utility poleat the ward the ground.Nestswith more favorable sameheight;eachbox facedoneof the four cardinal thermal

characteristics were those located in

large,live limbsthat facedeast;datasupporting theselattertwo associations arepresented below.)(4) Is thereany evidencefor constraints on nest-siteselectionin AcornWoodpeckers? STUDY SITE AND METHODS

We studied a color-bandedpopulationof Acorn Woodpeckersin and around HastingsNatural History Reservation in central coastal California (36ø22'N,121ø32'W),a populationthat hasbeenunder continuousstudy since 1971 (MacRobertsand MacRoberts1976, Koenig and Mumme 1987). Data were collected

on nests used between

1972 and 1987.

ThevegetationatHastingsconsists of oakwoodland interspersedwith grassland,oak savanna,riparian woodland,and chaparral(Griffin 1974).Valleyoaks (Quercuslobata),blue oaks (Q. douglasii),and coast

directions. Temperatureswere measured simulta-

neouslyin all fourboxesusingthermocouples placed in thecenterof thebox.An additionalthermocouple was used to measureambient temperature.Thermocouples wereattachedto a CampbellMicro Data Loggerthatrecordedthefive temperatures every5 s and averagedreadingsat 5-min intervals.Thermocoupleswere also used to record temperaturesin Acorn Woodpecker nest cavities in five matched pairsof live and deadlimbsin June1986.Eachthermocouplewas placedat the bottomof the nesting cavityapproximately 2 cmabovethefloorof thenest, and the temperatureswere averagedas above.Each pair of replicatedcavitieswas in the sametree and wasmatchedascloselyaspossiblefor direction,surroundingvegetation,height,and dnh. Availability of treeswasestimatedfrom vegetation surveysperformedthroughoutthe study area in suitablewoodpecker habitatbetween1977and 1979. Surveysconsistedof 0.04-hacircularplotsin thecenter of a 60-mgrid overlaidon thestudyarea.Theestimatedproportionand sizedistributionof eachtree species,aswell asthenumberof snagspresent,were basedon all 631 vegetationplots(corresponding to

live oaks(Q. agrifolia)are commonthroughoutthe reserve,and Californiablackoaks(Q. kelloggii)and canyonlive oaks(Q. chrysolepis) arelargelyrestricted to higherelevations.Californiasycamores (Platanus racemosa) and willows (Salixspp.)are commonin the a sampledareaof 227 ha) coveringapproximately riparian woodland, and madrones(Arbutusmenzie- 76% of the study area over which nestswere sam-

January1999]

AcornWoodpecker NestSites

pled. We estimatedthe proportionof suitablenest sitesthatwerein deadlimbsbasedon 100randomly

47

150

[]

Observed

[] Expected

chosenlimbs (4 eachfrom 25 trees) that were the size

typicallyused by nestingwoodpeckers. All of the treeswere largeand were distributedacrossspecies in the sameapproximatefrequencyas actualnest cavities.

Becausenest holes are often reused, we used nest

holesratherthannestattemptsasindependent sampies.In order to deal with annualdifferences in reproductivesuccess, which canbe considerable (KoenigandMumme1987),clutchsizeandfledgingsuccessfor eachyearwerestandardizedby subtracting the meanand dividingby thestandarddeviationof the samevariablesaveragedacrossall groupsstudied for that year. These standardizedvalueswere

Valley oak

Blue California Coast Sycamore oak

black oak

live oak

Species

thenaveraged across all nestingattemptsin thesame F•G.1. Distributionof Acorn Woodpeckernests cavity,whichrangedfrom 1 to 12 (œ= 2.0 _+SD of acrosstreespecies(excludingonenestbuilt in a wil1.7 attempts). low) comparedwith the averagenumericalabunData on nestorientationand entranceangle were danceof thefive treespecies in the studyarea.Nests analyzed using angular statistics(Zar 1984). The werefoundsignificantly moreoftenthanexpectedin tightnessof the dispersionaroundthe meanvector valley oaks and sycamoresand lessoften than exis represented by a relativeindex(r) in which1.0in- pectedin blue and coastlive oaks(X2 = 1,134,df = dicatesthe sameorientationfor all pointsand 0 in- 4, n = 221, P < 0.001). dicatesrandomorientation.Thesignificance of r was determined usingtheRayleightest(Batschelet 1965). The nonparametrictwo-sampleWatsonU2 test (Zar 1984)wasusedto comparedifferences in meanvectors.Skewness wastestedusingthe t statistic(Sokal and Rohlf1981).To testfor therelationshipbetween the most frequently used nest-sitecharacteristics and reproductive success,nestswere divided into

usedas granaries,alsofar morethanexpected (P < 0.001) given that granariescomprised only about0.12% of treesin our study site. Nestsinvariablyoccurredin relativelylarge trees:of all treesin thestudyarea,anestimated 76.3% were smaller than the smallest tree in

two or threecategories dependingon the variable and theshapeof thefrequencydistributionwith re- which a nest was located (i.e. 35.6 cm dbh). spect to the character.Statisticaltests were two- However,evenif theanalysisis restrictedto the tailed when possible. RESULTS

subsetof treeslargerthan 35.6cm dbh, birds preferredlarger treesfor nest cavities(MannWhitney U-test, z = 15.1, P < 0.001, n = 202 nest trees vs. 414 randomly selectednon-nest

Nest-site preferences.--The six speciesof trees trees). in which we found woodpeckernest cavities Characteristics of nests.--Descriptivestatistics constituted

more than 90% of the trees in the

of nest sites are summarized in Table 1, and fre-

study area. Basedon the estimatedabundance quencydistributionsfor characters are plotted of treespecies,AcornWoodpeckers clearlypre- in Figures2 and 3. Nestheightwas positively ferred valley oaksand sycamoresand avoided correlated with treeheight(rs= 0.45,n = 126,

blue oaksand coastlive oaks (Fig. 1). Of 222 trees used for nesting, only 7 (3.1%) were in snags.However,thisnumberwassignificantly higherthanexpectedbasedon the estimated availabilityof snags(0.65%;X2 = 21.3, df = 1, P < 0.001).

Despitetherarity of deadtrees,49.8%of nest cavities (n = 221) were in dead limbs, which was significantlyhigher than expectedbased on our sampleof randomlychosenlimbs(12% dead; X2 = 291, df = 1, P < 0.001). Forty-two percent of nest cavitieswere locatedin trees

P < 0.001);13%of nestswerelocatedin thetop 20% of the tree, and no nests were in the bottom

20%. The frequencydistributionof nest-hole orientationwas biased toward the east (67.2% were oriented between 0ø and 180ø; binomial

test,z = 5.2, n = 232, P < 0.001;Fig. 2A) and was significantlynonuniform(Rayleightest,z = 5.0, n = 194,P < 0.01).Almostall of the nest holes(95.4%)faceddownward(Fig.2B). Correlates of reproductive success.--Four measures of reproductivesuccesswere recorded for nest cavitiesand averagedover all known

48

HOOGE, STANBACK,AND KOENIG

TABLE

1.

[Auk, Vol. 116

Descriptivestatisticsfor nestcavitiesusedby AcornWoodpeckers at HastingsReservation,1972

to 1987.

Variable dbh (cmp dnh (cm)b

Hole depth (cm) Nest height (m) Relativenestheightc Distanceto granary (m) Nest-hole orientation (ø)

Nest-holeangled

œ+_SD

Range

n

98.3 _+36.8 31.1 ñ 13.4

35.6-254.6 13.1-114.0

203 167

22.0-70.0 2.3-18.0 0.21-0.99 0-1,080

105 196 126 161

37.9 ñ 8.1 ñ 0.58 ñ 140.9 ñ

9.8 2.7 0.17 217.5 78.5 ñ 109.5 -34.2 ñ 22.5

--

194

-87 to +30

175

Diameter of tree at breastheight.

Diameterof limbat nestheight. Nest height divided by tree height. Relative to horizontal.

times that individual cavities were used: (1) mean standardized clutch size (mean clutch size), (2) mean standardizednumberof young fledgedfrom all nests(total meanfledged),(3) mean standardizednumber of young fledged from successfulnests only (successfulmean

fledged),and (4) proportionof nestsin thecavity that successfully fledgedyoung(cavitysuccess).Nest holesthat were relativelyhigherin trees were more successful(r•,= 0.23, n = 120,

P < 0.01) and fledgedmoreyoung (total mean fledged,rs = 0.30, n = 111,P < 0.001;successful mean fledged,rs = 0.28, n = 92, P < 0.01) thanholesthatwererelativelylowerin trees.In addition,cavitiesin live limbstendedto fledge more youngthan thosein deadlimbs (MannWhitneyU-test,z = 2.3, n = 193,P < 0.05),relatively successful cavitieswere orienteddifferentlythanrelativelyunsuccessful cavities(Wat-

Relationship between optimalandmostfrequently usednestcharacteristics.--Five marginallysignificantresults(P < 0.05) emergedfrom analysestestingwhetherthe mostfrequentlyused categoryfor a particularnestcharacteristic was relatedto increasedreproductivesuccess; three of theseindicatedthat the modalcategorywas the most successful(Table 2). For dnh, both meanclutchsizeand meantotal youngfledged werelargestfor intermediate-sized limbs20 to 30 cmdnh,whichwerealsothemostfrequently usedin thepopulation(Fig.3). Bothof theseeffects were due primarily to poor successof nestsin limbs 100

DNH (cm)

0.3

0.3

0.2

0.2

Holedepth(cm)

Nestheight(m)

Relativenestheight(%)

Distanceto granary(m)

FIG. 3. Frequencydistribution for six characteristicsof cavitiesused by nestingAcorn Woodpeckers.

January1999]

AcornWoodpecker NestSites

49

TABLE 2. Relationships betweennest-site characteristics andfourmeasures ofreproductive success in Acorn Woodpeckers. Characteristics are dividedinto categories basedon the frequencydistributions in Figures 1-3 and listedin the far-rightcolumn;modalcategories are listedin bold. Listedare teststatisticswith samplesizesin parentheses; valuesare z-scores(Mann-Whitneytests)for two-categoryvariableand X2 (Kruskal-Wallis) for three-category variables. Forclutchsizeandnumberof youngfledged,valuesarethe averageof standardizedvaluesfor all yearsa particularcavitywasused. Meanno.of youngfledged Mean

Variable

Successful

clutchsize

All nests nestsonly

Tree species -0.1 (142) Nest tree a granary 1.5 (111)

-0.4 (179) -0.6 (137)

Tree viability 1.0 (149) dbh -3.4 (143) dnh 7.0 (131)* Nest-holedepth 0.0 (89) Nestheight 0.3 (144) Relativenestheight -3.6 (91) Nest-holeorienta- -0.2 (140)

-0.9 (194) 0.0 (159) -1.3 (178) 0.3 (145) 8.2 (151)* 4.3 (127) 0.8 (98) 0.0 (84) -2.0 (177) -2.1 (148) -8.7 (111)* -4.7 (92) 2.2 (177)* 1.1 (148)

Cavity

success

0.6 (145) -0.1 (197) 1.0 (114) -0.7 (155)

-0.6 (210) -3.8 (191) 4.7 (156) 1.3 (99) -0.3 (186) -5.8 (120)* 1.5 (185)

Categories Valley oak + sycamorevs. others Granary vs. non-granary Alive

vs. dead

100 cm 30 cm 40 cm 10 m 0.6 0 to 180' vs. 180 to 360 ø

tion

Nest-holeangle

-1.3 (136)

-1.6 (160)

-5.1 (133) -1.1 (165)

20 ø

*, P < 0.05.

west-facingholes (Table2). However,nestsin the upper 40% of trees fledged more total youngand were the mostsuccessful, whereas

0.50'

cavities occurred most often in the middle 20%

Clutch size (n = 131)

of trees (Table2).

0.25'

Factorsinfluencingnest microclimate.--From temperature measurementsin artificial nest

cavities,east-facingcavitieswarmedup faster in the morningsand maintainedwarmer temperaturesuntil early afternoonthan did holes facing in other directions(Fig. 5). We deter-0.50mined how microclimatewas influencedby -0.75 nest-holeorientationby averagingtemperatures throughoutthe daylighthours (0600 to Totalyoung fledged (n= 151) 1800) and then comparingthese meansand their standarddeviationsfor the five days of data.East-facing holeswerewarmerthaneither 0.00 west-facingor north-facingholes on all five days (Wilcoxon matched-pairssigned-rank -0.25 test, both P < 0.05) and were warmer than south-facingholeson four of five days (P = 0.08). Similarly,on all five daysthe standard deviations of daytime temperaturesfor east-0.75 facingholeswere smallerthan thosefor either 30 south-facingor west-facingholes(both P < 0.05) and smallerthan north-facingholeson DNH (cm) threeof fivedays(P = 0.22).Thus,east-facing FIG.4. Standardizedmean(-+SE)clutchsize(upholesgenerallywere warmerand experienced per panel)and total meannumberof youngfledged (lower panel) from nestcavitiesbuilt in variouslimb lower overall temperature fluctuationsthan diameters(dnh). Differencesare significant(Krus- holesfacingthe otherdirections. We testedfor the effectof limb viability on kal-Wallistests,clutchsize,X2 = 7.0, df = 2, P < 0.05; youngfledged,X2 = 8.2, df = 2, P < 0.05). nestmicroclimateby pairing five live and five

0.z

-0.50-•

T

50

HOOGE,$TANBACK, ANDKOENIG 30.0.

25.0'

20.0 '

15.0'

10.0'

5.0

7.5

-5.0

4•0

8•0

1.•0

ldO0 20•0 2400

Time of day FIG. 5. Effectsof orientationon daily temperature patternsin artificial nest boxesat HastingsReservation.Graphedaremeantemperaturesof boxesfacing the four cardinaldirections,averagedoverfive daysin June1986(upperpanel)and differences between east-facingboxesand matchedboxesfacing the other three cardinaldirections(lower panel).In the lower panel, positivevaluesindicatethat eastfacingboxesarewarmer,negativevaluesthereverse.

[Auk, Vol. 116

the converse. The meanangleof nestholeswas well below horizontal (Table 1) and was not skewedsignificantly(œ= -0.02 ñ 0.18,ts= 0.1, P > 0.5, Fig. 2B). Two variablesthat were likely to be associated with greaterprotectionfrom predators were positivelyassociated with increasedsuccess:smoothbark and relativenestheight.Of the nest trees used by Acorn Woodpeckers, only sycamoreshave relatively smoothbark that makes them more slippery than oaks. Nests in sycamorestended to fledge more youngthan did nestsin all othertree species c-ambined (Mann-WhitneyU-test,z = 1.96,n = 188,P = 0.05)and containeda higherproportion of neststhat were successful(z = 2.2, n = 206,P < 0.05).Regardlessof the species,nests that wererelatively(but not absolutely) highin trees fledged significantly more young and were more likely to be successfulthan nests that were relativelylow in trees. Relationshipbetweennest-siteselectionand group/territory characteristics.--The ability to build suitablenestscavitiespotentiallycould be influencedby group size and continuityof territoryoccupation. Weexaminedtherelationship betweenthesetwo variablesand the five characteristics of nesting cavitiesassociated with differencesin reproductivesuccess(i.e. limb viability, nest orientation, relative nest height,dnh, and tree species).Groupsizewas averaged over all occasionsthe cavity was used, and territorial occupancywas categorized into thoseoccupiedcontinuously versus thosenotoccupiedcontinuously. Nest-holeorientation,relativenestheight,and dnh did not differ significantlywith group size or continuity of territory occupation.However, mean groupsizewassignificantlylargerwhencavi-

dead limbs while controllingfor nest orientation,exposure,andheight.Temperatures were significantlyhigher in live limbs during the non-daylighthoursovera five-dayperiod(Wilcoxonmatched-pairs signed-rank test,z = 2.02, P < 0.05).Averagingall temperaturesrecorded duringthefivedaysof measurements, theover- ties were in live limbs (œ= 4.5 __-SD of 1.0, n all variancein temperaturewas considerably = 101) versus dead limbs (œ= 3.8 +- 1.7, n = lower in live limbs (variance= 26.0ø) than in 95; z = 2.5, P = 0.01) and when cavitieswere in sycamores (g = 4.8 - 2.0,n = 37)versusothdead limbs (variance= 161.3ø). Factors affecting predation.--Neithernest er speciesof trees(œ= 4.0 ñ 1.8, n = 156;z = depth,absolutenestheight,nor nestanglevar- 2.2, P < 0.05). Also, the proportionof holes ied linearly with the incidenceof nestingfail- built in live limbs was significantlyhigherin ure.Whentestedagainsta normaldistribution, territories that were continuouslyoccupied nestdepthand nestheightwere significantly (60.2%,n = 128) than in thosethat were not skewedtowardsmallervalues(depth:œ= 0.84 continuouslyoccupied(40.5%,n = 74;X2 = 6.5, ñ SE of 0.24 for skewness,ts= 3.7, P < 0.001;

df = 1, P < 0.05).

Onepossibleconsequence of thislatterresult height:œ= 0.38 _ 0.17,ts= 2.2, P < 0.05;Fig. of nestingin live 3), indicatingthat birds tended to use holes is that the fitnessadvantages lower in trees and in shallower limbs more than limbs and in sycamoresmight be due to the

January1999]

AcornWoodpecker NestSites

confoundingeffectsof groupcomposition rather than any microclimaticadvantageof live limbs.We checkedfor thisby performingANCOVAswith mean groupsize (the covariate) enteredprior to the main factor(limb viability or tree species).Evenafter controllingfor the effectof meangroup size (F = 5.2, df = 1 and 172,P = 0.02),fledgingsuccess wasinfluenced by limb viability (F = 4.3, df = 1 and 172, P = 0.04). Similarly, nest cavities in sycamores fledged more young (F = 7.5, df = 1 and 190,

51

frequently in our population (Table 2). Two

preferredcharacteristics (nestlimbs20 to 30 cm dnhandholesfacingeast)wereassociated with slightlyincreased reproductive success. Theeffectof limb sizewasdue primarily to relatively low clutchsizeandfledgingsuccess in cavities in very small limbs, an effect comparableto that foundin severalothercavity-nesting species(L•hrl 1973,Ludescher1973,Karlssonand Nilsson 1977).

Of the eightadditionalvariablesexhibitinga clearmodality,sevenwere not associated with = 1 and 190,P < 0.05)evenafter controllingfor differences in reproductivesuccess. Theeighth, mean group size. Thus, cavitiesin live limbs relativenestheight,washighlysignificantly asand in sycamoreswere more successfuleven sociatedwith increasedreproductivesuccess, aftercontrollingfor thegenerallylargergroups but only 13% of the nestswere in the reprowith whichtheywere associated. ductively most successfulupper 20% of the P < 0.01) and were more successful(F = 4.2, df

DISCUSSION

AcornWoodpeckers at HastingsReservation prefer to build nestsin large valley oaks and sycamores, which alsoare frequentlyused as granariesfor storingacorns.Snagsare rare in our study areabut are preferredwhenpresent, a result also found for severalcavity-nesting speciesin centralArizona (Li and Martin 1991). Birds also use dead limbs far in excess of their

availability. This almost certainly is due to choiceof limbsthat are easilyexcavated by the birds; we have no evidence that limbs die ow-

ing to the excavatingactivitiesof the birds. Nest-sitequalityhypothesis.--Out of the five preferred nest-site characteristics, only one (treespecies) wasassociated with higherreproductive success:nests in sycamoresfledged more young and were more successfulthan nestsin other tree species.However,the combined reproductivesuccessfor nestsin valley oaksand sycamores (bothof whichwereused in excessof their relative abundance)did not

differ from that in all other tree species(Table 2). Moreover,eventhoughAcornWoodpeckers preferredto nestin largertrees,treesthatwere dead, and trees that were used as granaries, thesecharacteristics did not havea significant positive influence on reproductive success. Dead limbs were actuallyassociated with lower reproductivesuccess despitebeingpreferred relativeto their availability. We obtained

similar

mixed results in tests of

associationbetween reproductivesuccessand nest-site

characteristics

that were used most

trees. These results indicate that neither modal nest characteristics nor nest characteristics cho-

senby birdsmorefrequentlythanexpectedby chance are necessarilyassociatedwith increasedreproductivesuccess. A similarlackof concordanceamong preferred, modal and most successful

nest-site

characteristics

has

beenreportedin otherstudiesof both secondary (Brawn 1988) and primary (Li and Martin 1991) cavity nesters. Theroleof nestmicroclimate.--Afavorablenest microclimate may increasereproductive successby decreasingthe energeticcostsof incubation (White and Kinney 1974) and by preventing death by exposureand/or increasing the growthrate of young(Quinneyet al. 1986). Data relevantto thehypothesisthatnestmicroclimate plays an important role in nest-siteselection in our populationare summarizedin Table3. All three variablesknown or likely to be associated with improvednestmicroclimate were also associatedwith higherreproductive success.Two of these (larger limbs and eastfacingcavities)werealsomodalcharacteristics of nest cavities. In contrast, live limbs were not

preferredrelativeto availabilityand wereused in the samefrequencyas dead limbs, despite being microclimaticallyfavorableand associated with increased total fledging success. However, for two of three variables, the data

suggestthat nestcavitieswith morefavorable microclimatic

characteristics

are

associated

with increasedreproductivesuccessand are usedmostfrequentlyby AcornWoodpeckers. Theroleof nestpredation.--Nestingsuccessin our populationexceeds70%(KoenigandMum-

52

HOOGE,STANBACK, ANDKOENIG

[Auk, Vol. 116

TABLE 3. Summaryof evidencefor the importanceof nestmicroclimate and accessibility to predatorsin nest-siteselectionby AcornWoodpeckers. Most

Variable

Preferred?

frequent?

Most

successful?

Variables known or likely to influencenest microclimate -Yes Yes

Largerlimb Live limb

East-facing hole

No

No

Yes

--

Yes

Yes

Related

to

success? Yes Yes

Yes

Variables known or likely to influence accessibilityto nest predators Smooth-barked

Yes

No

Yes

Yes

Holerelativelyhighin tree Deeperhole Higherhole

tree

----

No No No

Yes No No

Yes No No

Nest orientedvertically

--

Yes

No

No

me 1987).However,predationseemsto be the some manner. One commonly invoked conmain causeof nestloss(M. T. Stanbackunpubl. straintis interspecificcompetitionfor nestsites (Nilsson 1984, Brawn 1988, Rendell and Robdata).Gophersnakes(Pituophis melanoleucus) arethe mostcommonnestpredators(Eichholz ertson 1989, Li and Martin 1991). However, beandKoenig1992),andour datasupportthehy- causeAcornWoodpeckers constructtheirown pothesisthatnestsitesarechosen to reduceac- cavities,theyare presumably notsignificantly cessibilityto snakes(Table3). Nests in syca- constrained by otherspecies'nestpreferences. mores,whichhaverelativelysmooth,slippery More likely constraintsare of two kinds. First bark exceptwhen very old, were preferred are "ecological" constraints thatlimit theabilwhen availableand tended to have higher ity of birdsto choose nestsiteswith particular fledgingsuccessand lower nest failure than characteristics. Examples includethespecies of nestsin other speciesof trees.More signifi- tree in which nestscanbe built (which is concantly, nesting successincreasedas relative strainedby the absence of certainspeciesin height of nestsincreased.Why relativenest particularterritories),and the rarity of snags height(vs. absolute)showedthis effectis un- (whichlimits the numberof neststhat can be clear

excavatedin dead trees). An additional exam-

Wefoundno relationship betweenreproductive successand either hole depth, absolute nestheight,or angleof the holerelativeto the ground,factorsthatare alsolikelyto influence accessibility to predators.Nonetheless, these resultssuggestthat choosingrelativelyinac-

pleis relativenestheight.Despitea highlysignificantreproductiveadvantagein nestslocated relativelyhigh in trees,thevastmajorityof nestswere locatedin the "upper middle"of trees(Fig. 3). The mostlikely explanationfor this discrepancy is thatrelativenestheightis cessible nests is a factor in nest-site selection in determinedprimarily by the locationof large, AcornWoodpeckers, despitethelow incidence deadlimbs,whichinevitablydecreasein numof nestpredationrelativeto thatof manyopen- ber as one ascends a tree. nesting species. More interestingare "demographic"conConstraintson optimalnest-siteselection.--If straintsthatlimit theabilityofbirdsto excavate predictablefeaturesof nestsitesare associated cavitieswith optimalcharacteristics. Our best with higherreproductivesuccess, what pre- exampleof sucha constraintis the relatively

ventsbirds from selectingsitesand construct- low incidenceof nestsin live limbs (51.2% vs. ing cavities with those characteristics? Con- anestimatedavailabilityof 88.0%)despitetheir versely,why are certain nest characteristicsmicroclimaticadvantagesand correlationwith preferredor usedmorefrequentlywhen they high totalmeanfledgingsuccess. Mostlikely, do not appearto be associated with higherre- this reflectsthe difficultyof excavating in live productivesuccess? wood,especially hardwoodssuchasthe oaks Mostlikely,the answerto bothof theseques- that dominateour studysite.That excavation tions is that nest-site selection is constrained in difficultyconstitutes a demographic constraint

January1999]

AcornWoodpecker NestSites

is supportedby the differencein meangroup

53

CONNER, R. N. 1975. Orientation of entrances to

woodpeckernest cavities.Auk 92:371-374. size of birds nestingin live versusdead limbs (group size is higher when nesting in live CONNER,R. N., R. G. HOOPER,H. S. CRAWFORD,AND H. S. MOSBY.1975.Woodpeckernestinghabitat limbs)andby the differencein the probability in cut and uncut woodlandsin Virginia. Journal of nestsbeingin live limbsin territoriesthatare of Wildlife Management39:144-150. continuouslyoccupiedversusthose that are EICHHOLZ, M. W., ANDW. D. KOENIG.1992.Gopher not continuouslyoccupied(continuousoccusnake attraction to birds' nests. Southwestern Naturalist 37:293-298. pancyis associated with nestingin live limbs). Apparently,largerand morestablegroupsal- GRIFFIN,J. R. 1974. Botanical resourcesof the Haslow the increased cost of nest construction in

tingsReservation, MontereyCounty,California.

Madrofio 22:329-332. live limbs to be shared amongmultiple indiGUTZwILLER, K. J., AND S. H. ANDERSON.1987. Mulvidualsand overmultipleyears. tiscale associations betweencavity-nestingbirds Theseresultsindicatethatsignificantecologand features of Wyoming streamsidewoodical and demographicconstraintsexiston oplands. Condor 89:534-548. timal nest-siteselectionin AcornWoodpeckers, INOUYE, R. S., N.J. HUNTLEY, AND D. W. INOUYE. despitethe relativelystrongnest-buildingca1981. Non-random orientation of Gila Wood-

pabilitiesof this species.This conclusion has

peckernest entrancesin saguarocacti. Condor

83:88-89. important implicationsfor the conservation of J., AND S.G. NILSSON.1977. The influence primary cavitynesters.For example,although KARLSSON, of nest-box area on clutch size in some hole-nestAcorn Woodpeckersapparentlyprefer to nest ing passerines.Ibis 119:207-211. in snagsand deadlimbs,to providedeadlimbs KOENIG, W. D., ANDR. L. MUMME.1987. Population as artificial nestinghabitat would be to offer ecology of the cooperativelybreedingAcorn suboptimalnesting sites. Instead, providing Woodpecker. PrincetonUniversityPress,Princelive limbswith potentialnestcavitiespartially ton, New Jersey. excavated wouldbe morelikely to enhancethe KOENIG, W. D., P. B. STACEY,M. t. STANBACK,AND R. reproductiveoutputof the population.Because L. MUMME.1995.AcornWoodpecker(Melanerpes therelationships amongpreferred,modal,and formicivorus). In The birdsof North America,no. optimal nest-sitecharacteristics are complex, 194 (A. Pooleand E B. Gill, Eds.).Academyof Natural Sciences,Philadelphia,and American assumptions aboutnest-sitechoiceshouldnot

be made without careful evaluation, even for

Ornithologists'Union,Washington,D.C.

LI, P., AND T. E. MARTIN. 1991. Nest-site selection

speciesthat excavatetheir own cavities.

and nesting successof cavity-nestingbirds in high elevationforestdrainages.Auk 108:405-

ACKNOWLEDGMENTS

418.

LOHRL, H. 1973. Einfluss der Brutraumfl•che auf die

We thank Dick Conner, Elizabeth Ross Hooge, Gelegegr/Ssse der Kohlmeise(Parusmajor).JourWalt Klenner,Kathy Martin, Frank Pitelka,and an nal ffir Ornithologie114:339-347. anonymousreviewerfor constructivecommentsand LUDESCHER, E B. 1973. Sumpfmeise (Parusp. palusDeniseFruitt for help in the field.Thisresearchwas tris) und Weidenmeise(P.montanussalicarius) als supportedby BettyDavisFellowshipsto PNH and sympatrische zwillingsarten.Journalffir OrniMTS providedthroughthegenerosityof FannyHasthologie114:3-56. tingsArnoldandher family,anNSFgraduatefellow- MACROBERTS,M. H., AND B. R. MACROBERTS.1976. ship to MTS, and NSF grantsto WDK. Socialorganizationand behaviorof the Acorn Woodpeckerin centralcoastalCalifornia.OrniLITERATURE CITED thologicalMonographsNo. 21. NILSSON,S.G. 1984. The evolution of nest-site selec-

AUSTIN,G. T. 1974. Nesting successof the Cactus Wren in relation

to nest orientation.

Condor

76:

216-217.

BATSCHELET,E. 1965. Statistical methods for the

tion amonghole-nestingbirds:The importance of nest predationand competition.Ornis Scandinavica

15:167-175.

QUINNEY, t. E, D. J. T. HUSSEL,AND C. D. ANKNEY.

analysisof problemsin animal orientationand 1986. Sourcesof variation in growth of Tree Swallows. Auk 103:389-400. certainbiologicalrhythms.AmericanInstitute RENDELL,W. B., AND R. J. ROBERTSON.1989. Nest-site of BiologicalSciences,Washington,D.C. BRAWN,J. 1988. Selectivityand ecologicalconsecharacteristics, reproductivesuccess and cavity quencesof cavitynestersusingnatural vs. artiavailabilityfor Tree Swallowsbreedingin natficial nest sites. Auk 105:789-791.

ural cavities. Condor

91:875-885.

54

HOOGE, STANBACK, ANDKOENIG

RICKLEFS,R. E., AND E R. HAINSWORTH. 1969. Tem-

perature regulation in nesting Cactus Wrens: The nest environment.

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SOKAL,R. R., ANDF.J. ROHLF.1981. Biometry.W. H. Freeman, San Francisco.

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WACHOB,D. G. 1996. A microclimate analysis of nest-site selection by Mountain Chickadees. Journalof Field Ornithology 67:525-533. WHITE, F. N., AND J. L. KINNEY. 1974. Avian incubation. Science 186:107-115.

TROETSCHLER, R. G. 1976.AcornWoodpeckerbreed- ZAR,J.H. 1984.Biostatisticalanalysis,2nd ed. Prentice-Hall, EnglewoodCliffs, New Jersey. ing strategy as affected by Starling nest-hole Associate Editor: K. Martin competition.Condor 78:151-165.