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242 GP•ANTET AL. [Auk, Vol. 99 --, A. OLSZOWKA, & A. AR. 1974. The avian WHITTOW.1976. Regulationof incubation water egg: surfaceareaß volume and density. Condor lossin sevenspecies of terns. Physiol.Zool. 49: 76: 319-325. 245-259. PETTIT, T. N., G. S. GRANT, G. C. WHITTOW, H. RICKr•FS, R. A., & H. RAHN. 1979. The incubation RAHN,• C. V. PAGANELLI.In press.Respiratory temperature of Leach's Storm-Petrel. Auk 96: gas exchangeand growth of Bonin Petrel em- 625-627. bryos. Physiol.Zool. ROB•I•TS,B. 1940. The life cycle of Wilson's Petrel RAHN,H., • A. AR. 1974. The avian egg:incubation Oceanitesoceanicus (Kuhl). Scient. Rep. Br. Gra- time and water loss. Condor 76: 147-152. ham Ld Exped. 1934-37, 1: 141-194. , & --. 1980. Gas exchangeof the avian ROUDYBUSH,T., L. HOFFMAN, & H. RAHN. 1980. egg: time, structure, and function. Amer. Zool. Conductance,pore geometry and water loss of 20: 477-484. eggsof Cassin'sAuklet. Condor82' 105-106. , & W. R. DAWSON. 1979. Incubation water TUrL•TT, S. G., & R. G. BoAar>. 1977. Determinants loss in eggsof Heermann'sand Western Gulls. of avian eggshellporosity. J. Zool., London 183: Physiol. Zool. 52: 451-460. 203-211. ß C. V. PAGANErrS, & A. AR. 1975. Relation VL•CK, C. M., & G. J. KœNAG¾.1980. Embryonic of avian egg weight to body weight. Auk 92: metabolism of the Fork-tailed Storm-Petrel: 750-765. physiologicalpatterns during prolongedand in- , R. A. ACKERMAN,& C. V. PAGANELLI. 1977. terruptedincubation. Physiol. Zool. 53: 32-42. Humidity in the avian nest and egg water loss WHITTOW,G. C. 1980. Physiologicaland ecological during incubation. Physiol. Zool. 50: 269-283. correlatesof prolongedincubation in seabirds. , C. V. PAGANELLI, I. C. T. NISBET, & G. C. Amer. Zool. 20: 427-436. The sixthannual meeting of the ColonialWaterbird Group will be held 4-7 November1982 in Washington, D.C. A symposiumon the feedingbiology of waterbirdsis planned.Papers given at the meetingare eligible, after refereeing,for publicationin ColonialWaterbirds. Anyone wishing to contributeto either the sym- posium (deadline 1 September)or general session(deadline 15 September)should contactDr. Michael Erwin, U.S. Fish and Wildlife Service,Patuxent Wildlife ResearchCenter, Laurel, Maryland 20708.In- formationconcerning registration can alsobe obtained from Dr. Erwin. THE ROLE OF MIGRATION AND WINTER MORTALITY IN THE LIFE HISTORY OF A TEMPERATE-ZONE MIGRANT, THE DARK-EYED JUNCO, AS DETERMINED FROM DEMOGRAPHIC ANALYSES OF WINTER POPULATIONS Errv.• D. KETTERSONAND VAL NOLA• JR. Departmentof Biology,Indiana University, Bloomington, Indiana 47405 USA ABSTR•CT.--Thisstudy of migratoryDark-eyed Juncos (Junco h. hyemalis)investigates dif- ferencesin winter populationstructure and dynamicsassociated with north-southvariation in wintering site and considersthe relationshipsbetween these winter differencesand population dynamics at other seasons.Banded juncos were captured and recapturedin several winters at four latitudes (42ø-33.5ø),and results were integrated with long-term United StatesFish and Wildlife Service recordsof recoveriesthroughout the winter range. The data suggest: (1) Populationswintering in the north experience higher winter mortality than populationswintering in the south. (2) Sex and age do not affectthe prob- ability of overwintersurvival; therefore, dominance may not affectthat probability.(3) Some individuals do and others do not return to the samewintering site year after year. Individ- uals that do not return tend to be those that have spent the first winter of life in the north, and theseshift southward.(4) Annual mortality of northernand southernwinter populations probablyis the same. This conclusionis based in part on a comparisonof north-south annual return rates and on logic arising out of the geographicaldistribution of the sexes during winter. In juncos, becausemales tend to winter north of females,greater annual mortalityof northernwinter populationswould be expectedto skewthe primary and sec- ondarysex ratiosin favor of males.No evidenceof suchskews has been detected.(5) If annual mortality is the same,it is likely that southernpopulations suffer heavier mortality duringtheir longermigrations and that thesemigration losses offset the advantagedescribed in (1), above. In north temperate-zonebreeding species that migratesouthward into an extensivewinter range, distancetraveled probably is correlatedclosely and positively with both mortality rateduring migrationand survivalrate duringwinter. If theserates offset each other, annual survivorshipamong winter migratory populations(and among sedentaryand migratory populationsof partially migrant species)can be equal. Therefore, it is unnecessaryto pos- tulate for different winter populationseither unequal reproductivesuccess (von Haartman's hypothesis)or unequalyear-to-year survival rates that balanceout overthe longterm (Lack's hypothesis). Migration can causebreeding populations to be redistributedin variousways in winter; e.g. they may mix randomly, or northern breedersmay leapfrogsouthern breeders. Each variationin migrationpattern would producewinter populationswith predictablecharac- teristics.We considerthe extent to which winter junco populationshave these character- istics.Received 26 January1981, accepted30 September1981. THE Dark-eyed Junco(Junco h. hyemalis),an ers, by migrating farther, move into progres- abundant ground-feedingemberizid, migrates sively more moderateand even mild climates. between a breeding range lying largely in the Male juncostend to winter north of females boreal forests of Canada and, in eastern North (Ketterson and Nolan 1976, 1979). Assuming America, a winter range reachingfrom south- an equal sex ratio on the breedinggrounds-- ern Ontario and the northern boundary of the and both field study and theoreticalconsider- United States almost to the Gulf of Mexico ations of sex ratio and mating systemssupport (Bent 1968). Becausethe winter range is so ex- this assumption(see below)--the winter sexual tensive, northern-winteringjuncos spend that separationnecessarily implies that members of critical season(Lack 1954, Fretwell 1972) in cold southern-winteringpopulations have migrated and often snow-coveredregions, whereas oth- a greateraverage distance than membersof 243 The Auk 99: 243-259. April 1982 244 KETTERSOtaAtaD NOLAta [Auk, Vol. 99 northem populations. Further, in both sexes In this paper we present data on minimum individuals produced in the preceding breed- overwinter survivorshipand minimum annual ing seasonand adults exhibit differences,on survivorship in northern- and southem-win- the average, in winter distribution. The pat- tering populations that we sampled by cap- tern of age distribution is stable from year to ture-recapturetechniques at the beginning and year and is complex (Kettersonand Nolan in end of single winters and also in successive prep.), but its most interestingfeature is that early winters. Interpretation of such data re- the young of eachsex tend to concentratenorth quires that death and dispersalbe separated of their adult counterparts.As with the sexual out as factorsaffecting the recapturedata. We separation, this pattern implies differential thereforeanalyze United StatesFish and Wild- migratory behavior associatedwith age, with life Service(USFWS) Bird BandingLaboratory adults traveling farther. When age classesof recordsbearing on the questionof north-south bird speciesseparate in the winter range, it is variation in the winter dispersaltendency of usual for adults to make the shortermigration juncos in the eastern United States. We also (Gauthreaux1978), although the pattern we examinethese USFWS records for time elapsed find in the junco has been observed in the betweenbanding and recovery,which we take American Goldfinch (Carduelistristis) (A. Mid- as an estimateof survivorship,and compare dleton pers. comm.) and the White-crowned them with a long-term estimateof survival of Sparrow(Zonotrichia leucophrys gambelii) (King an Indiana sample. et al. 1965). Our results, like the USFWS data that we Predicting that juncos that winter in the consider, are limited to easternwinter popu- north would experience greater overwinter lations. The breeding sitesof thesebirds were mortality than those that migrate into the unknown, and all references to northern and south, we also expectedthat annual survivor- southernjuncos are to winter populations,ex- ship of southernwinter populationswould be cept where it is explicitly statedotherwise. higher. A corollaryof this latter prediction is that members of such southern populations METHODS should have lower reproductive success,as- Capture-recapturedata.--Juncos end their autum- suming that fitnessof juncosdoes not vary ac- nal migration about 1 Decemberand begin spring cordingto the latitude of the wintering site. As migration about 1 March (evidence for this is re- we here report, the expectationabout unequal viewed in Ketterson and Nolan 1976), with indica- overwinter mortality appearsto be correct,but tions that few move very far in the interim (see be- the evidence does not support the prediction low). Therefore, individuals found from December that wintering in the southconfers greater an- through February we consider to be winter resi- nual survivorship.This implies that southern dents.We capturedjuncos at a seriesof sitesin early winterers have higher mortality than northern winter (December, except that an occasionaleffort winterers at some other stage of the annual extended into the first few days of January), then cycle, and the most probableoccasion