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Factors Affecting Nesting Success of Wood Thrushes in Great Smoky Mountains National Park

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Factors Affecting Nesting Success of Wood Thrushes in Great Smoky Mountains National Park The Auk 116(4):1075-1082, 1999 FACTORS AFFECTING NESTING SUCCESS OF WOOD THRUSHES IN GREAT SMOKY MOUNTAINS NATIONAL PARK GEORGE L. FARNSWORTH • AND THEODORE R. SIMONS 2 CooperativeFish and Wildlife Research Unit, Department of Zoology,North Carolina State University, Raleigh, North Carolina 27695, USA ABSTRACT.--Recentevidence suggests that the nestingsuccess of forest-interiorNeotrop- ical migrantsis lower in fragmentedhabitat. We examinedthe nestingsuccess of Wood Thrushes(Hylocichla mustelina) in a largecontiguous forest from 1993 to 1997.From a sample of 416nests we testedfor predictorsof daily nestsurvival rates, including activity at the nest andvegetation parameters at the nestsite. We tested whether disturbance during nest checks (asmeasured by the behaviorof the adults)was relatedto subsequentnest predation. Fe- maleswere more likely to vocalizewhen brooding chicks than when incubating eggs. How- ever,we found no evidencethat observerdisturbance or WoodThrush activity influenced daily nestsurvival rates. Wood Thrushes nested predominately in smallhemlocks, generally surroundedby many other small hemlocks.However, survival ratesof nestsin hemlocks were not significantlydifferent from thosein othersubstrates. Overall, neither activity at the nestnor habitatin the vicinityof the nestwas a goodpredictor of nestingsuccess, and only onevegetation characteristic, a measure of concealment,was significantly correlated with successfulnesting. Brood parasitism by Brown-headedCowbirds (Molothrus ater) was extremelylow (<2% of nestsparasitized). However, nesting success was moderate(daily survivalrate = 0.958)when comparedwith otherpublished studies from more-fragmented landscapes.Our resultssuggest that daily nestsurvival rates do not increasemonotonically from smallto very largeforest patches. Received 31 August1998, accepted 22 March1999. GLOBALDECLINES in Wood Thrush (Hylocich- Midwest probably is insufficient to maintain la mustelina)populations are evident from an- the breedingpopulations in many fragments. alysesof BreedingBird Surveydata (Robbins et Theirfindings suggest that populations of Neo- al. 1989,Peterjohn et al. 1995).One possible tropicalmigrants exhibit source-sink popula- causeof this declineis low nestingsuccess in tion structure (sensuPullJam 1988) and that isolated forest remnants (Hoover et al. 1995, largecontiguous tracts of forests,such as that Robinsonet al. 1995b).In a landmark study, within GreatSmoky Mountains National Park, Wilcove (1985) identified an inverse relation- are important in sustainingregional popula- shipbetween forest patch size and rates of pre- tions. dationon artificialnests. In that study,preda- Nestparasitism by Brown-headedCowbirds tion ratesreported for GreatSmoky Mountains (Molothrusater) is anotherthreat that could in- National Park were the lowest recorded across fluencethe sustainabilityof WoodThrush pop- a spectrumof sites that ranged from small ulations.Populations of Brown-headedCow- fragmentsof suburbanforest to large contig- birds haveincreased markedly in this century uous tracts of forest. Hoover et al. (1995) also (Terborgh1989, Robinson et al. 1995a).Studies found a positive relationshipbetween forest in forest patchesin the Midwest have found patchsize and WoodThrush nesting success. thatrates of parasitismare negatively correlat- Robinsonet al. (1995b)concluded that the nest- ed with the degreeto whichthe surrounding ing successof Neotropicalmigrants (including landscape is forested (Donovan et al. 1995, the WoodThrush) in forestfragments in the Robinsonet al. 1995b).Even within the largest remainingforest tracts in Illinois (1,500to 3,000 ha), Trine (1998)found high ratesof nestpar- • Present address:Department of Biology and Health Sciences,Meredith College,3800 Hillsbor- asitismand a high incidenceof multiplepara- ough Street,Raleigh, North Carolina27607, USA. sitism on Wood Thrush nests. In some cases, E-mail: [email protected] however, Wood Thrush nestsin small, isolated 2 Addresscorrespondence to this author E-mail: forestpatches experience consistently low rates [email protected] of cowbirdparasitism (Roth and Johnson 1993). 1075 1076 FARNSWORTHAND SIMONS [Auk, Vol. 116 In fact,in a Maryland woodlot,Link and Hahn discoveredby predators.In a reviewof 36 stud- (1996) consideredthe Wood Thrush to be the ies from a number of bird taxa, Martin (1992) host speciesleast likely to be parasitizedby found29 studiesreporting an inverserelation- cowbirds. ship between nest concealmentand rates of Becauseof the large size of Great Smoky nestpredation. Johnson (1997) found a signifi- Mountains National Park (202,000 ha) and its cant linear relationshipbetween an index of location in the predominatelyforested land- concealment and the number of Wood Thrush scape of the southernAppalachians, we ex- fledglingsproduced in Delaware,but this was pected to find high rates of survival and very only true in one year of a two-year study. We low ratesof broodparasitism for WoodThrush measuredvegetation at nests and at adjacent nests. We also investigatedadditional factors sites to characterize Wood Thrush nest sites that may influencenest survival. Specifically, and to determine the extent to which features we examined (1) whether Wood Thrush activ- of the vegetationexplained variations in nest- ity at the nest,both observer-inducedand nat- ing successin the park. ural, increased the chance that a nest was dis- coveredby a predator; and (2) whether the STUDY AREA AND METHODS characteristicsof the vegetationsurrounding the nestwere associatedwith nestingsuccess. Study area.--Great Smoky Mountains National Park straddles the border of North Carolina and Ten- Disturbancecaused by investigatorsvisiting nesseeat an elevationalrange of 300 to 2,020m. Since nestsmay increasethe chancethat nestsare its establishmentas a nationalpark in 1934,all log- discoveredby predators (Martin and Roper ging hasbeen prohibited and forestfires havebeen 1988). We used the behavior of the adults dur- controlled,creating one of the largest contiguous ing nestchecks as an index of disturbanceand tracts of forest in the eastern United States. We mon- examinedthe relationshipbetween the amount itored 416 Wood Thrush nests on the Tennessee side of disturbanceand predation.We also exam- of the park from 1993to 1997.Sites ranged from an ined whether natural levelsof activity at the old-growth hardwood forest near the upper eleva- nest affectednest survival. Skutch(1949) hy- tional limit of Wood Thrushesin the southernAp- pothesizedthat the activity of adults at a nest palachians(1,350 m), to a second-growthsite on the may make nestsmore obviousto predators.He park boundary(300 m) near the city of Gatlinburg. Nestmonitoring and nesting success.--We monitored proposeda positiverelationship between nest nestsevery three daysuntil they failed or the chicks predation rates and clutch size, a hypothesis fledged.Nest visits were brief (<1 min) to minimize that hasbeen tested with severalspecies. Larg- disturbance,and when possible,we observednests er clutchessuffered higher rates of nest pre- from a distance. Wood Thrush chicks normally dationin studiesof Great Tits (Parusmajor; Per- fledge at about 12 days (Roth et al. 1996),but they rins 1965) and Black-billedMagpies (Picapica; will leavethe nest asearly as 10 days if disturbed.To Redondoand Castro1992). Studies of the Least preventpremature fledging, we did not visit nestsif Flycatcher (Empidonaxminimus; Briskie and chickswere older than 10 days, and we considered Sealy 1989) and Western Slaty Antshrike chicksto have fledged if they survived at least 10 daysafter hatching. (Thamnophilusatrinucha; Roper and Goldstein We calculateddaily nest survival based on May- 1997),however, failed to showthis pattern.By field's (1975) method and calculatedstandard errors comparingpredation rates at natural and arti- and performedz-tests following Johnson (1979). We ficial nests,we testedthe followinghypotheses tested for nest-treepreferences using a G-test to relatedto naturalactivity at the nest:(1) thrush comparethe ratio of nestsin eachof the two most nests fail more often than nearby artificial frequentlyused tree specieswith the ratio of each nests,(2) nestswith chickshave lower survival speciesmeasured at non-nestsites. We alsoused lo- rates than nestswith eggs,and (3) nestswith gisticregression (SAS 1995, 1996)with a backward larger clutchesare more likely to fail than nests selectionprocedure and a 0.25 significancelevel cri- with smaller clutches. terion to test for the effectsof multiple factorson nestingsuccess (Table 1). Vegetationcharacteristics at nestshave been Activityat thenest.--In 1997,we recordedthe be- associatedwith nestpredation in somestudies haviorof adult thrushesduring each nest check. The (Martin 1988, 1996; Johnson1997). Nest con- behaviors observedwere assignedan activity score cealmentmay influencesurvival because high- ranging from 0 to 3. An activity scoreof 0 was as- ly concealednests would be less likely to be signedwhen a femalewas observedon the nest and October1999] NestingSuccess of WoodThrushes 1077 TABLE1. Variablesused in logisticregression to evaluatenesting success in WoodThrushes. Variable Description DENS Averageof the four densiometerreadings at nest NUMTREES Numberof treesrecorded in the wedgeprism NUMSHRB Number of shrubs(<10 cm dbh) on four 12.5-m transects radiating from nest NUMHEM Numberof hemlocksin the shrublayer PERHEM Percentageof shrubsthat were hemlocks YEAR Yearnest was monitored(entered as a categoricalvariable,
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