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 (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- 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 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 forestnear 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 days until 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 nestand 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, 1994 to 1997) HEM Nest tree was (1) or was not (0) a hemlock HEIGHT Height of nest (m) TREEHT Height of the nesttree (m) DISTTRNK Distance of nest from trunk (m) TREEDBH Diameterat breastheight of nesttree (cm) CLUTCH Size of clutch DAY Day from beginningof seasonon whichnest was initiated was not otherwise disturbed, or if the nest was un- more than one nest failed during the sameinterval attendedduring a checkand no thrusheswere seen betweennest checks.We useda G-testof indepen- or heardin the vicinity.An activityscore of 1 was denceto testwhether any of the threetypes of nests recordedif the female (or occasionallythe male was morelikely to fail first. perchedby thenest) was flushed from the nest with- Vegetationcharacteristics.--We characterized the out makingany audible vocalizafion. We recorded an vegetationat 400 nestsby recordingthe speciesof activityscore of 2 if the flushedbird emitteda call tree or shrub in which the nest was located, the describedas "bup bup" (Roth et al. 1996),which is heightsof the nest and the nesttree, the diameterat consideredto be the first-levelagitation vocalization. breastheight (dbh) of the nesttree, and the distance If the thrushgave the second-levelagitation call, de- from the trunk to the nest. We recorded additional scribedas "pit pit," the visit receiveda scoreof 3. vegetationdata at 355 of thesenests. We countedthe Weused a G-testof independenceto testwhether the number and recordedthe speciesof all trees in a activity scorewas relatedto the probabilityof nest wedge-prismpoint (basal-area factor 20; Husch et al. predationfollowing the visit. 1982) centeredat the nest. We made four estimatesof To test whethernatural activity at the nest in- canopy cover using a sphericaldensiometer and creasedthe likelihood of predation,we placeda pair countedthe number of vertical woody stemsless of artificialnests containing two NorthernBobwhite than 10 cm dbh and at least1.5 m high (i.e. shrubs) (Colinus virginianus) eggs near 65 active Wood within two transects(25 x 2 m) centeredat the nest Thrush nests in 1996 and 1997. One artificial nest and orientednorth-south and east-west.All vegeta- wasplaced 25 m from the activenest in a randomly tion samplingwas conductedafter nesting attempts chosendirection. The otherwas placed 100 m from had terminated. the activenest at an angleof 90ø from the first arti- We also measured vegetationat two "non-nesg' ficial nest.Artificial nests were placedat the same plots 50 m from eachof the above-mentioned355 heightand in the sametree species as nearby active nestsusing the samemethods as at the nests.Non- nests and were checked in the same manner as active nestplots were locateddue eastor west(chosen at nests. random)and north or south(also chosen at random) Wetested the hypothesis that activity near the nest from each nest. increasesthe probabilityof nestpredation and pre- dictedthat nestfailure would be highestat active nests,next-highest at artificialnests 25 m away,and RESULTS lowestat artificialnests 100 m away.Each set of three nestswas monitoredevery three daysto determine Nestingsuccess and daily survivaL--Sevenof predationrates. We ended the experimentas soon as the 416 Wood Thrush nests monitored con- the first nest failed because if the Wood Thrush nest tained oneBrown-headed Cowbird egg.Five of failed,the "activity"ceased. theseseven nests fledged a total of 14 Wood We scoredthe resultsof eachexperiment by re- cordingwhich nest(s) failed first. At the time oneor Thrushesand 4 cowbirdchicks, suggesting that morenests failed, the failednest(s) received a score nestparasitism had only a negligibleeffect on of 1 and the other nest(s) receiveda scoreof 0. Be- Wood Thrush nesting successon our study causewe checkednests every three days, oftentimes area. Similarly, only nine nests were aban- 1078 FARNSWORTHAND SIMONS [Auk,Vol. 116

1.00

0.98

0.96

0.94

0.92 1 2 3 4 5 6 7 8 9 10 11 Week

FIG. 1. Mayfielddaily survivalrates (_+ SE) for WoodThrush nests throughout the nestingseason (n = 416) pooledfor 1993to 1997.Horizontal line depictsthe overallaverage. donedwith no evidenceof predation(mostly daily survivaldid not differ betweenyears (all duringthe egg-layingstage). In contrast,the P > 0.20). Moreover,daily survival did not dif- majority of nests(225 nests)failed due to nest fer amongsites (Table 2) at thethree study sites predation. wherewe had morethan 50 nests(all P > 0.15). Of the 400 thrush nests where we recorded Activity at the nest.--Wechecked nests, re- nestingsubstrate, 336 (84%)were in hemlocks cordedactivity scores, and returned three days (Tsugacanadensis) and 45 (11.3%)were in rho- later on 390 occasionsduring 1997. Activity dodendrons(Rhododendron maximum). No more was scored 0 on 185 occasions, 13% of which thanthree nests were found in anyother plant were followed by predation.Scores of 1 were species(complete list in Farnsworth1998). followedby predationon 10%of 122occasions, Slightlymore than 84% of nestswere in trees scoresof 2 were followedby predationon 15% lessthan 10 cm dbh,so we usedthe proportion of 41 occasions, and scores of 3 were followed of shrub speciesrecorded at non-nestsites as by predationon 10%of 42 occasions.Therefore, our measureof the availabilityof nestingsub- the probabilityof nestfailure appearedto be strates. independentof the activityof adult birdsdur- The overallMayfield daily survival rate from ing our nestchecks (Gadi = 1.23,df = 3, P = 1993 to 1997 was 0.958 + SE of 0.003. The total 0.75). of 5,407 exposuredays comprised3,351 days Of the 185 occasions when we scored nest for nestswith eggsand 2,056 daysfor nests disturbance0, 74% occurred during incuba- with chicks.Of the225 nesting failures, 146 oc- tion. Similarly,of the 122 occasionswhen we curredduring incubation and 79 duringbrood scored nest disturbance 1, 72% occurred dur- rearing.The resultingsurvival rates for nests ing incubation.However, of the 41 occasions with eggs(0.956 -4- 0.004) and nests with chicks whenwe scorednest disturbance 2, only 54% (0.962 - 0.004)were not significantlydifferent occurredduring incubation,and of the 42 oc- (z = 0.933, P = 0.35). When all nestsfrom 1993 casions when we scored nest disturbance 3, to 1997 were pooled, daily nest survival was only 52%occurred during incubation. Activity fairly constantthroughout the breeding season scoreswere significantlyhigher for nestswith (Fig. 1). No two weekswere significantlydif- chicksthan for nestswith eggs(Gaai = 12.7,df ferentfrom eachother (all P > 0.05).When all = 3, P < 0.01). nestswere combinedfor eachyear (Table2), Among the 65 setsof activeand artificial October 1999] NestingSuccess of WoodThrushes 1079

TABLE2. Spatialand temporal variation in daily survivalrates (+SE) of WoodThrush nests.

No. of active No. of failed Site/year nests nests Daily survival All years combined Albright 81 43 0.9608+ 0.0059 Grassy 70 41 0.9564 _+0.0067 Cosby 114 70 0.9499 +_0.0058 Other sites 151 71 0.9641 +_ 0.0042

All sites combined 1993 49 25 0.9614 +_ 0.0076 1994 78 39 0.9605 +_ 0.0062 1995 101 49 0.9622 +_ 0.0053 1996 107 66 0.9529 _+ 0.0057 1997 81 46 0.9571 +_ 0.0062 Total 416 225 0.9584 +- 0.0027 nests, active nests were the first to fail 31 times, nestedin smallhemlocks more frequently than artificial nests 25 m from active nests were the wouldbe expectedby chance(Gaai = 377,df = first to fail 25 times, and artificial nests 100 m 1, P < 0.001).The next most commonnesting from active nests were the first to fail 22 times. substrate, rhododendron, was not used more Theseresults are in line with our predictions, frequentlythan expected (Gaai = 1.34,df = 1, P but the differencesamong treatments were no = 0.51). Nevertheless,we found no difference largerthan what wouldbe expectedby chance betweendaily survivalrate of nestsin hem- alone(Gadi = 2.66,df = 2, P = 0.26).Therefore, locks(185 failed nestsin 4460.5exposure days) we cannotreject the null hypothesisthat the and nestsin all otherspecies of shrubs(40 fail- probability of nestfailure is independentof ac- ures in 946.5 days;z = 0.11, P = 0.90). tivity at the nest. We had sufficient data to include 284 nests in Nest-sitecharacteristics.--We observed signif- a logisticregression analysis. All variableslist- icantly more shrubsat nest sitesthan at non- nest sites (two-tailed t-test, P < 0.01; Table 3). ed in Table1 were requiredfor a nestto be in- The compositionof the shrubspecies also dif- cluded in the analysis.Of these nests, 116 fered significantlybetween nests and non-nest fledged young and 168 failed. Six variables sites.Hemlocks comprised more than 51% of were included in the regressionmodel and the shrubsrecorded near nests but only33% of only one,the densiometerreading, was signif- thoseat non-nestsites (t-test, P < 0.01). icantly associatedwith nestingsuccess (Table Using the proportionof hemlocksin the 4). Overall concordance(which measures how shrub samplefrom non-nestsites as an esti- well the modelpredicted fledging success) was mate of hemlockavailability, Wood Thrushes 65%, indicatingthat the variablesused in the

TABLE3. Comparisonof vegetationmeasurements at WoodThrush nests and non-nest locations. Values are •-+SD.

Non-nests Vegetationmeasure Nests(n = 355) (n = 722) P Trees Number of trees 8.19 +_ 2.82 8.16 + 3.44 0.90 % Tulip poplar 38.6 -+ 26.2 36.7 _+28.9 0.30 % Hemlock 14.8 -+ 16.4 13.7 -+ 19.5 0.34 % Red maple 14.7 +_16.8 14.6 + 17.1 0.90 Shrubs Number of shrubs 37.7 _+ 20.8 28.0 _+ 18.2 <0.01 % Hemlock 51.2 +_ 29.2 33.4 + 29.8 <0.01 % Rhododendron 9.0 _+ 20.0 11.3 _+ 23.1 0.11 1080 FARNSWORTHAND SIMONS [Auk, Vol. 116

TABLE4. Resultsof logisticregression to evaluatenesting success in WoodThrushes.

Variable df Parameterestimate SE X2 P INTERCEPT i -1.94 1.12 2.97 0.08 DENS 1 -0.13 0.06 4.10 0.04 NUMSHRB 1 0.01 0.01 2.22 0.14 PERHEM 1 -0.73 0.44 2.77 0.10 YEAR 3 -- -- 6.49 0.09 HEIGHT 1 0.18 0.12 2.19 0.14 CLUTCH 1 0.33 0.25 1.69 0.19 model provided only partial explanation for esis,daily survival rate did not differ between variation in nesting success. nestswith eggsand nestswith chicks,and we found no correlation between clutch size and DISCUSSION probability of fledging. This suggeststhat WoodThrush activity is not relatedto nestsur- Daily survivalrates of WoodThrush nests in vival. our study area did not vary significantly Hemlockwas the most abundantpotential amongsites or years(Table 2), and they were nestingsubstrate, making up 33%of the shrubs similarover the courseof the breedingseason within Wood Thrush territories. We found that when data were pooledfor all years(Fig. 1). WoodThrushes used hemlocksas a nesting Thelevel of disturbanceto adultscaused by our substratemore frequently (84%) than would be nestchecks was not associatedwith a higher expectedby chance.It is possiblethat we found probability of nestingfailure. In similar stud- more nests in small hemlocks because we de- ies, Martin and Roper (1988) and Mayer et al. velopeda searchimage for nestsin theselo- (1997) also found no differencesbetween sur- cations.Although we attemptedto locateall vival rates of nests visited by observersand Wood Thrush nests in the areas we searched, nests observed from a distance. We also found we cannot be certain that such a bias was ab- no evidence for an association between the ac- sent. However, 9 of the 10 nests that we found tivity of breedingadults at the nest and nest using radio telemetry(which presumablyis predationrate. free of suchbias) also were placed in hemlocks Our predictionthat predation rates would be (Farnsworth1998). The apparent preference for higherfor activethrush nests than for nearby hemlocksas a nest substratesuggests that (25 to 100m) artificialnests was not supported. Wood Thrushesemploy a "needle in a hay- This result differsconsiderably from that re- stack"nesting strategy. This hypothesispro- ported by Wilsonet al. (1998).Using an exper- imentaldesign that was similar to ours,Wilson posesthat if predatorspreferentially search the et al. (1998) placedone artificial nest within 3 plant speciesused for nesting,nests should be to 4 m of 58 active Wood Thrush nests in Penn- placedin the mostabundant plant speciesin sylvania and found that the artificial nests order to minimize the chanceof discovery failedwith greaterfrequency. This couldhap- (Martin and Roper 1988, Filliater et al 1994). pen if artificial nests were more visible (i.e. Hemlocks were more abundant surrounding poorly concealed)than natural nests, or if Wood Thrush nests sites than at non-nest sites. adults actively protected or concealedtheir Thus,birds appearto be placingtheir nests in nests. local "haystacks"within the larger "hay- Interestingly,our disturbancescores during stack." However, the daily survival rate of the nestlingperiod were significantlyhigher Wood Thrush nests in hemlocks did not differ than thoseduring incubation, suggesting that from that of nests in other substrates. More- parentalinvestment accrued during later stag- over, the relative abundance of hemlocks es of the nestingcycle makes adult birds more aroundnests was not correlatedwith nesting proneto riskybehavior in defenseof theirnests success(Table 4). (Trivers 1972). However, we found no evidence With the exceptionof the densiometermea- thatactivity at thenest increased the probabil- sure of canopycover, no variablestested by lo- ity of predation.Contrary to Skutch'shypoth- gisticregression were stronglycorrelated with October1999] NestingSuccess ofWood Thrushes 1081 nesting success.Densiometer values represent low levelsof brood parasitism,probably allow a measure of nest concealment because read- the park to actas a populationsource for Wood ingsare made directly below the nest.This is a Thrushes(Farnsworth 1998). However, the con- crude index of nest concealment because it is tribution of the park to sustainingWood impossibleto separateprecisely which vege- Thrush populationson larger spatial scales tationlayers are mostresponsible for the over- warrantsfurther investigation. all densiometerreading. In most cases,how- ever,lower valueswere associatedwith vege- ACKNOWLEDGMENTS tation immediatelyabove the nest. Despite many studiescited as evidencethat nestcon- Fundingfor thisresearch was provided by theNa- cealment affectsnest survival rates (seeMartin tional ParkService and the United StatesGeological Survey.We thank the staff of Great SmokyMoun- 1992),few studieshave taken an experimental tainsNational Park, in particularJoe Abrell, Keith approach.Howlett and Stutchbury(1996) ma- Langdon,and Carroll Schell, for assistancethrough- nipulatedthe concealmentof HoodedWarbler out all phasesof the study. ChuckParker and Mi- (Wilsoniacitrina) nests and failed to showan ef- chael Kunze provided technicalassistance with fectof removingcover from aroundthe nests. mapsand data, and Jeremy Lichstein helped with the The relativelylow concordance(65%) of the logisticregression and provided commentson ear- logisticregression model indicates that lier versionsof the manuscript.We especiallythank variablescannot explain all of thevariability in our dedicated field crew: Soffian Adam, David nestingsuccess of WoodThrushes. Monitoring Aborn, Charles Bower, Kathy Brumm, Charlotte Chiswell, Anna Ciecka, Jeff Clark, Ken Draves, Joan- nests with remote cameras revealed that the na Fitzgerald,John Garesche, Brad Goodner,Scott relatively pristine in Great Smoky Hansen,Owen Heine,Sam Hightower, Tracy John- MountainsNational Park support an abundant son, SusanMoegenburg, Kimi O'Halloran, Rusty and diversepredator assemblage that includes Painter,Lori Peoples,Susan Shriner, John Simon, small rodents,squirrels, weasels, black bears Pam Simons, Vanessa Sorensen, Barbara Stedman, (Ursus americanus),snakes, , and corvids PaulSturm, Erik Trojnar,Liz Workman,John Young, (Farnsworth and Simons 2000). Our results and ChristaZweig. supportthe conclusionsof Filliateret al. (1994) that passerineshave evolved under conditions LITERATURE CITED of high nestpredation, which provides strong selectivepressure for the ability to renest BRISKIE,J. V., AND S. G. SEALY. 1989. Determination of clutchsize in the LeastFlycatcher. Auk 106: quickly. 269-278. Nest survival rates reported for the Wood DONOVAN, t. M., E R. THOMPSON Ill, J. FAABORG, Thrushesfrom a varietyof forestedareas led us ANDJ. R. PROI•ST.1995. Reproductivesuccess of to expectthat survival would be relatively high migratorybirds in habitat sourcesand sinks. in Great SmokyMountains National Park. Rob- ConservationBiology 9:1380-1395. inson et al. (1995b)found that nest survival FARNSWORTH,G. L. 1998. Nesting successand sea- ratesfor Neotropicalmigrants in general,and sonalfecundity of the WoodThrush, Hylocichla Wood Thrushesin particular,increased with mustelina,in GreatSmoky Mountains National Park. Ph.D. dissertation, North Carolina State the percentageof forestcover in the landscape. University,Raleigh. Similarly,Hoover et al. (1995)found that nest- FARNSWORTH,G. L., AND t. R. SIMONS. 2000. Obser- ing successof WoodThrushes in Pennsylvania vationsof WoodThrush nest predators in a large increasedas the size of forest fragmentsin- contiguousforest. Wilson Bulletin 112: in press. creased.Based on thesetrends, daily nest mor- FILLIATER,t. S., R. BREITWISCH,AND P.M. NEALEN. tality ratesin GreatSmoky Mountains National 1994. Predation on Northern Cardinal nests: Park,which is a large,contiguous forest patch Does choice of nest site matter? Condor 96:761- within a mostly forestedlandscape, should 768. havebeen 0.03 or lower (Robinsonet al. 1995b: HOOVER,J. P, M. C. BRITTINGHAM,AND L. J. GOOD- RICH.1995. Effects of forestpatch size on nesting figure4), and nestingsuccess should have ex- success of Wood Thrushes. Auk 112:146-155. ceeded70% (Hoover et al. 1995:table 1). How- HOWLETT, J. S., AND B. J. STUTCHBURY.1996. Nest ever,our nestmortality rateswere consistently concealmentand predation in HoodedWarblers: near 0.04(33% nesting success). This moderate Experimentalremoval of nest cover.Auk 113: levelof nestingsuccess, combined with current 1-9. 1082 FARNSWORTHAND SIMONS [Auk, Vol. 116

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