Microhabitat Factors Influencing Predation and Success of Suburban Blue Jay Cyanocitta Cristata Nests Author(S): Keith A

Nordic Society Oikos

Microhabitat Factors Influencing Predation and Success of Suburban Blue Jay Cyanocitta cristata Nests Author(s): Keith A. Tarvin and Kimberly G. Smith Source: Journal of Avian Biology, Vol. 26, No. 4 (Dec., 1995), pp. 296-304 Published by: Wiley on behalf of Nordic Society Oikos Stable URL: http://www.jstor.org/stable/3677044 . Accessed: 04/08/2014 16:20

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This content downloaded from 132.162.168.180 on Mon, 4 Aug 2014 16:20:56 PM All use subject to JSTOR Terms and Conditions JOURNAL OF AVIANBIOLOGY 26: 296-304. Copenhagen1995

Microhabitat factors influencing predation and success of suburban Blue Jay Cyanocittacristata nests

Keith A. Tarvin and Kimberly G. Smith

Tarvin,K. A. and Smith,K. G. 1995. Microhabitatfactors influencing predation and success of suburbanBlue JayCyanocitta cristata nests. - J. AvianBiol. 26: 296-304.

We examinedhow microhabitatfeatures surrounding nest-sites influence probability of nest predationby testingunique predictions generated from the "distanceto human activity,"the "habitatcomplexity," and the "multiplenest-site" hypotheses. During springand summer1990, we quantifiednine featuresof nest-sitesand 11 featuresof 0.1 ha nest-patchessurrounding 42 nests of suburbanBlue JaysCyanocitta cristata in Fayetteville,Arkansas, to determinedifferences associated with depredatedand successful nests. Overallnesting success was 52% and meandaily survivalrate of nests was 0.983 (? 0.004 SE). Twenty-fournests fledged at least one young, and 18 failed as a resultof nest predation.Features of thenest-site, including nest height,size of the nest tree, andnest concealment,were not relatedto nest predation,although distance from a nest to the nearestbuilding was inverselyrelated to durationof nest survival. The latterrelationship suggests human activity may decreasenest predationin suburban areas.Density of vegetationsurrounding nests did not influencenest fate, thus providingno supportfor the habitatcomplexity hypothesis. Instead, failed nests oc- curredin patchescontaining fewer trees of the samespecies, size class andvine cover as the nest tree. Becausethis patternwas not an effect of size or species of eitherthe nest tree or neighboringtrees, nor an effect of vines in the nest tree or neighboring trees, it providessupport for the multiplenest-site hypothesis.

K. A. Tarvinand K. G. Smith,Department ofBiological Sciences, University of Arkan- sas, Fayetteville,AR 72701, USA.Present address of K. A. Tarvin:Department of Biology,University of SouthFlorida, Tampa, FL 33620, USA.

Because nest predationis a majorfactor limiting breed- ris 1980) suggests increasingspatial habitat complexity ing success in open nesting passerinebirds (e.g., Rick- reducespredator search efficiency by impedingtravel or lefs 1969, Martin1988, 1992), naturalselection should by inhibitingtransmission of visual, auditory,or chemi- favorbirds that behave in ways thatreduce the probabil- cal cues that predatorsuse to find nests. The "multiple ity of nest predation.If qualityof nesting microhabitats nest-site"hypothesis (Martin and Roper 1988) suggests differs amongpotential nest-sites, such that nests differ thatincreasing numbers of potentialnest-sites surround- in the likelihood of being found by a predator,and if ing an actual nest may reduce predatorefficiency be- such differencesare identifiable,selection should favor cause predatorsare more likely to encounterempty nest- birdsthat choose nestingmicrohabitats associated with a sites, and, being rewardedless frequently,are more low probabilityof nest predation(Martin 1992). likely to move to a differentarea. The "distanceto hu- A numberof hypotheseshave been proposedto ex- man activity" hypothesis (Collias and Collias 1984) plain how nesting microhabitatmay influencenest pre- states that nests closer to human activity suffer lower dation (Filliateret al. 1994). Here we examinetwo hy- predationrates because humanactivity deterspredators potheses that addressthe mannerin which structureof fromusing such areas. the habitatsurrounding nests may affect predatorsearch In this study we quantifiedmicrohabitat features sur- efficiency, and a thirdthat addressesthe way in which roundingnests of suburbanBlue Jays Cyanocittacris- the suburbanlandscape may influence nest predation. tata to determinedifferences associated with failed and The "habitatcomplexity" hypothesis (Bowman and Har- successfulnests. We also tested uniquepredictions gen-

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This content downloaded from 132.162.168.180 on Mon, 4 Aug 2014 16:20:56 PM All use subject to JSTOR Terms and Conditions erated from the habitat complexity,multiple nest-site, summer, 1990, in neighborhoodscontaining park-like and distance to human activity hypotheses to examine standsof oaks Quercusspp., elms Ulmusspp., and sugar how microhabitatfeatures may influencethe probability maples Acer saccharum that formed patchy canopies of nest predation.A unique predictionof the habitat over lawns, scatteredshrubs, and copses. Lawns often complexityhypothesis is thatthe probabilityof nest pre- were borderedby overgrownbrush along propertylines, dation should decrease as vegetationdensity increases. small streams,or vacant lots. Several dense woodlots, The multiplenest-site hypothesisuniquely predicts that rangingin areafrom a few to tens of hectares,were in- the probabilityof nest predationshould decreaseas the terspersedthroughout the city. numberof potential,but unoccupied,nest-sites within an Commonpotential nest predatorson our studysite in- area increases.Because potentialnest-sites are difficult cluded gray squirrelsSciurus carolinensis, house cats to identifyfor species that nest in a varietyof nest sub- Felis domesticus,American Crows Corvusbrachyrhyn- strates,we tested the corollaryprediction that variables cos, and other Blue Jays. Less common predatorsin- thatreflect similaritiesbetween the actualnest-sites and cluded black rat snakes Elaphe obsoleta, House Wrens the surroundingvegetation are good predictorsof nest Troglodytesaedon, raccoonsProcyon lotor, and Virginia fate. Finally, we tested a predictiongenerated from the opossums Didelphis virginiana.We did not attemptto distanceto humanactivity hypothesis, that probability of assess relative impact of each predatorspecies on our nest predationincreases with distancefrom nest to near- study site; however,Blue Jays usuallyresponded vigor- est building. ously to the presenceof AmericanCrows and gray squirrels nearnest-sites, indicating jays view those species as threatening.We observedAmerican Crows taking eggs from one jay nest, and an unmarkedBlue Jay flying Methods fromanother nest with an egg in its beak.Eggs of a third jay nest containedpuncture holes characteristicof egg Study species destructionby House Wrens (Belles-Isles and Picman Blue Jays nest in deciduousand mixed coniferous/decid- 1986). We observedno otherpredation events. uous forest habitatsacross the easternUnited States and southernCanada. In moderntimes, Blue Jays have be- come numerousin suburbanhabitats, where build they success nests in a wide arrayof tree species.Nests may be as low Nesting as 1 m above the groundor near the top of the nest tree, Upon discovery,nests were checkedfor activityevery 1 althoughtypical nest height is between 3 and 8 m (Tay- to 6 days; usually every 2-3 days dependingon number lor 1965, Woolfendenand Rohwer 1968). Nests consist of activenests on any given day.We used Student'st test of a ratherbulky outershell made of sticks surrounding to determinewhether frequency of nest checks influ- a compactcup. They may be placed near the end or at enced probabilityof nest predationby comparingthe ra- the base of a branch,directly on a largehorizontal limb, tio of numberof nest checks per nest observationperiod or in a fork in the trunkof the nest tree. Nests may or (in days) betweensuccessful and depredatednests for all may not be concealedby vegetation,and adultsusually nests that were checked more than once. If a nest ap- vigorouslydefend their nests from potentialnest preda- peared inactive during a nest check, contents were tors. Females incubate eggs and brood nestlings, and checked with a mirrormounted on a pole. Nests were food for incubatingfemales and young nestlingsis pro- consideredactive if at least one egg or nestlingwas seen vided by males. Regardlessof whethernests are well in a nest, or if an adult was observedbrooding on the concealed by vegetation,provisioning males approach nest over a spanof at least threedays. the nest indirectly,usually moving to severalperches in Nests were consideredsuccessful if they survivedat the vicinity of the nest before deliveringfood to the sit- least 34 days beyond the laying of the penultimateegg. ting female or nestlings. Hardy (1961) concludedthat Throughoutthe geographicrange of the Blue Jay, the Blue Jays build "false"nests, small platformsof sticks time betweenlaying of the penultimateegg and success- that are abandonedprior to completion.We know of no ful fledging of young jays is about 34 days (e.g., Tyler published studies addressingthe effects of false nests, 1946, Woolfendenand Rohwer 1968, this study).When tendencyof adults to defend the nest, or approachpat- we couldnot determinethe age of a nest, success was as- ternsof provisioningmales on probabilityof nest preda- sumedif (1) fledglings were foundin the areasurround- tion, nor did we attemptto quantifythem in the present ing a nest after it was empty; (2) adults became ex- study. tremely agitatedas we searchedfor fledglings, but no fledglings were found; or (3) nestlings were seen perchedon the nest rim. Active nests were assumedto have failed due to pre- Study area dation if contents disappearedbefore young were ex- Our study was conductedwithin the city limits of Fay- pected to fledge or if puncturedor broken eggs were etteville, WashingtonCo., Arkansasduring spring and found. If a predatorremoved only a portionof the nest

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This content downloaded from 132.162.168.180 on Mon, 4 Aug 2014 16:20:56 PM All use subject to JSTOR Terms and Conditions contentsand the nest continuedto survive,it was consid- strate.A "nest-patch"was definedas a 0.1 ha circle (ra- ered active until the remaining contents later disap- dius = 17.8 m) centeredaround the nest. Nest-patchfea- peared.When dead nestlingswere found below a failed tures were featureswe felt potentiallycould influence nest and showed signs of predation(such as puncture predatoroccurrence, or predatorforaging or searcheffi- holes), the nest was assumedto have been preyedon. If ciency within the area (Bowman and Harris 1980, no signs of predationwere found on fallen young, the MacKenzieet al. 1982, Martinand Roper 1988). nest was assumedto have failed as a result of unknown Nine nest-site variablesreflected nest-substrate char- causes. Active nests were consideredabandoned when acteristicsand nest concealment.For each nest, we de- adults were not seen tendingthem for several consecu- terminednest-tree species, height,and diameter at breast tive nest checks, even thoughthey still containedeggs. height (dbh);height of the nest and whetherit was situ- Only successfulnests andnests knownto have been dep- ated in a clump of vines; distancefrom the nest to the redatedwere includedin nest predationanalyses. Nests centerof the nest tree; distancefrom nest to the top of that were abandonedbefore completion were not in- the nest tree;compass orientation of the nest relativeto cluded in analyses of nest success because Blue Jays the centerof the nest tree; distancefrom nest to nearest may build "false"nests (Hardy1961). building;and degreeof nest concealment. We estimatedthe week thatthe first egg of each nest Nest concealmentwas determinedby visually esti- was laid to determineif temporalvariability existed in matingpercent of the nest that would be concealedby nest failure, and hence whetherthe Mayfield(Mayfield foliage or branchesif viewed from 1 m away in both 1961, 1975) methodwas appropriatefor estimatingover- vertical (above/below) and horizontal(cardinal direc- all nest successfor the entirenesting period (Johnson and tions) planes relative to the nest (Martin and Roper Shaffer1990). We were unableto calculatesurvival rates 1988, Kelly 1993). Because of the subjectivityinvolved for the egg andnestling periods separately because many in these measurements,especially when nests were too nests werenot readilyaccessible due to theirheight or po- high to be examinedfrom 1 m away,we performedthree sition withinthe nest tree, and we fearedfrequent check- independentmanipulations on concealmentdata to cre- ing of nest contentswould cause abandonment(personal ate threetypes of concealmentindex. Concealmentindi- observation).Nests thatlikely failed due to observerdis- ces representedthe sum of concealment values in a turbancewere excluded from analyses. given plane, the mean concealmentacross all measured The minimumduration of survivalof each nest was directionsof a given plane, and the directionexhibiting calculatedto provide an index of susceptibilityto nest the lowest percentconcealment in the horizontalplane predation.Duration of nest survivalwas determinedby (Martinand Roper 1988). The "lowest value"conceal- assumingthat for successfulnests, the post-layingincu- ment index was createdfor the horizontalplanes only. bation and nestling periods each lasted 17 days (Tyler The "summed"and "mean"concealment indices were 1946). Unsuccessfulnests found afterclutch completion createdfor both the horizontaland vertical plane. Be- were assumedto have survivedone day for each egg in cause of their relative subjectivity,the above nest con- the clutch plus the numberof days the nest was under cealmentindices were not includedin any multivariate observationafter it was discovered. analyses. Eight measurementswere used to provide descrip- tions of the nest-patch.We countedand measured dbh of all stems in the and noted the or Habitat woody patch, presence sampling absenceof vines on them (hereafterreferred to as "vine We measurednest-site and nest-patchmicrohabitat fea- status").Clumps of stems less than2.5 cm dbh thatwere tures (MacKenzieet al. 1982, Martinand Roper 1988, obviouslyramets of the same genet were consideredas a Kelly 1993) between 20 June and 5 September1990. single shruband tallied separately.Woody stems were Nest-site features reflected qualities of the nest sub- grouped into size classes for most statisticalanalyses Table1. Sizeclass categories of woodystems used for statistical (Table1). Percentof the groundsurface in the nest-patch analyses. coveredby buildingsand shrubswas estimatedvisually, as was the percentof the ground surfacethat was not Sizeclass Growthform Diameteratbreast height coveredby buildings,shrubs or trees (hereafterreferred (cm) to as "open ground").Percent canopy was estimatedat 40 as described Jamesand All Shrub clumpeda < 2.5 points by Shugart(1970). Tiny single < 2.5 percentagevalues were arcsinetransformed. Small single 2.5 -7.4 To test the multiple nest-site hypothesis,we had to Mediumb single 7.5- 30.5 identify potentialnest-sites within each We > 30.5 nest-patch. Largeb single defined a potentialnest-site as any tree large enough to supporta Blue Jay nest (> 7.5 cm dbhbased on observed a Clumpsof stems were assumedto comprise single shrubsonly when the stems appearedto be rametsof the same genet. nest trees),regardless of tree species. Two indices of the numberof potentialnest treesthat were similarto the ac- b Stems in these size classes were large enough to supporta Blue Jay nest. tual nest tree in the nest-patchwere obtainedfrom the

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This content downloaded from 132.162.168.180 on Mon, 4 Aug 2014 16:20:56 PM All use subject to JSTOR Terms and Conditions stem frequencydata. The first index was the numberof trees in the nest-patchthat were of the same species and Results in the same size class as the nest tree. The second index success was the numberof trees in the nest-patchthat were of Nesting the same species, in the same size class, and of the same Sixty-seven Blue Jay nests were found between 22 vine status as the nest tree. Although the two indices Marchand 24 July 1990. Fortynests were found before were similar,the latter was a more refined estimate of egg laying began; five were found during laying; two the similarityof the othertrees to the actualnest tree. were found after completionof the clutch, but before hatching;and six were found afterhatching, but before fledging. One nest was foundthe day the young fledged. We found 12 othernests with eggs, but could not deter- Statistical analysis mine if they were foundbefore the clutchwas completed Statisticalanalyses were conductedusing SAS (SAS In- becausetheir contentscould not be checked(due to the stitute,Inc. 1985). Because we tested for differencesin presenceof an adulton the nest) until severaldays after 20 microhabitatfeatures associated with successful and discovery.We were unableto determinethe contentsof depredatednests (continuous nest-site and nest-patch one other active nest. Of the 67 nests, only 58 were variablescombined), we should expect at least one sig- knownto have containedeggs; of the 58 nests, we were nificant,yet perhapsspurious, relationship at an a level able to determinesuccess or failurefor 47. Twenty-one of 0.05. We correctedfor this problemby using a "pro- of 47 nests whose fate was known failed to producea tected ANOVA" technique (Scheiner 1993). We first fledgling. Predationaccounted for 18 (90.0%)of the 20 conducteda MANOVA,which allowed us to examine failed nests for which cause of failure could be deter- for an overall differencein microhabitatwith a single mined. Virtuallycomplete habitat data sets were ob- test, therebyreducing the chanceof committinga type I tained on 42 nests, 24 of which were successful and 18 error(Scheiner 1993). We then proceededwith univari- of which failed as a resultof predation.Because most of ate tests, using the Wilcoxon Rank Sums test to deter- our habitatanalyses concerned only successfuland dep- mine which continuousmicrohabitat variables differed redatednests, sample sizes range from 38 to 42 nests, betweensuccessful and depredatednests. Linearregres- dependingon the numberof nests for which the appro- sion was used to examinethe relationshipof all continu- priatehabitat characteristics were measured. ous variableswith durationof nest survivalto determine Frequencyof nest checks did not differ between suc- relativeability of single variablesto predictnest fate. cessful and depredatednests (all nests checked at least Categoricalnest-site variables were not included in twice: N = 23 successfulnests, 15 depredatednests; Stu- the MANOVA.Rayleigh's test and the Watson-Williams dent's t = -1.635; df = 30.4; P > 0.11). Mean week of test (Zar 1974) were used to test effects of nest orienta- nest initiationdid not differbetween depredated and suc- tion on fate of nests having distinctdirectional orienta- cessful nests (Wilcoxon Rank Sums: Z = 0.678, P > tion relativeto the centerof the nest tree. Fisher'sexact 0.40), nor did it explain significantvariation in duration test was used to comparepredation frequency of nests of nest survival (r2 = 0.025, P > 0.30), suggesting the having distinct orientationwith those for which direc- Mayfieldmethod was appropriatefor calculatingnesting tional orientationrelative to the center of the nest tree success (Johnsonand Shaffer1990). Of the 47 nests for was not distinct. Chi-squarecontingency analysis and which we determinedfate, 45 were observedover a pe- Fisher'sexact test were used to determineif nests were riod of at least 24 hours and thus were appropriatefor preyedupon more or less often thanexpected by chance Mayfield analysis (Mayfield 1961, 1975). Based on in a particularnest substrate,nest tree size class, or in those 45 nests, population nesting success was 52% commonlyused nest tree species. (1099 nest-days)and mean daily survivalrate of nests A stepwise logistic regression procedure (Harrell afterthe first egg was laid was 0.983 (S.E. = 0.004). 1986) was used to identifyinteracting variables that were good predictorsof nest fate and that discriminatedbe- tween depredatedand successfulnests. Stepwiselogistic Nest-site characteristics regressionwas chosen over discriminantfunction analysis becausethe formerdoes not assumemultivariate nor- We found Blue Jay nests in 18 tree species, and on two mality or equal covariance matrices between groups houses (Table2). No differencesin probabilityof preda- (Press and Wilson 1979). After creating a model with tion were detected in common nest tree species (i.e., stepwiselogistic regression,we used non-stepwiselogis- those in which at least five nests were found;Table 2) vs tic regressionto examine the contributionof particular otherspecies (N = 40, X2= 0.009, df = 1, P = 0.923), nest variablesto the originallogistic regressionmodel. Co- trees with vines vs trees without vines (N = 37, X2= hen'skappa statistic (Titus et al. 1984) was used to deter- 0.035, df = 1, P = 0.852), or in large vs mediumnest mine whetherclassification produced by logistic regres- trees (Table 1; N = 39, X2= 0.010, df = 1, P = 0.921). sion was significantlybetter than expected by chance. Blue Jay nests were uniformlydistributed around the centers of nest trees (Rayliegh'sZ = 0.842, 0.20 < P <

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This content downloaded from 132.162.168.180 on Mon, 4 Aug 2014 16:20:56 PM All use subject to JSTOR Terms and Conditions Table2. Treespecies used as nest-sitesby BlueJays in Fayetteville,Arkansas during spring and summer, 1990, and number of nest-patches in whichat leastone individualof thespecies was present. Numberused as Numberof patchesin which nest-sites specieswas present

Sugar Maple Acer saccharum 7 19 Post Oak Quercusstellata 5 14 Winged Elm Ulmus alata 5 14 Red Elm Ulmus rubra 3 17 Red Mulberry Morus rubra 3 13 Black Jack Oak Quercusmarilandica 3 9 ShortleafPine Pinus echinata 2 4 Redbud Cercis canadensis 2 17 Hackberry/Sugarberry Celtissp. 2 18 Flowering Magnolia Magnolia grandiflora 1 3 Tree of Heaven Ailanthusaltissima 1 6 Honeysucklea Lonicera maackii 1 2 Osage-orange Maclurapomifera 1 4 Pecan Carya illinoensis 1 6 Plum Prunussp. 1 1 CherrybarkOak Quercusfalcata var.pagodaefolia 1 4 Chestnut Castaneasp. 1 2 Boxelder Acer negundo 1 5 Deadsnag Pinussp. 1 2 House 2 31 a Lonicera maackii is abundantas a shrubon our study site. However, we found individualshaving stems large enough to supporta nest in only 2 nest-patches. 0.50) andno differenceswere foundbetween depredated Distancefrom nest to nearestbuilding may reflect ef- and successfulnests in orientation(depredated nests, Z = fects of proximityof nests to buildings(a characteristic 0.356, P > 0.50; successfulnests, Z = 1.529, 0.20 < P < of the nest-site) or of relative openness of understory 0.50) or meanangle (Watson-Williamstest, F = 1.27, P > nearbuildings (a characteristicof the nest-patch;see be- 0.25). Nine nests were consideredto have no distinctori- low). To determinewhether vegetation density varied entation;they were depredatedat the same rate as those with increasingdistance from buildings, density of vege- positionedperipherally to the maintrunk or centerof the tation in each size class (Table1) was regressedagainst nest tree (Fisher'sexact test, N = 42, P > 0.20). distancefrom nest to nearestbuilding. In no size class of MANOVAindicated overall microhabitat differed be- vegetationwas density linearlyrelated to distancefrom tween successfuland depredated nests (N = 42, F = 2.43, buildings(r2 < 0.009, P > 0.50). df = 20, 16, P = 0.04). Continuous(i.e., non-categorical) nest-site variables,however, did not differ significantly between and successful nests when consid- depredated characteristics ered individually(Table 3). Distancefrom nest to near- Nest-patch est buildingwas inverselyrelated to durationof nest sur- Univariate tests revealed depredated nests were in vival (r2= 0.124, P = 0.04). No othernest-site variables patchesassociated with fewer trees of the same species, were significantlyrelated to durationof nest survival size class, and vine statusas the nest tree; however,no (linear regressionfor all other nest-site variables,r2 < other nest-patchvariables differed between depredated 0.025, P > 0.35). andsuccessful nests (Table4). Whenconsidered univari-

Table3. Relationshipof continuousnest-site variables to fateof suburbanBlue Jay nests (Wilcoxon Rank Sums test). Variable n SuccessfulX (S. E.) Depredated (S.E.) P

Nesttree height (m) 40 11.8 (1.1) 11.6 (1.1) 0.74 Nest treediameter (cm) 40 36.3 (5.3) 41.3 (6.8) 0.72 Nest height(m) 42 4.8 (0.4) 4.1 (0.5) 0.24 Distancefrom nest to centerof nesttree (m) 42 1.4 (0.4) 1.5 (0.5) 0.69 Distancefrom nest to nearestbuilding (m) 42 9.6 (1.2) 15.9 (2.7) 0.14 "Summed"concealment indexa (horizontal) 42 13.7 (0.79) 13.8 (0.64) 0.84 "Summed"concealment indexa(vertical) 41 7.2 (0.36) 7.9 (0.41) 0.41 "Mean"concealment indexa (horizontal) 42 58.6 (4.10) 61.1 (3.24) 0.85 "Mean"concealment indexa(vertical) 41 62.8 (3.60) 69.4 (4.28) 0.57 "Lowest value" concealmentindexa (horizontal only) 42 28.7 (5.60) 28.7 (4.83) 0.56 Distancefrom nest to topof nesttree (m) 42 6.97 (0.90) 7.52 (0.82) 0.77 a See text for explanationof these indices.

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This content downloaded from 132.162.168.180 on Mon, 4 Aug 2014 16:20:56 PM All use subject to JSTOR Terms and Conditions Table4. Relationshipof nest-patchvariables to fateof suburbanBlue Jay nests (Wilcoxon Rank Sums test). Stem size classes are defined in Table1.

Variable n Successfulx (S. E.) Depredatedx (S. E.) P

Numberof shrubsin thepatch 41 14.3 (3.4) 16.0 (4.7) 0.68 Numberof tinystems 41 20.1 (5.1) 20.1 (6.0) 0.82 Numberof smallstems 41 17.5 (3.8) 16.8 (6.0) 0.63 Numberof mediumstems 41 20.1 (4.8) 12.9 (3.1) 0.33 Numberof largestems 41 4.1 (0.4) 4.4 (0.3) 0.35 Totalnumber of trees> 7.4 cm dbh 41 24.2 (4.70) 17.3 (3.03) 0.35 Totalnumber of stems 41 74.9 (14.8) 70.3 (17.7) 0.84 Numberof treesof the samespecies 39 9.1 (3.4) 1.8 0.07 andsize classas thenest tree (0.4) Numberof treesof the samespecies, 39 8.1 (2.9) 1.4 (0.3) 0.03 size class,and vine statusas thenest tree Numberof treeswith vines 41 8.0 (2.4) 6.2 (2.1) 0.85 %open ground 42 79.4 (2.9) 74.3 (5.1) 0.52 %ground covered by buildings 42 14.7 (2.8) 14.4 (4.5) 0.30 %ground covered by shrubs 42 4.5 (0.9) 10.4 (3.7) 0.32 %canopy cover 42 66.8 (3.3) 69.7 (3.2) 0.74 ately, no nest-patchvariables accounted for significant numberof treesin the patchthat were of the same species, variationin durationof nest survival(all variables,r2 < in the samesize class, andof similarvine statusas the nest 0.086, P > 0.088). tree (Table5). Toexamine the importanceof the similarity of othertrees to the nest tree, logistic regressionmodels wererun again, but with variables that were less refinedin their estimatesof the of othertrees to the nest Multivariate similarity analyses tree.Those logistic regression models were not as good at Four variableswere selected by the initial stepwise lo- predictingnest fate (i.e., they hadlower multiplecorrela- gistic regressionas good predictorsof nest fate (Table tion coefficients)as were the initialmodels, althoughthe 5). The model predictednest success (non-predation) overall ability of the models to classify successful and better than nest predation:76.5% of depredatednests depredatednests varied little (Table6). and 86.4% of successful nests were correctlyclassified. Distancefrom nest to centerof the nest tree also was This classification is 63% better than expected by selectedby the originalstepwise logistic regressionpro- chance (Cohen'stest for chance correctness,Pc = 0.511, cedure. However,its effects may have reflectedthe in- Po = 0.821, Kappa= 0.633 ? 0.252, P = 0.0001). fluence of other variables.Distance from nest to center The stepwiselogistic regressionprocedure selected as of the nest tree was positively correlatedwith nest tree an importantpredictor of nest fate a variabledescribing dbh, nest tree height,and nest height (for each variable,

Table5. Initialstepwise logistic regression analysis of therelationship of fateof suburbanBlue Jay nests with continuous nest-site and nest- patchvariables (n = 39 nests).(a) Parametersof variablesselected by theforward stepwise procedure. (b) Overallmodel parameters. (a)Parameters of individualvariables Variables Beta S. E. x2 r P

Intercept -9.96 4.68 4.52 0.03 Distancefrom nest to nearestbuilding -0.02 0.01 4.58 -0.22 0.03 Percentof openground in patch 13.85 6.46 4.60 0.22 0.03 Numberof treesof the samespecies, size class, 1.19 andvine statusas the nesttreea Distancefrom nest to centerof nesttree 0.02 0.01 3.87 0.19 0.05

(b) Overallmodel parameters Model Residual

X2 d.f. P x2 d.f. P -2 LogL r

31.33 4 0.00 17.40 15 0.30 22.09 0.66 aBeta was assumed to be infinite duringat least one iterationand parameterestimation was consequentlysuppressed (Harrell 1986).

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This content downloaded from 132.162.168.180 on Mon, 4 Aug 2014 16:20:56 PM All use subject to JSTOR Terms and Conditions Table6. Comparisonof theability of threelogistic regression models to predictfate of suburbanBlue Jay nests. Four variables were forced intoeach model: distance from nest to nearestbuilding, percent of openground in the nest-patch,distance from nest to centerof thenest tree,and one of thevariables below. Each of thevariables below represents an estimateof thenumber of potentialnest-sites within the nest- patch.They differ in thedegree of similarityof thepotential nest-sites to theactual nest-site.

Overallmodel parameters Variable Similarity N X2 r P %correct classification

Numberof treesof the samespecies, size class,and vine statusas thenest tree. greatest 39 31.3 0.661 < 0.01 82.1 Numberof treesof the samespecies and size classas thenest tree intermediate 39 28.3 0.617 < 0.01 84.6

Numberof treeslarge enough to support a BlueJay nest, regardless of species least 41 22.4 0.510 < 0.01 82.9 r > 0.51, P < 0.0007), and was negativelycorrelated with probabilityof nest predationon our study site, nor did number of mediumtrees in the nest-patch(r = -0.355, numberof stems withinparticular size classes (Table4). P < 0.025). Because distancefrom the nest to the center Bowmanand Harris (1980) found supportfor the habitat of the nest tree was not significantin univariatetests, we complexityhypothesis using artificialground nests anda are unsureof its importance. mammalianpredator (raccoon). Perhaps the important predators on our study site (most likely American Crows,gray squirrels,and Blue Jays)find nests by cuing on particularkinds of vegetation or on movementsof Discussion breeding adults, rather than by randomly moving throughvegetation in searchof nests. In this study, we tested unique predictionsgenerated Our results provide some supportfor the multiple from the "habitatcomplexity" (Bowman and Harris nest-site hypothesis.Number of trees in the nest-patch 1980), the "multiplenest-site" (Martin and Roper1988), thatwere similarto the nest tree was the most important andthe "distanceto humanactivity" (Collias and Collias predictorof nest fate. Furthermore,logistic regression 1984) hypotheses. Each hypothesis is designed to ex- modelsthat included variables representing more refined plain how microhabitatsurrounding nests may influence estimatesof the similarityof othertrees to the nest tree nest predation,although they are not necessarilymutu- consistentlywere betterat predictingnest fate thanwere ally exclusive. The distanceto humanactivity hypothe- models using variablesthat represented a wider arrayof sis (Collias and Collias 1984) was supportedby our re- potentialnest trees. sults. Blue Jay nests near buildingssuffered less preda- The patternswe found supportingthe multiplenest- tion than those furtheraway from buildings (Table5). site hypothesismay reflecteffects of variablesrelated to Proximity to buildings probablycan be equated with those describingthe similarityof other trees to the nest proximityto humanactivity, which may deterpredators tree. However,we found that importanceof numberof from approachingnest-sites and prey items (Baeyens trees of the same species, size class, and vine statusas 1981, Collias and Collias 1984, Doncaster1994). An al- the nest tree did not reflect an influenceof tree size be- ternativeexplanation is that,because suburban buildings cause nest-tree size did not differ between successful are usually associatedwith lawns, the relationshipbe- and depredatednests. Likewise, this variableincluded tween probabilityof nest predationand proximity to only large trees or only mediumtrees, dependingon a buildings may in fact reflect the effects of the "open- particularnest tree. Thus, it did not reflect effects of ab- ness" of vegetation on predation;i.e., predatorsmay solute numbersof mediumor large trees in the patch. avoid areaswith little cover becausethey have a greater Neithernumber of mediumtrees, nor of large trees, dif- chance of being detectedby prey, or because predators fered betweenpatches surroundingsuccessful and dep- themselves may be subjectedto greaterpredation pres- redatednests, nor did the total numberof trees in the sures in such areas.However, no linearrelationship ex- patch(Table 4). isted between density of vegetation and distance from Similarly,number of trees of the same species, size nest to nearestbuilding (r2 < 0.009, P > 0.50), suggesting class, and vine status did not reflect an effect of vines the presenceof buildingsdid not affect the opennessof arounda nest or on othertrees because presence of vines the surroundingvegetation. aroundnests had no effect on nest fate. Likewise, num- Ourresults do not supportthe habitatcomplexity hy- ber of vine-cladtrees did not differ amongpatches con- pothesis. The total numberof stems within a nest patch, taining successful and depredatednests (Table4). Nest a good indicator of the complexity of the vegetation concealmentalso appearedto have no influenceon the structure(Bowman and Harris 1980), had no effect on likelihoodof nest success (Table3). Finally,the variable

302 JOURNALOF AVIANBIOLOGY 26:4 (1995)

This content downloaded from 132.162.168.180 on Mon, 4 Aug 2014 16:20:56 PM All use subject to JSTOR Terms and Conditions probablydid not reflect an effect of nest tree species on for commentsthat greatly improvedthe manuscript.The De- nest fate, as we detectedno patternsof associationbe- partmentof Biological Sciences at the Universityof Arkansas tween nest tree and of nest and the ArchboldBiological Stationprovided computer facili- species probability predation. ties. We also wish to extendthanks to the manypeople of Fay- Few studies have examined the effect of potential etteville who allowed us to work in theiryards and neighbor- nest-sites on nest predationrates. Martin and Roper hoods. (1988) found nests of HermitThrushes Catharus gutta- tus were surroundedby more small white firs Abies con- color, the nest substrateused almost exclusively by the thrush,than were randomlyselected non-use sites. Fur- References thermore,nests surroundedby a lower density of white Baeyens, G. den 1981. Magpiebreeding success and carrion firs had a higherprobability of predationthan those sur- crowinterference. - Ardea69: 125-140. and House wren nest-de- roundedby a greaterdensity of firs. However,nests as- Belles-Isles,J.-C. Picman,J. 1986. stroyingbehavior. - Condor88: 190-193. sociated with a low probabilityof predationalso were Bowman,G. B. andHarris, L. D. 1980.Effect of spatialhetero- better concealed than were high predation-risknests, geneityon ground-nestdepredation. - J. Wildl.Manage. 44: making evaluationof the importanceof multiple nest- 806-813. E. Nest and bird sites difficult. (1993), on the otherhand, found no Collias, N. E. and Collias, C. 1984. building Kelly behavior.- PrincetonUniversity Press, Princeton, NJ. supportfor the multiplenest-site hypothesis in a popula- Doncaster,C. P. 1994. Factorsregulating local variationsin tion of Dusky FlycatchersEmpidonax oberholseri, but abundance:field test on hedgehogs,Erinaceus europaeus. - found vegetationdensity was positively correlatedwith Oikos69: 182-192. Filliater,T. S., Breitwisch,R. andNealen, P. M. 1994.Predation nest success. Mountainjunipers Juniperus scopulorum, on northerncardinal nests: does choice of nest site matter?- the most commonnest substrateof the flycatchers,were Condor96: 761-768. less dense around successful than unsuccessful nests, Hardy,J. W. 1961. Studiesin behaviorand phylogeny of certain - and other,structurally different, tree species were more New Worldjays (Garrulinae). Univ. KansasSci. Bull. 42: dense aroundsuccessful nests. Neitherof the lattertwo 13-149. Harrell,F. E. Jr. 1986.The LOGISTprocedure. - In: SAS Insti- trendswas statisticallysignificant, however. tute, Inc. 1986.SUGI supplementallibrary user's guide, ver- The Hermit Thrushesstudied by Martinand Roper sion 5 edition.- SAS Institute,Inc., Cary, N.C. nested in white James,F C. andShugart, H. H. Jr.1970. A quantitativemethod exclusively firs, potentially allowing of habitat - AudubonField Notes 24: 727-736. for thrushnests to cue on a description. predatorssearching explicitly Johnson,D. H. andShaffer, T. L. 1990.Estimating nest success: particularnest substrate.Blue Jays, however, typically whenMayfield wins. - Auk 107:595-600. place nests in a varietyof substrates,even withina local Kelly, J. P. 1993. The effect of nest predationon habitatselec- Woolfendenand Rohwer tion by duskyflycatchers in limberpine-juniper woodland. - population(Tyler 1946, 1968; Condor95: 83-93. Table 2). The wide arrayof nest-sites chosen by Blue MacKenzie,D. I., Sealy, S. G. and Sutherland,G. D. 1982. Jays in our study may reflect a strategyby which Blue Nest-sitecharacteristics of the aviancommunity in the dune- Jays preclude predatorsfrom cuing on potentialnest- ridgeforest, Delta Marsh, Manitoba: a multivariateanalysis. sites, thus the of nest If - Can.J. Zool. 60: 2212-2223. reducing probability predation. Martin,T. E. 1988. On the advantageof being different:nest so, perhapswe should not expect a strong relationship predationand the coexistenceof bird species. - Proc.Natl. betweenincidence of nest predationand similarityof po- Acad.Sci. USA 85: 2196-2199. tentialnest-sites and actualnest-sites. - 1992. Breedingproductivity considerations: what are the ap- Inferencesfrom studies of this the propriatehabitat features for management?.- In: HaganIII, nature,including J. M. andJohnston D.W. (eds). Conservationof neotropical presentone, should be made with caution, as foraging migrants.Smithsonian Inst. Press, Washington,DC, pp. methodof potentialpredators will influencethe impor- 455-473. - and J. J. 1988.Nest and tance of spatial complexity on foraging efficiency. To Roper, predation nest-siteselection of a westernpopulation of theHermit Thrush. - Condor90: understandhow differingmicrohabitats affect probabil- 51-57. ity of nest predation,we must betterunderstand the cues Mayfield,H. 1961.Nesting success calculated from exposure. - that specific predatorsuse to locate bird nests (Martin WilsonBull. 73: 255-261. a diverse of nest - 1975. Suggestionsfor calculatingnesting success. - Wilson 1988). Indeed, array potential predators Bull. 87: 456-466. the of birds to a may impede ability choose particular Press,S. J. andWilson, S. 1978. Choosingbetween logistic re- strategythat allows them to avoid nest predation(Fil- gressionand discriminantanalysis. - J. Amer. Stat. Assoc. liateret al. 1994). The findings in the presentstudy are 73: 699-705. Ricklefs,R. E. 1969.An analysisof nestingmortality in birds.- importantbecause they demonstratethat microhabitat Smithson.Contr. Zool. 9: 1-69. may influence patternsof nest predationin multiple SAS Institute,Inc. 1985. SAS user'sguide: statistics, version 5 ways, and thathuman activity may actuallydecrease the edition.- SAS Institute,Inc., Cary,N.C. probabilityof nest predationin suburbanhabitats. Scheiner,S. M. 1993. MANOVA:multiple response variables and multispeciesinteractions. - In: S. M. Scheinerand J. Gurevitch(eds.). Design and analysisof ecological experi- Acknowledgements- We extend great appreciationto L. D. ments.Chapman and Hall, New York,pp. 94-112. Walkerfor assistancein the field;T. E. Martin,D. R. Petit,L. J. Tarvin,K. A. 1991. Microhabitatinfluences on predationof Petit and D. A. Jamesfor discussion;and T. E. Martin,D. A. suburbanBlue Jay nests. - M.S. Thesis, Universityof Ar- James,M. C. Garvin,J.-A. Nilsson,and anonymous reviewers kansas,Fayetteville.

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This content downloaded from 132.162.168.180 on Mon, 4 Aug 2014 16:20:56 PM All use subject to JSTOR Terms and Conditions Taylor,W. K. 1965. Nestingheights of some Louisianabirds. - Woolfenden,G. E., andRohwer, S. A. 1968. Breedingbirds in a WilsonBull. 77: 146-150. Floridasuburb. - Bull. FloridaState Mus. 13: 1-83. Titus,K., Mosher,J. A. andWilliams, B. K. 1984. Chance-cor- Zar,J. H. 1974. Biostatisticalanalysis. - Prentice-HallInc., En- rectedclassification for use in discriminantanalysis: ecolog- glewoodCliffs, NJ. ical applications.- Am. Midl.Nat. 111:1-7. Tyler,W. M. 1946. Cyanocittacristata bromia: Northern Blue (Received29 December1994, revised 24 April1995, accepted 3 Jay.- In: Bent,A. C. (ed.). Life historiesof NorthAmerican May1995) jays, crows,and titmice.U. S. Natl. Mus. Bull. 191, pp. 32- 52.

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