Warbling Vireo Nesting Ecology in the Northern Sierra Nevada

WARBLING VIREO NESTING ECOLOGY IN THE NORTHERN SIERRA NEVADA

JULIA I. SMITH, Department of BiologicalSciences, Holy Names College, 3500 Mountain Blvd., Oakland, California94619 MARK D. REYNOLDS, The NatureConservancy, 201 MissionStreet, 4th Floor,San Francisco,California 94105 GRETCHEN LeBUHN, Department of Biology, San FranciscoState University, 1600 HollowayAve., San Francisco,California 94132

ABSTRACT: In California,for unknownreasons, the WarblingVireo (Vireogilvus swainsonii)has poor reproductivesuccess, and its numbershave declined over the past 20 years. From June throughAugust 1998 we monitored70 nestsof the WarblingVireo in a previouslyunstudied population on the easternslope of the northernSierra Nevada. Nests were generallyplaced 7 m or higherin maturetrees, situatedat over50% of the nest-plant'sheight, and mostoften in the peripheryof the nest-plant'sfoliage. The two most commonnest-plant species were the Quaking Aspen(Populus trernuloides, n -- 30) andLodgepole Pine (Pinus contorta, n = 30). Such heavy relianceon a conifer by the WarblingVireo has not been reported previously.Nest successwas not significanfiyassociated with habitat or nest-site characteristics;thus successful sites did not differdramatically from unsuccessfulsites. Mostegg dates were concentrated within a singleweek (2-9 July),and mostof the successfulnests were in the egg-incubationphase during the firsttwo weeksof July. Eighteennests fledged at leastone Warbling Vireo; one nest fledged a Brown-headed Cowbird (Molothrus ater). Seventy-fourpercent of nestingattempts failed; most failednests showed signs of avianrather than mammalian predation. We recommend that vireo conservationefforts in the northernSierra Nevadafocus on increasingthe availabilityof suitablenest sitesby promotingmature standsof aspensand pines offeringwell-concealed nest sitesin the peripheryof the foliage,limiting forest disturbancein Julyduring the criticalnesting period, and minimizingenvironmental modificationsthat favor avian nest predatorssuch as Steller's Jay (Cyanocitta stelleri).

Severalspecies of North Americanvireo have endured drastic population declinesor regionalextirpation in recent times (e.g., Bell's, Vireo bellii, Black-capped,V. atricapillus, and Gray, V. vicinor, Gardaliet al. 2000). Despiteconsiderable research attention focused on endangeredvireos, the basicnatural history and populationtrends of the muchmore widespread WarblingVireo (V. gilvus)are not well known (Gardaliand Ballard2000, Ward and Smith 2000). Various lines of evidencesuggest that Warbling Vireo(V. g. swainsonii)populations are declining in California(Gardali et al. 2000, Gardaliand Jaramillo 2001), despitemoderately positive population trendsin North Americaas a whole(+1.2% per year,P < 0.05; Saueret al. 2001). BreedingBird Surveysimply that Californiapopulations of the WarblingVireo have declinedannually by 1.0% (P < 0.04, Sauer et al. 2001). In addition,at Palomarin,Marin County,autumnal capture rates of the WarblingVireo have decreasedover the past two decadesby 9% per year (Ballardet al. 2003). Furthermore,in the last century,the Warbling Vireo has been largelyextirpated from the SacramentoValley and San Diego County (Gaines 1974, Unitt 1984, Gardali and Ballard 2000).

32 WesternBirds 35:32-41, 2004 WARBLING VIREO NESTING ECOLOGY IN NORTHERN SIERRA NEVADA

Demographicstudies suggest that low reproductivesuccess rather than low adultsurvivorship is the mostimportant contributor to the decline(Gardali et al. 2000, Gardali and Jaramillo2001). Becausethe species'basic nesting ecologyhas been characterized for onlya few populationsin westernNorth America(Gardali and Ballard2000), the ultimatecauses of thislow produc- tivityremain unknown. For conservationplans for decliningneotropical migrants such as the WarblingVireo to be effective,the stageor stagesin the annualcycle that limit populationgrowth, as well as habitatfeatures that directlyinfluence reproductionand survival,must be identified(Martin 1992, Gardali et al. 2000). Towardthat end, we analyzedin detailthe breedingcycle and nest- site characteristicsof a previouslyunstudied population of WarblingVireos nestingin broadleafand mixedconiferous forests on the easternslope of the northern Sierra Nevada. Avian habitat relationshipsin the northern Sierra Nevadaare poorlyunderstood in comparisonto thosein the southernSierra Nevada and Sierra foothills,for which there is extensiveliterature, e.g., Vernerand Boss(1980) and numerousreferences therein. Thus identifying keyenvironmental characteristics for vireosin thisarea will not onlydeepen our understandingof avian habitatassociations in westernNorth America but will alsoadvance the developmentof land managementappropriate to benefitthe WarblingVireo in California.

METHODS

We studiedWarbling Vireo breedingecology from June through August of 1998 in three riparian areas on the easternslope of the Sierra Nevada in northern Californiain the Tahoe National Forest:Sagehen Creek (7 km northwestof Hobart Mills in Nevada County at 39 ø 25.852' N, 120 ø 14.481' W, elevation1937 m), DaviesCreek (12 km north of Hobart Mills in Sierra Countyat 39 ø 30.699' N, 120 ø 9.632' W, elevation1900 m), and KlondikeMeadow along East Martis Creek (17 km southeastof Hobart Mills in PlacerCounty at 39 ø 18.357' N, 120 ø 03.206' W, elevation2120 m). Eachstudy area consistedof Sierranmontane riparian and meadowhabitats with standsof willow(Salix sp.) and occasionallyalder (Alnus sp.) bordered by conifers,primarily Lodgepole Pine (Pinuscontorta), and broadleaftrees, primarilyQuaking Aspen (Popu lus tremu Ioides) and FremontCottonwood (P.fremontii). We locatedWarbling Vireo nestsby followingadult vireos(Martin and Geupel1993). We monitoredthe progressof eachnest at leastonce a week and no more than twice a week. To identifycritical periods in the nesting cycle,we recordeddates on whichpairs were observedbuilding nests, laying eggs, incubatingeggs, or brooding nestlings.For unsuccessfulnesting attempts,we notedthe stageat which failureoccurred (i.e., abandonment while buildingor laying, egg incubation,or nestlingperiod). Failed nesting attemptswere thosein which the adult pair, the nest, or all of its contents disappearedbefore fledging was possible.Whenever possible, we classified failednests to the mostlikely type of predatoras outlinedby Morton et al. (1993). We identifiedthe nest predator as "avian" if the nest contents

33 WARBLING VIREO NESTING ECOLOGY IN NORTHERN SIERRA NEVADA disappearedbefore fiedgingbut the nest cup appearedundisturbed, and "mammalian"if the nestwas knocked down or the nestcup lostits integrity. Clearly, there are limitationsto this indirectmethod of identifyingnest predators(Larivi•re 1999, Thompsonand Burhans2003). We examineda suiteof nest-sitecharacteristics, many of whichare likely to be affectedby proposedmanagement to restorelate-seral forest structure and regenerateaspen stands throughout the Sierra Nevada(SNFPADSEIS 2003). For each nest, using protocols from Ralph et al. (1993), we determinedthe plantspecies containing the nest,height of the nest,height of nest plant, nest-plantdiameter at breastheight (dbh),nest orientation measuredas the compassdirection from the main stemto the nest,number of branchessupporting the nest,average diameter of the branchessupporting the nest, nest distancefrom central stem, nest distancefrom the periphew of the plant, nest concealmentmeasured in each of the four cardinaldirections from the nest(0-25% concealmentwas ranked "1," 26- 50% concealmentwas ranked "2," 51-75% concealmentwas ranked "3," and 76-100% concealmentwas ranked "4"), andcanopy cover measured in eachof the four cardinaldirections from the nest(percent canopy cover was ranked in a fashionsimilar to nest concealment).We tabulatedsummary scoresfor both nestconcealment and canopycover by averagingthe scores in each of the four cardinaldirections. To enhancethe reliabilityof all measures,early in the fieldseason we trainedthe study'sparticipants until they achieveda high degreeof repeatability. To characterizegeneral habitat associations for WarblingVireos nesting in the northernSierra Nevada,we investigateddifferences in nest-sitecharac- teristicsamong nest-plantspecies. Nest-site data were first tested for normalityand then analyzedwith a one-wayanalysis of variance(ANOVA, Sokal and Rohlf 1995). A Bonferroni correctionwas used to maintain an alpha level of 0.05 for the overallanalysis (Lehner 1996). The variables "averagediameter of the branchessupporting the nest"and "nestdistance from the periphew of the plant" were log-transformedfor normality.The "number of branchessupporting the nest" and the "nest canopy cover summary score" did not conform to a normal distributioneven after transformationand were thereforeanalyzed with a Kruskal-Wallisone-way analysisof variance(Lehner 1996). To highlighthabitat characteristicscritical to vireo productivityin the northern Sierra Nevada,we determinedpercent nest successby studysite and nest-plantspecies. We then analyzedthe likelihoodof nestsuccess using logisticregression. Independent variables included study site (Sagehen Creek, Davies Creek, and Klondike Meadow), nest-plantspecies, nest height,and withina nest-plantspecies the plant'sheight and diameter.The dependentvariable "nest success" (Kus 2002) was codedas "0" for pairs failingto fledgeyoung or fiedginga Brown-headedCowbird (Molothrus ater), and "1" for pairs where we either observedfiedging, observed adults feedingfledglings in the natal territow, or observedfledglings within the natalterritow accompaniedclosely by adults.Significance was accepted at P < 0.05.

34 WARBLING VIREO NESTING ECOLOGY IN NORTHERN SIERRA NEVADA

RESULTS

We found WarblingVireos nestingin QuakingAspen, LodgepolePine, Fremont Cottonwood,and willow;the two most common nest plantswere the aspenand pine (Table1). Nest heightsranged from 1.2 m (in a willow) to 19.0 m (in a LodgepolePine), reflectingthat the plantsin which these nestswere placedranged in height from 1.8 m (a willow)to 28.0 m (a LodgepolePine). Among the four nest-plantspecies nest-site characteristics were very similar.Five of the nine habitatcharacteristics compared did not differsignificanfiy (Tables 1-3). Most nestsfound (68%, 46 of 68) were in maturetrees (10 to 17 m tall) and positionedin the peripheryof the nest- plant'sfoliage (Tables 1 and 3); 51% (33 of 64) were placedat leastthree timesas far fromthe mainstem as they were from the peripheryof the plant. Furthermore,the distanceof the nestfrom the peripherywas on average lessthan 1 m and invariantamong the plant species(Table 3). In general,the nestswere well hidden by foliage: 63% (41 of 65) had more than 50% concealmentaround the entirenest, and 74% (40 of 54) had more than 50% canopycover. Most nests(81%, 50 of 62) were situatedin a fork between two branchesof relativelysmall diameter (0.20-2.50 cm); 19% of nests(12 of 62, fivein QuakingAspen and sevenin LodgepolePine) were supported by three branches. Four of the nest-sitecharacteristics differed significantly by nest-plant species(Tables 1-3). For all but one of these nest-sitecharacteristics, nests in willows(a shrub)varied the mostfrom the generalpattern. The "average diameterof branchessupporting the nest"was typically small (< 2 cm), but for nestsin willowsit was especiallysmall (Table 2). The generalpattern for the variables"nest height" and "distancefrom the main stem to the nest" was that the tallerthe nest plant the higherthe nestwas placedin it and the fartherit wasfrom the main stem.Nests in willowsalso showed this pattern, but the distanceswere disproportionatelysmall in comparisonto those in othernest plants (Tables 1 and 3). The fourthdiffering nest-site characteristic was "nestheight as a percentageof tree height."In thiscase, the Lodgepole

Heightsof WarblingVireo Nestsby Type of Nest Plant•

Numberof Nest Plant NestHeight NestPlant Nests Height(m) Height(m) (% of TreeHeight)

Aspen 30 7.0 + 3.3 10.9 + 5.0 65.1 + 17.0 Cottonwood 4 6.4 + 4.0 10.6 + 5.8 57.9 + 12.4 LodgepolePine 30 8.2 + 4.2 17.6 + 5.6 46.6 + 19.0 Willow 6 2.0 + 0.6 2.7 + 0.6 70.2 + 9.8 All nests 70 7.0 + 4.0 b 13.0 + 5.0 57.2 + 19.4 b

aMean + standard deviation. bp_< 0.05 for single-factorANOVA examining differences in nest-sitecharacteristics among tree and shrub species.(No statisticalanalysis was done on number of nests, and becauseof pronounceddifferences in growthform among plant species no analysiswas performed on plant height).

35 WARBLING VIREO NESTING ECOLOGY IN NORTHERN SIERRA NEVADA

Talkie 2 Nest-PlantDiameters, Nest Orientations, and Numbersof Supporting Branchesof WarblingVireo Nestsby Type of NestPlant a

AverageDiameter of Branches Nest Numberof Supporting NestPlant Diameter(cm) b Orientation(o) SupportingBranches Nest(cm)

Aspen 28.7 + 15.6 163.0 + 80.5 2.2 + 0.4 1.0 + 0.6 Cottonwood 38.0 + 24.8 156.8 + 128.5 2.0 + 0 1.1 + 0.5 LodgepolePine 49.3 + 21.4 227.9 + 104.6 2.3 + 0.4 1.5 + 0.6 Willow -- 244.0 + 128.9 2.0 + 0.0 0.3 + 0.1 All nests 38.2 + 21.6 197.3 + 102.0 2.2 + 0.4 1.1 + 0.7 c

aMean + standard deviation. bOftrunk at breastheight (dbh). cp _<0.05 for single-factorANOVA or Kruskal-Wallistest examining differences in nest-sitecharacteristics amongtree and shrubspecies. (Because of pronounceddifferences in growthform amongplant speciesno statisticalanalysis was performedon plant diameter).

Pinewas the onlynest-plant species in whichnests were situated on average at lessthan 50% of the heightof the nestplant (Table1). Nest successwas low; 74% (52 of 70) of nestingattempts failed to producefledglings. Of the neststhat failed, 12% (n -- 6) were abandoned duringbuilding/laying, 44% (n -- 23) failedduring incubation, 42% (n -- 22) failedduring the nestlingstage, and one (2%)fledged only a Brown-headed Cowbird.Ten of the failednests were positionedlow enoughthat we could see the contentsof the entire nest. Of those ten, three were abandoned duringincubation, six showed signs of avianpredation, and one showedsign of mammalianpredation. At an additionaleleven nests we were ableto see the headsof nestlingsabove the nest'srim: eight showedsigns of avian predationand three showedsigns of mammalianpredation. Because the contentsof thesenests were not completelyvisible we cannotrule out the possibilitythat some were abandonedand not depredated.However, we

Positionand Concealmentof WarblingVireo Nestsby Type of NestPlant a

Distancefrom Main Distanceof Nest NestConcealment NestCanopy Cover NestPlant Stemto Nest(m) fromPeriphery (m) SummaryScore SummaryScore

Aspen 1.5 + 1.0 0.8 + 0.6 3.31 + 0.6 3.2 + 0.9 Cottonwood 1.0 + 0.8 0.8 + 0.2 3.94 + 0.1 3.92 + 0.1 LodgepolePine 2.1 + 0.8 0.6 + 0.5 2.94 + 0.7 3.21 + 0.7 Willow 0.06 + 0.03 1.1 + 1.2 3.45 + 0.6 NA b All nests 1.6 + 1.0 c 0.7 + 0.6 3.2 + 0.7 3.2 + 0.8 aMean + standard deviation. •NA, not applicable. cP _<0.05 for singlefactor ANOVA or Kruskal-Wallistest examiningdifferences in nest-sitecharacteristics among tree and shrubspecies.

36 WARBLING VIREO NESTING ECOLOGY IN NORTHERN SIERRA NEVADA haveno evidenceof abandonmentduring brooding from those nests whose contentswere visible.Combining these two groupsof neststogether (n -- 21) yieldsan estimateof 67% (n -- 14) failuredue to avianpredation, 19% (n -- 4) failuredue to mammalianpredation, and 14% (n = 3) failuredue to abandonment. SuccessfulWarbling Vireo nests(26%, 18 of 70) were not associated stronglywith specifichabitat characteristics. The likelihoodof an attempt's beingsuccessful did not vary significantlyby studysite (log-likelihoodof study-sitemodel •2 __3.9, P -- 0.14): 35% successful(13 of 37) at Sagehen Creek, 17% successful(4 of 24) at DaviesCreek, and 11% successful(1 of 9) at KlondikeMeadow. The likelihoodof a successfulnesting attempt was alsonot significantlyaffected by nest-plantspecies (log-likelihood of nest- plantspecies model •2 = 3.0, P -- 0.40): 27% (8 of 30) successfulin Quaking Aspen,30% (9 of 30) successfulin LodgepolePine, 17% (1 of 6) successful in willow,and zero(of 4) successfulin FremontCottonwood. Neither was the likelihoodof a successfulattempt significantly affected by nestheight (log- likelihoodof nestheight model •2 = 2.5, P = 0.11), nest-plantheight (log- likelihoodof nest-plant-heightmodel •2 -- 3.2, P -- 0.08), or nest-plant diameter(log-likelihood of nest-plant-diametermodel • -- 1.0, P -- 0.31). Inclusionof studysite and nest-plantspecies (i.e., performingthe logistic regressionusing both study site and nest-plantspecies as independent variables)did not improvethe model (log-likelihoodof study-siteand nest- plant-speciesmodel • -- 6.9, P -- 0.23), nordid including nest-plant height and nest-plantdiameter (log-likelihood of nest-plant-heightand nest-plant- diametermodel • -- 1.7, P -- 0.43). The nestsurveys allowed us to detailthe WarblingVireo's nesting cycle on the easternslope of the northern Sierra Nevada.The peak period for incubationwas the firsttwo weeksin July;46 of the nestswe studiedwere discoveredduring egg incubation,and 56% (n -- 26) of thosewere found duringthe first two weeksin July. Sixteenof the successfulnests were monitoredduring incubation,and of those 75% (n -- 12) were being incubatedduring the firsttwo weeksin July.Fifty-three percent of eggdates (26 of 49 nestsdiscovered with at leastone egg)ranged from 2 to 9 July. The firstWarbling Vireo nestwas found 19 Junein the egg-incubationstage at DaviesCreek; the lastwas discovered on 10 August,also at DaviesCreek. When discovered,the last nestcontained one WarblingVireo egg and one Brown-headedCowbird nestling. Thus, the eggsfor thislast nesting attempt were probablylaid near the end of July. One pair nestingin a willowat SagehenCreek incubatedtwo apparentlyinfertile eggs for at least25 days. Fourof the 70 nests(6%) were renesting attempts; we observedrenesting only at Davies Creek. Only pairs whose first nestingeffort failed during incubationand earlyin the breedingseason (between 19 Juneand 24 June) attemptedto renest.One femalewas observeddismantling the firstunsuc- cessfulnest and usingthose pieces to builda secondnest. Another female builta nestcomposed of a greatdeal of whitematerial, and afterthis nest failed she then built a second white nest that also failed. We did not observe any doublebrooding. Our studypopulation was not color-banded,so it is possiblethat we failedto detectsome renesting attempts and that somepairs couldhave dispersed and renestedoutside of the studyarea.

37 WARBLING VIREO NESTING ECOLOGY IN NORTHERN SIERRA NEVADA

DISCUSSION

On the easternslope of the northern Sierra NevadaWarbling Vireos nestedpredominantly in Quaking Aspen and LodgepolePine; other plant speciesless commonly used were FremontCottonwood and willow.Despite differencesin growthform betweentrees and shrubs, in all of the nest-plant speciesnest-site characteristics were very similar (Tables1-3). Warbling Vireos generallynested 7 m or higher in maturetrees, placingtheir nestsat over 50% of the nest-plant'sheight and mostoften in the periphewof the nest-plant'sfoliage. This preferencefor nestingin the periphewof the tree is especiallyevident in nestsin LodgepolePine beingsituated at lessthan 50% of the tree'sheight (Table 1). Becauseof the pyramidalshape of this conifer,the branchesbelow the midpointextend farther from the mainstem and only nests on those branchesmay be positionedin the outermost periphew of the plant. Althoughthe characteristicsof the neststhat we studiedconform to the generalpattern reported for the species(Gardali and Ballard2000), we did discovertwo differences.Warbling Vireos in the northernSierra Nevada nestregularly in LodgepolePine (n -- 30, 43%), in contrastto the preference for deciduoustrees describedin the literature (Gardali and Ballard 2000). Studies of the vireo's nest-siteselection in southern Ontario, Arizona, coastalCalifornia, and the foothillsof the OwensValley of easternCalifor- nia, reportedonly a singlenest (0.6%, n -- 162) in a conifer(Ponderosa Pine, P. ponderosa;reviewed by Gardaliand Ballard2000). Also, in contrastto the generalpattern describedin the literature,19% of the WarblingVireo nestswe foundwere supportedby three branchesrather than two. The percent nest successwe observedwas low (26%) in comparisonto other high-elevationpopulations (62% successin Arizona, Martin and Li 1992). In contrastto otherwestern populations (Ward and Smith 2000), this poor successwas apparently not the resultof pressurefrom broodparasites. Of the 19 neststhat fledgedoffspring only a singlenest fledged a cowbird. Similarly,the analysisof nest successillustrated that successfulWarbling Vireo nest sites do not differ dramaticallyfrom unsuccessfulsites; the likelihoodof a successfulnesting attempt was not significantlyaffected by studysite, nest-plantspecies, nest height, or within a nest-plantspecies by nest-plantheight or diameter.This lack of differentiationmay signifythat less obvioushabitat featuresplay an important role in determiningthe WarblingVireo's nestsuccess. The nestsite's microclimate (a subtlefeature of nestinghabitat) may be an important factor (Smith, Reynolds,and LeBuhnunpubl. data). The lackof differentiationalso suggests that preserv- ing and enhancingmature aspensand pineswith well-concealedsites in the periphew of the foliage (i.e., those featuresof the habitat most often associatedwith vireo nests)might benefit vireo productivitysimply by increasingthe availabilityof suitablesites. Eggdates on the easternslope of the northernSierra Nevada range from 19 Juneto the end of July.Previous estimates of WarblingVireo egg dates for California range from 26 April through 25 July (Gardaliand Ballard 2000). Altitudinaleffects on local climatic conditionsmay explain the delayedinitiation of egglaying in the northernSierra Nevada. The majority

38 WARBLING VIREO NESTING ECOLOGY IN NORTHERN SIERRA NEVADA of egg datesin the northernSierra Nevada fell within a singleweek of July (2 July through9 July). This contrastsdramatically with other western populations,in whichthe majorityof egg dateshave been reportedto span over two or three weeks (Gardali and Ballard 2000). Most (75%) of our successfulnests were in the egg-incubationphase during the firsttwo weeks in July;this was also the peakegg-incubation period for the populationas a whole. On the basisof an incubationperiod of 12 or 13 days(Gardali and Ballard2000), the peaknestling phase falls during the lasttwo weeksof July. In terms of offspringmortality there is little differencein risk between incubation(44% failed)and the nestlingstage (42% failed).Because reproductivesuccess is thought to be the primary factor limitingthe vireo's populationgrowth in California(Gardali et al. 2000), minimizingactivities that disturbnesting during the monthof Julyshould be a focalconsideration in the developmentof managementplans in the northern Sierra Nevada. We observedno doublebrooding, which Tewksburyet al. (1998) consid- ered common among Warbling Vireos nesting in Montana. In addition, unlikeWarbling Vireos near Point Reyes(Gardali and Ballard2000), very few pairswe studiedattempted to renestafter a failure.A highlysynchro- nized pulseof egg layingcombined with a lack of doublebrooding and renestingargues that the breedingseason for WarblingVireos at high elevationin Californiais shorterthan at lower elevations,making the birds more sensitive to disturbance. Far morefailed nests apparently suffered predation by birds(67%) than by mammals(19%). Among the avian nest predatorsoccurring in the study area, suchas Clark'sNutcracker (Nucifraga columbiana), American Crow (Corvusbrachyrhynchos), and Common Raven (C. corax), Steller'sJay (Cyanocittastelleri) is by far the mostnumerous and widespread (Reynolds and Smith unpubl.data). Steller's Jays are knownto exert tremendousnest- predation pressureon passerinesand neotropicalmigrants in particular (Sievingand Willson1999). We did not observeSteller's Jays preying on WarblingVireo nestsbut we did observevireos scolding jays near nests,a reactionto a nest predator(Gardali and Ballard2000). Throughoutits rangethe WarblingVireo prefersopen parkland,forest edge, and forestopenings (James 1976, Gardaliand Ballard2000, Ward and Smith2000). Open and parklikeconiferous and riparian habitats in the SierraNevada have declined significantly over the last 150 yearsas a result of fire suppressionand the encroachmentof smallertrees (SNEP 1996). In addition,photographic analysis has revealedthat all Sierranriparian corri- dors have been interruptedby human factors like roads, railroads,and grazing (SNEP 1996). Jay populationsincrease dramatically near man- modifiedenvironments (Sieving and Willson1999), and Steller'sJays have increasedsignificantly in Californiaduring the sameperiod in whichWar- bling Vireos have declined(Gardali et al. 2000). Human alterationof the landscapeof Californiamay have contributed to the declinein the Warbling Vireo by limitingsuitable breeding habitat and increasing populations of nest predators. Our findingssuggest that maturestands of QuakingAspen and Lodgepole Pineare importantfeatures of WarblingVireo nestinghabitat on the eastern slope of the northern Sierra Nevada. In addition,our data highlightthe

39 WARBLING VIREO NESTING ECOLOGY IN NORTHERN SIERRA NEVADA restrictedand synchronousnature of the WarblingVireo's breeding season at high elevationsand thusthe importanceof reducingforest disturbance duringJuly, the criticalnesting period. Our resultsalso argue that the vireo's low reproductivesuccess in the northernSierra Nevadais currentlydeter- mined more by nest predationthan by habitator brood parasites.Thus managementpractices that promote avian nest predatorsshould be mini- mized. Future studiesshould focus on the relationshipbetween proposed habitatchanges in the SierraNevada (SNFPADSEIS 2003) andthe feedback on the populationdynamics of nestpredators and the reproductivesuccess of WarblingVireos.

ACKNOWLEDGMENTS

Thiswork was supported by a grantfrom the Partnersin FlightProgram of the U.S. ForestService. The Universityof California,Santa Barbara's Natural Reserve System, providedadditional support. We thank the staff of the Universityof California's SagehenCreek Field Station for assistancein the field and accessto the station's resources.Genny Wilson, Lydia Mann, and Barbara LiSanti providedtechnical support.We thank Scott Carroll, Tom Gardali, Wait Koenig, Peter Karieva,Jenella Loye, and one anonymousreviewer who providedhelpful comments on an earlier draft of the manuscript.

LITERATURE CITED

Ballard,G., Geupel,G. R., Nur, N., and Gardali,T. 2003. Long-termdeclines and decadalpatterns in populationtrends of songbirdsin westernNorth America 1979-1999. Condor 105:737-755. Gardali,T., andBallard, G. 2000. WarblingVireo (Vireogilvus), in The Birdsof North America(A. Pooleand E Gill, eds.),no. 551. BirdsN. Am., Philadelphia. Gardali,T., and Jaramillo,A. 2001. Furtherevidence for a populationdecline in the westernWarbling Vireo. W. Birds32:173-176. Gardali,T., Ballard,G., Nur, N., and Geupel,G. R. 2000. Demographyof a declining populationof WarblingVireos in coastalCalifornia. Condor 102:601-609. Gaines,D. F. 1974. A new look at the riparianavifauna of the SacramentoValley, California. W. Birds 5:61-80. James,R. D. 1976. Foragingbehavior and habitatselection of threespecies of vireos in southern Ontario. Wilson Bull. 88:62-75. Kus, B. E. 2002. Fitnessconsequences of nestdesertion in an endangeredhost, the Least Bell's Vireo. Condor 104:795-802. Larivi•re, S. 1999. Reasonswhy predatorscannot be inferred from nest remains. Condor 101:718-721. Lehner,P. N. 1996. Handbookof EthologicalMethods, 2nd ed. CambridgeUniv. Press,Cambridge, England. Martin, T. E. 1992. Breedingproductivity considerations: What are the appropriate habitatfeatures for management?,in Ecologyand Conservationof Neotropical Migrant Landbirds(J. M. Hagan, III, and D. W. Johnston,eds.), pp. 455-471. SmithsonianInst. Press,Washington, DC. Martin, T. E., and Geupel,G. R. 1993. Nest-monitoringplots: Methods for locating nestsand monitoringsuccess. J. Field Ornithol. 64:507-519.

4O WARBLING VIREO NESTING ECOLOGY IN NORTHERN SIERRA NEVADA

Martin, T. E., and Li, P. 1992. Life-historytraits of open- vs. cavity-nestingbirds. Ecology73:579-592. Morton, M. L., Sockman,K. W., and Peterson,L. E. 1993. Nest predationin the mountainWhite-crowned Sparrow. Condor 95:72-82. Ralph, C. J., Geupel, G. R., Pyle, P., Martin, T. E., and DeSante, D. F. 1993. Handbookof field methodsfor monitoringlandbirds. U.S. ForestServ. Gen. Tech.Rep. PSW-GTR-144.Pac. SouthwestRes. Sta., P.O. Box 245, Berkeley, CA 94701. Sauer,J. R., Hines,J. E., and Fallon,J. 2001. The North Americanbreeding bird survey,results and analysis1966-2000, version2001.2. U.S. Geol. Survey, PatuxentWild•ife Res. Center, Laurel, MD. SierraNevada Ecosystem Project (SNEP). 1996. Finalreport to Congress:Status of the SierraNevada. Volume II: Assessmentsand ScientificBasis for Management Options.Wildland Resources Center Rep. 37. Centersfor Waterand Wfidland Resources,Univ. of Calif., Davis. SierraNevada Forest Plan AmendmentDraft SupplementalEnvironmental Impact Statement(SNFPADSEIS). 2003. U.S. Dept. Agric.Forest Serv. Pac. Southwest Reg. Publ.R5-MB-019. Sieving,K. E., andWillson, M. E 1999. A temporalshift in Steller'sJay predation on birdeggs. Can. J. Zool. 77:1829-1834. Sokal,R. R., andRohlf, E J. 1995. Biometry,3rd ed. W. H. Freeman,San Francisco. Tewksbury,J. J., Hejl, S. J., and Martin,T. E. 1998. Breedingproductivity does not declinewith increasingfragmentation in a westernlandscape. Ecology 79:2890- 2903. Thompson,F. R., and Burhans,D. E. 2003. Predationof songbirdnests differs by predatorand betweenfield and foresthabitats. J. WildlifeMgmt. 67:408-416. Unitt,P. 1984. The birdsof SanDiego County. San Diego Soc. Nat. Hist. Mem. 13. Verner,J., and Boss,A. S. 1980. Californiawildlife and their habitats:Western Sierra Nevada.U.S. ForestServ. Gen. Tech. Rep. PSW-37. Pac. SouthwestForest and RangeExp. Sta., P.O. Box 245, Berkeley,CA 94701. Ward, D., and Smith,J. N.M. 2000. Brown-headedCowbird parasitism results in a sinkpopulation in WarblingVireos. Auk 117:337-345.

Accepted 13 November 2003