TheAuk 115(4):929-936,1998
EFFECTS OF PRIOR NESTING SUCCESS ON SITE FIDELITY AND BREEDING DISPERSAL: AN EXPERIMENTAL APPROACH CAROLA A. HAAS •
Department of NaturalResources, FernowHall, CornellUniversity,Ithaca,New York14853, USA
ABSTRACT.--Based on morethan300individuallymarkedAmericanRobins(Turdusmigratorius) and BrownThrashers(Toxostoma rufum),I testedthreehypotheses to explainlow return ratesof birds whosenestingattemptsare unsuccessful: (1) birds with low reproduc-
tive success are low-qualityindividualsthat are morelikely to suffermortalitybetween breedingseasons; (2) nestingfailureincreases reproductiveeffortby causingbirdsto renest, and thisenergeticstressincreases theprobabilityof mortality;and (3) birdsusea "decision rule" basedon prior experience to selectnestingsites,suchthatindividualsthatexperience low reproductivesuccess aremorelikelyto moveto analternatebreedingsite,whereasbirds that nestsuccessfully are morelikely to breedin the samesite again.Birdssubjectedto experimentalnestingfailurereturnedat a significantlylowerrate(robins18%,thrashers12%) thanbirdsthatnestedsuccessfully (robins44%,thrashers 29%).Birdsthatnestedmorethan oncein a seasonreturnedat rates(robins43%,thrashers21%) indistinguishable frombirds that nestedonly oncein a season(robins36%,thrashers23%).Theseresults,aswell assupplementarydata,wereinconsistent with hypothesesI and 2 and consistent with hypothesis 3. Thisstudyprovidesstrongevidencethatlow returnratesresultfromdispersalin response to nestingfailure.Received 11 August1997,accepted 12 March1998. DESPITE THE OBVIOUS AND
IMPORTANT
DIF-
FERENCES between dispersaland mortality, we
are unable to distinguishbetween them in many natural populations(e.g. Delius 1965, Harvey et al. 1985). Dispersal is difficult to study,and dataon immigrationand emigration largelyarelackingfor bird populations(Greenwood and Harvey 1982, Clobert and Lebreton
1991, Brawn and Robinson1996). Birds that breed successfully in one year are more likely to return to the samesite the followingyear
dence suggestsa link between reproductive success and survivorship(Pugesekand Diem 1990) and between reproductive successand breedingdispersal(Harvey et al. 1979,Drilling and Thompson 1988, Jakobsson1988, Nur 1988a),a causalrelationshipbetweenan individual bird'sfailure to produceyoungand an increasedprobabilityof thatbird'ssubsequent dispersalor mortality has not been demonstratedexperimentally.
In this study,I examinethreehypotheses to than are thosethat fail to fledgeyoung(e.g. explainlow returnratesby birdsthatnestunHarvey et al. 1979, Gratto et al. 1985, Drilling successfully. Thelow qualityhypothesis posits andThompson 1988,GavinandBollinger1988, that birds with low reproductivesuccess are P•irtandGustafsson 1989,ThompsonandHale low-qualitybirds that are morelikely to suffer 1989, Haas 1990, Payneand Payne 1993, Mur- mortalitybetweenbreedingseasons andtherephy 1996;but seeBollingerand Gavin 1989). fore are lesslikely to return to their previous When markedbirds are not resightedon a breeding site (Coulson1968, Lambrechtsand studyarea,however,we generallydonotknow Dhondt1986,PugesekandDiem 1990).Therewhethertheyhavedispersedor died. nestingstresshypothesisstatesthat nesting Some authors have attributed low return failureincreasesreproductiveeffortby causing ratesof bandedbirds to mortality (e.g.Asken- birds to renest;this energeticstressincreases mo 1979, Harvey et al. 1985, Johnsonand the probabilityof mortality. Lastly,the prior Marzluff 1990),whereasothershave invoked experience hypothesisstatesthat birds use a dispersal(e.g.H6gstedt1981).Althoughevi- "decisionrule" basedon prior experienceto select nestingsites.Individualsthat experience are morelikely to dis•Present address:Department of Fisheries and low reproductivesuccess Wildlife Sciences, Virginia Polytechnic Instituteand perseto anotherbreedingsite,whereasthose State University,Blacksburg,Virginia 24061,USA. that fledge young are more likely to breed E-mail:
[email protected] againin thesamesite(Darleyetal.1977,Gratto 929
930
CAROLA A. HAAS
[Auk,Vol. 115
TABLE1. Responses of birds (centeredin bold) in three experimentalcategoriesof nestingoutcome(successful,naturalfailure,experimentallyinducedfailure)aspredictedby threealternativehypotheses. Predicted
outcome
Natural
Hypothesis
Successful
Induced
failure Return
failure
Test
rate
Low quality (LQH) Renestingstress(RSH) Prior experience(PEH)
High High High
Low Low Low
High Low Low
Low quality Renestingstress Prior experience
Good Good Good
Low quality Renestingstress
Equal Equal
Equal Equal
Equal Equal
Priorexperience
Short
Long
Long
Body condition Poor Good Good Good Good Good
Reject LQH if return rates of induced-failure birds are lower than those of successful birds
RejectRSH and PEH if body conditionearly in seasonis lower in adults that fail
Dispersal distance Reject LQH and RSH if dispersal distancebetween
nestsis longerafter nesting failure
et al. 1985,Gavin and Bollinger1988).I tested in (Turdusmigratorius)and Brown Thrasher (Toxopredictionsof thesehypotheses(seeTable 1) stomarufum), breeding in shelterbeltsand woody usingtwo speciesof passerines that breedin a draws in Sioux and Morton counties, south-central
habitatthat was easyto censuscompletely.I North Dakota. Becausetrees were planted in wellchoseto work with tree- and shrub-nestingspacedrows,it was possibleto searcheverytree for birdsin a prairiehabitatsothatI couldidentify a nest, completelycensusingthe study area. The study was conductedduring the breedingseasons and searchall potentialnest siteswithin the (April to August)of 1984to 1989.The studyareaenstudy area.
I lookedat additionalparametersthat could supportor challengeone or more of the hypotheses.I measuredtwo indicesof condition of breedingbirds (fat scoreand a ratio of body massto body size)to determinewhetherbirds startingthe seasonin poorerconditionwould be more likely to experiencenatural nesting failure,becauselow energystoresoftenappear to limit reproduction in birds (see Martin 1987). I also examined dispersal distancebetween subsequent nests(both within and between years) to determine whether birds moved
different
distances
between
nest sites
compassed 52 shelterbelts at 16 sitesand the woody draws within an 8 x 11 km block of agricultural land. The most commonwoody speciesplanted in shelterbeltsincluded exotic species(Siberian elm [Ulmuspumila]and Russianolive [Eleagnus angustifolia]) as well as native species(green ash [Fraxinus pennsylvanica] and boxelder [Acer negundo]).Tree speciesoccurring in wooded creeksincluded American elm (Ulmusamericana), boxelder,and greenash.
Fora moredetaileddescription of thestudyarea,see Haas (1990, 1997) and Titus and Haas (1990). From 1984 to 1988,we captured 209 adult robins and 282 adult thrashers in mist nets and marked each
bird with a unique combinationof three colored plastic bands and one aluminum U.S. Fish and Wild-
afterdifferentoutcomes oftheirfirstnestingat- life Serviceband. From 1986to 1989,we alsopainted
tempts.In addition, I comparedreturn ratesof
the tailswith Testorsmodelpaint for easyidentifi-
birds that nestedonly onceper seasonwith cationwithina season. Sexwasdetermined bybrood thoseof birdsthat nestedmultipletimesper patchor cloacalprotuberance, or (robinsonly) by seasonto determinewhetherrenestingeffort plumageand wing chord.We estimatedbody conresulted
in reduced return rates.
ditionusingthe ratioof bodymassto tarsuslength (Murphy 1980) and interclavicularfat scores.We scoredfat on a scalefrom 0 (no visible fat) to 2 (fat filling the cavity)at intervalsof 0.5.
METHODS
My field assistantsand I monitoredthe nesting success and movements
of marked
individuals
of
two speciesof migratorypasserines, AmericanRob-
Weconductedregularsearches of everytreein the 52 shelterbelts
and numerous
wooded
creeks for
nestsand markedbirds.We locatednestsweeklyby
October1998] visual searches as we walked
Nesting Success andSiteFidelity
931
on both sides of each
differentclassesof reproductivesuccess using2 x 3 contingency tables.If I foundsignificantdifferences in the 2 x 3 tables,I partitionedeachtableinto two independent2 x 2 subtables(Siegeland Castellan 1988)and testedthem using Fisher'sexacttests.I used two-tailed t-teststo comparebody-condition the nests of banded birds that nested in shelterbelts indicesof birds that nestedsuccessfully with birds in the studyarea.Eachnestwasvisitedat leastonce that failed naturally. When testingthe effect of rea week(mostmorefrequently)to identifybothpar- nestingonreturnrate,I includedall birds(successful ents and to determine the nest's fate. We returned to single-broodedand multiple-broodedbirds and unbandyoungwhentheyweresevento eightdaysold successful birds) in the analysisfor the following and checkedthe area subsequentlyto determine reasons.It wasnot possibleto testonlybirdswhose whetherthe younghad survivedto fledging.A nest nests had failed, becausealmost all birds that failed or adultthatproducedat leastoneyoungto fledging renestedwithin the season.Testingonly successful was defined as successful. To determine the number birdswouldhaveignoredthe populationof interest of youngfledged,we usedthenumberof youngthat (birdsthat failed).If repeatednestingis stressful,its werebandedin nestsknowntohaveproducedfledg- effects should be visible in both successful and unlings.Fledgingusually occurredwithin two to four successful birds. daysof banding.Only adultsthat failed to produce any young in a given season(adults for whom all RESULTS nestingattemptsfailed)were definedasunsuccessrow of every shelterbeltor by following adults to nests.Most nestswere foundduring laying or incubation.Becausewe rarely encountereda markedbird without a nest, and becausenestsgenerallywere easyto locate,we believethat we found almostall of
ful for that season.
In 1987, we randomly selectedone out of every
Returnraterelativeto nestingsuccess andnest-
probabilitieswere three pairs of bandedthrashersand robinsnesting ing attempts.--Resighting returnratesapproximaton the studyarea for nestmanipulations, during high,sotheobserved whichwe removedall eggsor nestlingsfrom every nestof thesepairsduringthatseason. We continued thesemanipulations for thrashers in 1988,selecting one out of every two pairs for nest manipulations.
ed actual rates of survival and return with min-
imal bias (Martin et al. 1995). Out of 100 re-
sightings of eachspecies, only7 wereof robins notlocatedin anintervening year(93%resightMost nestswere subjectedto experimentalpredation ing probability), and only8 wereof thrashers at the egg stage.When we removednestlingsfrom notlocatedin anintervening year(92%resightthe nest, nestlingsmore than two days old were hand-reared,banded,and released.Nestlingsyounger than two daysold were euthanizedby thoracic compressionor by cervical dislocationfollowing publishedguidelines(AOU 1988). Collectingand hand-rearingwere conductedunder federal and statepermitsand wereapprovedby the University Animal
Care and Use Committee.
We measured the distance between consecutive nests of marked birds that bred more than once on
the studyareawith a metertape,by pacing,or, for distances>500 m, by mappingthe locationsof the nestson a U.S.G.S.topographicmap and measuring the distancebetweenthosetwo points.Within the study area,resightingprobabilitywasnot relatedto distance moved because all sites in the 8 x 11 km
ing probability).Somemissingobservations of robins(43%,3/7) and thrashers(25%,2/8) occurred when birds moved between sites on the
study area. For both robins and thrashers,return rate in
any given year was related to an individual's
nestingsuccess in the previousyear (Table2). Returnratesdifferedamongbirdsthatnested successfully, experiencednatural nest failure, and experiencedexperimentalnestfailure for both robins(X2 = 9.80, df = 2, P = 0.006)and thrashers (x2= 12.45,df = 2, P = 0.002).Return ratesof birds that failed experimentally were almost identical to those of birds that failed
area were searcheduniformly. Becausedistances naturally, bothforrobins(Fisherexacttest,P = moved between nests of a male and female that re-
mainedpairedwerenot independent,I did not analyze between-nestdispersaldistancesseparately for eachsex.I lumpeddatafor bothsexes,but when bothmembersof a pair movedtogether,I calculated inter-nestdistancefor onlyonebird. I calculatedbetween-season returnratesby dividingthenumberof bandedbirdsthatreturnedtobreedonthestudysite in year n + 1 by the total numberof bandedbirds breedingon the studysitein yearn. I comparedthe proportionsof returningbirdsin
0.999) and thrashers(Fisher exact test, P = 0.999).Return ratesof birds that nestedsuccessfullywere significantly higherthan those of birds whose nests failed (natural and induced failure combined;Fisher exact tests,rob-
ins, P = 0.002;thrashers,P = 0.0005).When sexeswerecompared separately, thetrendsremained the same, but differences in return
ratesdependingon previousreproductivesuccesswerenot significant for femalethrashers.
932
CAROLA A. HAAS
[Auk, Vol. 115
TABLE 2. Observedreturnratesof AmericanRobinsandBrownThrashersrelativeto nestingsuccess in the previousyear.Valuesin parentheses are the numberof birdsthat returned/totalnumber. Nesting outcome Sex
Successful American
Males Females
Induced failure
12% (2/16) 27% (8/30)
20% (1/5) 17% (1/6)
Robin
46% (33/72) 43% (39/90)
Sexes combined
Natural failure
44%
22%
18%
36% (31/87) 24% (22/90)
17% (5/30) 9% (3/32)
12% (4/32) 12% (4/32)
30%
13%
12%
Brown Thrasher
Males Females Sexes combined
For robinsand thrashers,the tendencyto re- 500 m within a season.For both species,disturn to the studyareain a givenyear wasnot persal distancesbetween nests tended to be relatedto thenumberof nestingattemptsmade shorterwithin a given breedingseasonthan in the previousyear (Table3). between breeding seasons,and tended to be Bodycondition.--During the yearswhennest fartherafter experimentaland naturalnesting manipulations were performed (1987 and failuresthan after successful nestingattempts 1988),fat scoresandratiosof bodymassto tar- (Figs.1A-D). Therelationshipbetweennesting suslengthdid not differamongbirdswith dif- outcome and dispersal distancewas signififerent nesting outcomes(Table 4). The only cant,however,only for movementsof Americomparison of bodyconditionthatapproached can Robins within a season. Robins that resignificance wasfor femalethrashers: meanfat nested after successfullyraising one brood scoreswere slightly higher for femalesthat moved shorterdistancesbetweennests(medinestedsuccessfully than for thosethat failed an = 42 m) than did robinsthatwererelaying naturally (1.0 vs. 0.7, respectively;t-test,P = after natural nestingfailure (median = 71 m; 0.18; Table4). Distance moved between nests.--Observed
dis-
persal distancesbetween nests on the study TABLE4. Indicesof body condition(ratio of body massto tarsuslength,and fat score)of American area ranged from 4 to 1,200m within seasons Robins and Brown Thrashersrelative to nesting and from 0 to 1,300 m between years. No success. Data for 1987 and 1988 combined. Values thrashers were observed
to move more than
are œ_+SD, with n in parentheses. P-valuesare from two-tailed
TABLE 3.
Observed
return rates of American
Robins
andBrownThrashersrelativeto numberof nesting attemptsin the previousyear.Valuesin parentheses are the number of birds that returned/total number. P-values are from X2 tests.
Number of nestingattempts Sex
One nest
> One nest
P
t-tests.
Nesting outcome Variable
Successful
Natural
American
failure
P
Robin
Mass / tarsus
Males Females
2.1 _+0.09 (22) 2.1 _+0.10 (8) 2.3 _+0.25 (29) 2.2 _+0.22 (7)
0.24 0.70
0.6 -+ 0.49 (20) 0.6 -+ 0.80 (6) 0.8 _+0.62 (26) 0.9 _+0.56 (7)
0.98 0.78
Fat score
American Males Females Sexes combined
38% (25/65) 35% (31/89)
39% (11/28) 46% (17/37)
36%
43%
Brown Males Females Sexes combined
Robin
>0.50
Thrasher
27% (31/113) 29% (8/28) 19% (24/125) 12% (3/25) 23%
1.00 >0.10
21%
1.00 >0.25 >0.50
Males Females
Brown
Thrasher
Mass / tarsus
Males Females
1.8 _+0.12 (21) 1.8 _+0.10 (17) 0.45 1.8 _+0.13 (24) 1.9 -+ 0.15 (21) 0.21
Fat score
Males Females
0.6 +- 0.58 (21) 0.6 _+0.52 (17) 0.80 1.0 _+0.77 (25) 0.7 _+0.66 (21) 0.18
October1998]
Nesting Success andSiteFidelity
933
Mann-Whitney U = 281.5, P = 0.017) or that were relaying after either natural or experimentalnestingfailure combined(median = 69
s=42 nf=71 if=69
m; U = 303.5, P = 0.020).
20'i$$$ Aßsuccess 15-1[ []natural failure
DISCUSSION
10-
Experimentalmanipulations of reproductive successdemonstratea causalrelationshipbetween an individual's failure to reproducesuccessfullyin a seasonand its failure to return the nextyear.Randomlyselectedpairssubjected to experimentalnestingfailure returned to the study area in lower proportionsthan did pairsthatfledgedyoungandin proportionsindistinguishablefrom thoseof pairs that experiencednaturalnestingfailure (Tables2 and 3). Thesefindingsallowrejectionof anynoncausal
•LE•• ilure s=66
nf=152
15
if=125
10
explanations, suchas the low qualityhypothesis.Bollingerand Gavin (1989)reachedthis sameconclusionfor Bobolinks(Dolichonyx oryzivorus)after experimentallycausingneststo fail by mowing the entire field in which the birds were nesting.Elsewhere,I have shown that the probabilityof BrownThrashersreturning to a site is significantlyrelatedto the proportion of thrashersnestingat that site successfullyin the previousyear (Haas 1997),but the results reported here demonstratethat birds alsoselectbreedingsitesbasedon their own previous nesting successat a breeding
5
0 s=43 nf=52 if=70
lO- •
C
site.
0 , ,
,[:[I....
s=104
nf=122 if=106
5-]
Lowqualityhypothesis.--The resultsof the experimental nest manipulationallowed me to rejectthis hypothesis.Dependingon the proportion of low-quality birds existing in the population(and therefore,in the randomlyse-
2
causedto fail. Valueson x-axisrepresentmediansof distancecategories(e.g. 150 representsvaluesbetween101and 200m). Distances->701m arelumped in the final category.(A) Distancesmovedby American Robins between
Distance Moved (m)
successive nests within
a breed-
ing season.(B) Distancesmovedby AmericanRobins between successivenests between years (between
the lastnestof the firstyearand the firstnestof the following year). (C) Distancesmoved by Brown Thrasherswithin a breedingseason.(D) Distances area, in relation to outcome of the first nest. Success- movedby Brown Thrashersbetweenyears.Values ful nestsproducedat leastonefledgling,naturalfail- abovebars are median dispersaldistances(m) for (s), ure nests failed naturally, and experimentallyin- birdswhosepreviousnestoutcomewassuccess ducedfailurenestswererandomlyselectedandthen natural failure (nf), or induced failure (if). FIG.1. Numberof individualbirds movingvariousdistances betweensuccessive nestson thestudy
934
CAROLA A. HAAS
lectedsample),it wouldbe possibleto interpret the results of the experimentalmanipulation differently.Assumingthat all survivingbirds are site faithful, by definitionany bird that is successful is a high-qualitybird, and all lowqualitybirds fail to produceyoung.(However, not all birdsthatfail to produceyoungarelow-
qualitybirds.)Specifically, thedifferencein return
rate between
successful
birds
and birds
that fail naturally shouldreflectthe proportion
of low-qualitybirdsexistingin the population if: (1) 100%of the high-qualitybirds that are alive return; (2) somehigh-quality birds lose theirnestsbut surviveto return(thisis theproportion of failed birds that return); and (3) all
low-qualitybirdslosetheirnestsandfail to return. Thus, the argumentthat my resultsare dependenton the proportionof low-quality birds in the populationcanbe testedwith my data.
For thrashers,the proportion of allegedly low-qualitybirds was 17% (i.e. 30 - 13; Table 2). The remainder (83%) allegedly are highqualitybirds and would return at a rate of 30% (24.9 birds). Therefore,we would expect an overall return rate of 24.9% in a randomly selected group of thrashers.However,the observed return rate, 12%, was less than half of
the expectedvalue(X2 = 5.60,P < 0.028).Similarly, for robins,22% of the birds (i.e. 44 - 22) allegedlyare of low quality (Table2). The remainder(78%)allegedlyarehigh-qualitybirds and would return at a rate of 44% (34.3birds). Therefore,we would expectan overallreturn rate of 34.3% in a randomlyselectedgroup of robins. However, the observed return rate, 18%,
wassignificantlylowerthanthe expectedvalue (X2 = 6.65,P < 0.015).Evenafter correctingfor the estimatedproportionof low-qualitybirds in the population,then,my resultsallow me to rejectthe low qualityhypothesis. Data on body conditionalsofail to support the hypothesisthatbirdswith low nestingsuccessare low-quality birds, at leastasmeasured by my indices (i.e. fat scoreand the ratio of body massto tarsuslength). Renestingstresshypothesis.--The hypothesis that nestingfailure inducesparentalmortality by causingan energeticstressassociated with renestinghas not been emphasizedin the literature,but it is a logicalexplanationfor therelationshipbetweennestingfailureandthesubsequentfailure to return to a site.Although a
[Auk, Vol. 115
tradeoff between reproductiveeffort and survival of parentshasbeen suggestedfor some species(Pugesek1987,Nur 1988b,Jacobsen et al. 1995),the influenceof renestingon survival is not clear.This hypothesisactuallyis just one of a setthat composes a largerhypothesis, that nestingfailure causesadult mortality. General knowledgeof the biologyof breedingpasserinesprovideslittle to suggestthat nestingfailure causesmortality of adult birds (Biedenweg 1983, Finch 1984, Bryant 1988, Weathersand Sullivan 1989, Ricklefs 1996). The only possibility that seemedplausibleenoughto warrant testing was that adults are subjectedto increasedrisk of mortality if they depletetheir energy reservesby renesting after a nesting failure.
This hypothesispredictsthat birdsthat nest more than once would have lower return rates
than birds that nestonly oncewithin a season (assumingthatthisindexof reproductiveeffort is correlatedwith reproductivecost).However, renestinghad no effecton subsequent return rate (Table3). Thus,my resultsdo not support the hypothesisthat the energeticstressof renestingcausesincreasedmortality in adults. Priorexperience hypothesis.--Results of the experimental nest manipulation indicate that nestingfailure causedbreedersto fail to return to theirpreviousbreedingsites,eitherby causing mortality or dispersal.Becausewe haverejected the hypothesisthat nesting failure increasesthe probabilityof mortalityof breeders (renestingstresshypothesis),we concludethat nestingfailure mustincreasethe probabilityof dispersalof breeders.The resultsof the experimentthereforeprovidestrongsupportfor the prior experiencehypothesis. For robins renestingwithin a season,there was a significantdifferencein dispersaldistances between
individuals
that were success-
ful versusthosethat failed.Theseresultsagain suggestthat individual birds responded to nestingfailure by moving longer distancesto breed. The low overall return rate of birds that
failedsuggests thatmuchof themovementoccurred on a scale greater than that encompassedby the study area(i.e. morethan 5 to 10 km). Observations of WoodThrushes(Hylocichla mustelina)movingmore than 4 km between nestswithin seasonsand up to 7 km between nesting and molting sites suggestthat this
October1998]
NestingSuccess andSiteFidelity
range of movement is not unusual for small passerines(Vega Rivera 1997). CONCLUSIONS
935
and Morton counties,North Dakota, includingthe StandingRock SiouxTribe, who so graciouslyallowed me to work on their property.I receivedfinancialsupportfrom NSF (includinga graduatefellowshipand dissertationimprovementgrant#BSR8800914), Andrew W. Mellon Student ResearchFund,
Thisis thefirst studyto demonstrate exper- graduateschoolof CornellUniversity,AmericanOrimentally in migratory passerinesthat failure nithologists'Union,SigmaXi, ManometBird Obserof an individual
nest causes that bird to dis-
vatory,CornellLaboratoryof Ornithology,Associa-
perse.The extentto which this relationship tion of Field Ornithologists, WilsonOrnithological holds in other birds and other vertebrates will
be an importantavenuefor future study.Obviously,manybirdsthatfail to returnto a study area are dead, and my resultsdo not rule out a weaker relationshipbetweennestingfailure and mortality. Thisworkhasimportantimplicationsfor our understandingand managementof bird populations.Althoughmanystudiesof avianpopulationdynamicsassumethatimmigrationbalancesemigration,and thereforethat dispersal is unimportant,the result that breedingdispersalmay be linked to anotherdemographic parameter,the productionof young,suggests that more attentionto detail is required.Protected populations may decline in reserves smallenoughto undergocross-population reproductivefailure in a givenyear not only (or even primarily) becauseof the lack of recruitment,but becauseof the dispersalof muchof the breedingpopulation.Learningmoreabout typicaldistancesand timing of dispersalmovements,as well as about the tendencyof landscapefeaturesto direct thesemovements(e.g. Villard et al. 1992;Haas 1995,1997;Matthysen et al. 1995;Machtanset al. 1996), would add greatlyto our knowledgeof and ability to manage populationsof migratorybirds. ACKNOWLEDGMENTS
I thankE. K. Bollinger,T.J.Cade,T.A. Gavin,E H. Pough,S. Sargent,and D. Winkler for commentson the designof this studyand on draftsof the manuscript.E. Cam, J. Fraser,N. Ingle, W. Piper,J. Rob-
inson,M. Sherfy,P.Stacey, R. Titus,andanonymous reviewersalsoimprovedthe manuscript.I thank E.
Society,and the GeorgeD. Harris Foundation. Some of thewriting wasconductedwhile I wassupported by an NSF-JSPS Fellowshipin the lab of Dr. Tsukasa Nakamura,Yamanashi University,Japan. LITERATURE CITED
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936
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