April 2001] ShortCommunications 513

Acknowledgements.--VicCarpenter, Bennie Cock- COULTER,M. C., W. D. MCCORT,AND A. L. BRYAN,JR. erel, Carol Eldridge, Lara Hopkins, William Hicks, 1987. Creation of artificial foraging habitat for Michael Reider and Tanya Youngbloodassisted with Wood Storks. Colonial Waterbirds 10:203-210. the various24 h observations.Dan Connellyand Paul DARNELL, R. M., AND R. R. MEIEROTTO.1965. Diurnal Koehler of the National Audubon Society'sSilver- periodicity in the black bullhead, Ictalurusmelas bluff Sanctuarymanaged and maintained the Kath- (Rafinesque). Transactions of the American wood foraging ponds. Keith Bildstein, Carol El- FisheriesSociety 94:1-9. dridge and two anonymousreviewers improved FASOLA,M., AND L. CANOVA.1993. Diel activity of earlier drafts of this manuscript.This projectwas resident and immigrant waterbirds at Lake Tur- funded by financial assistanceaward DE-FC09-96- kana, Kenya. Ibis 135:442-450. SR18546from the U.S. Department of Energy to the KAHL, M.P. 1964. Food ecology of the Wood Stork Universityof Georgia'sSavannah River EcologyLab- (Mycteriaamericana) in Florida.Ecological Mono- oratory,and the U.S. Fish and Wildlife Service--Jack- graphs 34:97-117. sonvilleField Office through a grant to the U.S. Fish KREBS,J. R. 1978. Optimal foraging: Decision rules and Wildlife Service--SavannahCoastal Refuges. for predators. Pages23-63 in BehavioralEcolo- gy, an Evolutionary Approach (J. R. Krebs, and LITERATURE CITED N. B. Davies, Eds.). Blackwell Scientific Publi- cations, London. BAUMAN,P. C., AND J. F. KITCHELL.1974. Diel pat- KUSHLAN,J. A. 1978. Feeding ecology of wading terns of distribution and feeding of bluegill (Le- birds. Pages249-297 in WadingBirds (A. Sprunt pomismacrochirus) in Lake Wingra, Wisconsin. IV, J. C. Ogden, and S. Winkler, Eds.). National Transactionsof the American FisheriesSociety Audubon SocietyResearch Report 7. New York. 103:255-260. KUSHLAN,J. A. 1979. Prey choiceby tactile-foraging BENT, A. C. 1926. Life histories of North America wading birds. Proceedingsof the Colonial Wa- marsh birds. Smithsonian Institution, U.S. Na- terbird Group 3:133-142. tional Museum Bulletin 135. KUSHLAN,J. A. 1981.Resource use strategiesof wad- BRYAN,A. L., JR., M. C. COULTER,AND J. c. PENNY- ing birds. Wilson Bulletin 93:145-163. CUICK. 1995. Foraging strategies and energetic MCNEIL, R., P. DRAPEAU, AND R. PIEROTTI.1993. Noc- costs of foraging flights by breeding Wood turnality in colonial waterbirds: Occurrence, Storks. Condor 97:133-140. special adaptations, and suspected benefits. COMER,J. A., M. C. COULTER,AND A. L. BRYAN,JR. Current Ornithology 10:187-245. 1987. Overwintering locations of Wood Storks SHENKER,J. M., AND J. M. DEAN. 1979. The utilization captured in east-centralGeorgia. Colonial Wa- of an intertidal salt marsh creek by larval and terbirds 10:162-165. juvenile fishes:Abundance, diversity and tem- COULTER,M. C., ANDA. L. BRYAN,JR. 1993. Foraging poral variation. Estuaries2:154-163. ecology of Wood Storks (Mycteriaamericana) in east-centralGeorgia. I. Characteristicsof forag- Received27 December1999, accepted 8 November2000. ing sites. Colonial Waterbirds 16:59-70. Associate Editor: K. Bildstein

The Auk 118(2):513-519, 2001

Timing of BreedingRange Occupancy Among High-latitude PasserineMigrants

ANNA-MARIE BENSON •'3 AND KEVIN WINKER 2 •AlaskaBird Observatory,P.O. Box 80505, FairbanksAlaska, 99708, USA; and 2Universityof AlaskaMuseum, 907 YukonDrive, Fairbanks Alaska, 99775, USA

ABSTRACT.--The brief subarctic summer limits the tumn migration among 18 passerinespecies to ob- time availablefor birds to completetheir reproduc- tain indirect estimatesof the time they occupytheir tive activities,yet the temporalrequirements of high- breeding ranges in northwestern North America. latitude passerinemigrants are not well understood. From 1992 to 1998, the Alaska Bird Observatory Our analysesexamined the timing of springand au- (64ø50'N, 147ø50'W) banded 31,698 individuals dur- ing the most intensive standardizedmist-netting study ever conducted in subarctic North America. 3E-mail: ambensøn@alaskabird'ørg Among the migrants examined, the estimated num- 514 ShortCommunications [Auk, Vol. 118

ber of daysthat specieswere presentin interior Alas- TABLE1. Net-hours of operationand total days of ka ranged from 48 days for adult Alder Flycatchers netting in spring and autumn at Creamer'sField (Empidonaxalnorum) to 129 days for American Rob- Migration Station in Fairbanks, Alaska (1992- ins (Turdusmigratorius). Adults departedsignificant- 1998). ly later in autumnthan immaturesin 10 of 18 species Spring Fall we examined and significantlyearlier than imma- turesin only onespecies, Alder Flycatcher.Breeding Year Net-hours Days Net-hours Days range occupancyof Nearctic-Neotropicmigrants oc- 1992 6,903 42 5,822 46 cursin this regionwithin the range of averagefrost- 1993 10,552 43 13,472 50 free temperaturesin Fairbanks,Alaska, and is sig- 1994 11,252 41 13,935 52 nificantly shorterin duration than amongNearctic- 1995 12,731 45 13,944 57 Nearctic("short-distance") migrants at this latitude. 1996 12,411 44 14,985 57 1997 7,548 42 14,617 66 The high latitudesof North America support a di- 1998 6,800 39 11,853 54 verse avifauna,because during the summermonths Total 68,196 296 88,627 382 the region is characterizedby a nutrient-richenvi- ronment,an extendedphotoperiod, and a hospitable climate.But summerat high latitudesis brief. For ex- ample, the average frost-free period in Fairbanks, The studysite, at Creamer'sField Migration Station Alaskawas 105days from 1905to 1999(from 20 May (CFMS), encompasses-20 ha of the southwestern to 2 September;National Weather Service data). portion Creamer'sField Migratory WaterfowlRefuge Birds that migrate to Alaska are therefore under (731 ha). It is owned by the State of Alaska and op- greatertemporal pressure to completebreeding-sea- eratedby the Alaska Departmentof Fish and Game. sonactivities (such as territory and mate acquisition, The northern portion of CFMS is dominatedby ma- nestbuilding, egg laying, incubation,care of young, ture willow (Salixspp.) and paperbirch (Betula papyr- and molt) than birds at lower latitudes. Yet, the total ifera)bordering a seasonallyflooded wetland domi- amount of time passerinesspend completing the nated by sedges(Carex spp.) and one grassspecies breeding effort is not well known. Few studieshave (Calamagrostiscanadensis). The central sectionof the been conductedover the entire breeding seasonat study area is characterizedby late successionalwhite high latitudes, and thosestudies that have examined spruce(Picea glauca) and balsam poplar (Populusbal- breedingchronology at northernlocations (e.g. Rim- samifera).The southernportion of the study area has mer 1988) may not accuratelydocument departure tremblingaspen (Populus tremuloides), balsam poplar, from the breedinggrounds because of small samples and willow growingnear an agriculturalfield. Except and postbreedingdispersal. for the agriculturalfield, that mosaicof habitattypes Studiessampling migrant passageduring spring is representativeof thehabitats occurring in theboreal and autumn provide an indirect measure of time forestfloodplains of interior Alaska. spenton the breeding grounds.The accuracyof that A standardized mist-netting protocol was con- measureincreases with proximity to a population's ducted at CFMS from 1992-1998using an array of breeding area.We sampledmigrants near the north- 22-50 standard mist nets (30 mm mesh, 2.6 x 12 m). ern and westernlimits of their migrationsduring Nets were arrangedin a north-south direction,per- spring and autumn.Here we examinetiming of pas- pendicularto the TananaValley migrationcorridor, sage at the specieslevel in that region to obtain in- and operateddaily from 0600 to 1300 (Alaska) dur- direct estimatesof the time birds occupytheir breed- ing spring migration (25 April-15 June).During au- ing ranges in the northwestern extreme of North tumn migration (15 July-30 September),nets were America. Additionally, we answer the following openedat approximatelysunrise and closed7 h later. questions:(1) How compressedare breeding-season Samplingat the endsof both seasons(10-15 Juneand activitiesamong high-latitude migrants? (2) Do Ne- 25-30 September)was limited to every other day. arctic-Neotropicmigrants differ from Nearctic-Ne- Nets were closed during inclement weather. Fewer arctic ("short-distance")migrants in the timing of nets were operated in 1992, 1997, and 1998 than in breeding-rangeoccupancy? other years (Table1), and netswere not operatedbe- Studyarea and methods.--Thestudy area is located tween 17 July and 2 August 1992. in Fairbanks, Alaska (64ø5'N, 147ø5'W), near the con- Birds were banded with U.S. Fish and Wildlife Ser- fluence of the Chena and Tanana rivers (elevation 130 vice bands,and data were collectedto determineage m). The TananaValley is a well documentedmigra- (autumnonly, using degree of skull ossification),and tion corridor for many speciesof birds, including breeding condition (spring only, using incubation SandhillCranes (Grus canadensis; Kesse11984), many patchesin femalesand enlarged cloacalprotuber- speciesof raptors(Mcintyre and Ambrose1999), wa- ancesin males).During autumnmigration, estimates terfowl, shorebirds,and passerines(Cooper and Rit- of the percentageof juvenal plumagewere recorded chie 1995). for first-year individuals. April 2001] ShortCommunications 515

To ensureindependence of recordsin thoseanalyses, ing thoseseven years, 31,698 birds of 58 specieswere we used only initial capturesof individuals(all sub- banded. We examined the timing of passage of sequentcaptures of individualswere eliminated).We 25,718individuals among 18 passerinespecies. Judg- also excluded local breeders on the basis of the follow- ing from capture distributions (Benson 2000), the ing criteria:females with incubationpatches, individ- netting periods spanned the entire spring and au- ualscaptured more than sevendays from first capture, tumn migration periods for all speciesanalyzed ex- andbirds captured in oneseason and recapturedin an- cept Alder Flycatcher.Spring netting was truncated other.First capturesof individualswere alsoremoved shortly after peak passageof that species.Therefore, if they were later recapturedwith breedingcharacter- for that speciesmedian spring passage dates may be istics.Finally, we excludedfirst-year birds with >30% slightly later than presentedhere. of their body in juvenalplumage, because we consid- There were significantdifferences in median dates ered them nonmigratoryat that time. of autumnpassage between adults and immaturesin We definedNearctic-Neotropic migrants as those 11 of 18 species(Table 2). The Alder Flycatcherwas specieswith all or part of their populationsbreeding the only speciesin which the departure date of north of the Tropic of Cancer and all or part of the adults precededthat of immatures;that difference samepopulations wintering southof that line (Rap- averaged 13 days (Table 2). In Ruby-crownedKing- pole et al. 1983). We defined Nearctic-Nearctic mi- lets, Swainson's Thrushes, American Robins, grants as those specieswhose breeding and winter- Orange-crowned Warblers, Yellow-rumped War- ing populationsare primarily north of the Tropic of blers, Yellow Warblers,Blackpoll Warblers, Wilson's Cancer.Those species are often categorizedas short- Warblers,American Tree Sparrows,White-crowned distance migrants. Sparrows,and Dark-eyed Juncos,immatures preced- A maximum estimate of time spent on breeding ed adults (Table2). The largest significantage-relat- grounds was obtained from the difference between ed differencesin passagedates occurred among Wil- median datesof spring and autumnpassage. We did son's Warblers and Dark-eyed Juncos; in both not standardizenumbers of birds captured by unit species, adults migrated on average 13 days later of nettingeffort for two reasons.First, net hours(nh) than immatures.In 6 of 18 species,there were no sig- were uniformly distributed among days when all nificant differences between adults and immatures years were combined during spring (Kolmogorov- in median datesof passage.Because there were sig- Smirnov-test, Dmax = 0.08, n = 68,196, P > 0.10) and nificantage-related differences in departuredates in autumn (KS-test, Dmax = 0.07, n = 87,627, P > 0.10). severalspecies, and becauseinference of departure Second,standardizing captures by unit of nettingef- is confiatedby large numbersof immatures (lower- fort can impart biasby artificiallyinflating or deflat- ing accuracy;Table 2), immatures were eliminated ing sample sizes. For example, 18 birds captured from all subsequentanalyses. during 80 nh could provide sample sizes of 22.5 Estimatesof the period that adults occupiedtheir birds/100 nh, 225 birds/I,000 nh, or .225/nh. Thus, breeding rangeswere more condensedfor Nearctic- an arbitrary number of net-hoursused to standard- Neotropic migrants (n = 12) than for Nearctic-Ne- ize capturesdirectly affects sample size, which af- arctic migrants (n = 6; Mann-Whitney U = 71.0, P < fects the power in statisticaltests. Finally, manipu- 0.05; Table 2, Fig. 1). The mean period of breeding- lating samplesize in our study was not necessary range occupancy for all Nearctic-Neotropic mi- given the uniformity of net-hourswithin seasons. grantswas 90.6 days(SE = 4.4 days),or <25% of the Differences in timing of passage were present annual cycle. Nearctic-Nearctic migrants averaged among years for some species(Benson 2000). Here 119.8 days (SE = 3.4 days), or 33% of the annual we combined all years to increasesample sizes be- cycle. causethat providesthe best species-levelestimate of The estimated number of days that adults were the duration of breeding-rangeoccupancy. Addi- present in interior Alaska ranged from 48 days for tionally,variation in the nettingeffort in earlieryears Alder Flycatchersto 129 days for American Robins was minimized when years were pooled because (Fig. 1). The next-briefesttimes occurringbetween nets were operated uniformly among days within median datesof spring and autumnpassage in Yel- each seasonwhen all years were combined. low Warblers, Hammond's Flycatchers,and North- We tested for differential timing of migration be- ern Waterthrushes,which spent 84, 86, and 86 days, tween adults and immatures during autumn using respectively,on their breeding ranges, or -23% of the Mann-Whitney U test. Differencesin timing of the annual cycle. migrationbetween age classescould affectspecies- Discussion.--Thelater departure datesof adults in level estimatesof the median date of autumn pas- many speciesmay be explainedby the timing of molt sage,because most autumn captures at that study in adults, which, unlike immatures, replace flight site are first-year birds. feathersin autumn.Differences in timing of passage Results.--During 1992-1998, staff of the Alaska between adult and immature Swainson's Thrushes, Bird Observatory accumulated 68,196 nh during although present here, were not at Long Point, On- spring and 88,627nh during autumn (Table1). Dur- tario, western Pennsylvania,or coastalAlabama in 516 ShortCommunications [Auk, Vol. 118

.•< April2001] ShortCommunications 517

American Robin

-- AmericanTree Sparrow

_ Ruby-crownedKinglet

_ Dark-eyedJunco

-- Fox Sparrow

Yellow-rumpedWarbler

White-crownedSparrow

Orange-crownedWarbler

Wilson's Warbler

-- Gray-cheekedThrush

-- Lincoln'sSparrow Nearctic-Nearctic Migrant --

Swainson's Thrush

_ Nearctic-Neotropic 1 Migrant BlackpollWarbler

-- SavannahSparrow [•] Frost-free

-- Temperatures Hammond'sFlycatcher

--

Northern Waterthrush

--

Yellow Warbler

-- Alder Flycatcher

--

Average

_

Extreme

I 125 150 175 200 225 250 275

Juliandates of springand autumn passage and frost-free temperatures FIG.1. Species-levelestimate of thenumber of dayspasserine migrants occupy breeding ranges in Alaska usingmedian dates of springand autumnpassage. Number of frost-freedays are calculatedfrom temper- ature records(n = 95 years) in Fairbanks,Alaska.

1990 (Woodreyand Chandler 1997). The same was in adults of that speciesprior to autumn migration true for 1991, with the exception of Long Point, (Dwight 1900)probably accounts for thisdifference. where the median date of passageof adults was sig- Studiesat lower latitudeshave suggested that the nificantly earlier than immatures (Woodrey and duration of the breedingseason for someNearctic- Chandler 1997). Neotropicmigrants is brief.Winker et al. (1992)stud- The Alder Flycatcherwas the only speciesin our ied passerinemigration in Minnesotaat --45øNand studyin whichadults preceded immatures in autumn. concludedthat many speciesspend <30% of their We estimatedthat adult Alder Flycatchersdeparted 13 annual cycle on their breeding grounds. In this days earlierthan immatures,which is similar to pre- study,at 64ø5'Nand muchnearer to the final desti- viousresults from Long Point,Ontario where a 14 day nationsof high-latitudemigrants, we estimatedthat differencewas found (Hussel 1991a).The lack of molt populations of Nearctic-Neotropic migrants cap- 518 ShortCommunications [Auk, Vol. 118

tured at our study site occupytheir breedingranges are not available and are therefore able to withstand for <25% of their annual cycles,and that Nearctic- early and late frostsin extremeyears. Nearcticmigrants average 33% of their annualcycles Summer at high latitudes is brief. Nearctic-Nearc- on thosehigh-latitude breeding ranges. tic migrantsaveraged 119.8 days between spring and The 48 day differencebetween median spring and autumn median datesof passage,and Nearctic-Neo- autumn passagedates of Alder Flycatchersin inte- tropic migrants averaged90.6 days. All but one Ne- rior Alaska suggeststhat that speciesspends just arctic-Neotropic species, the Yellow-rumped War- 13% of its annual cycle on thosenorthern breeding bier, appearedto time their arrivals and departures grounds. That is the shortest period of breeding to occurwithin the long-termaverage of 105 daysof range occupancyyet documentedfor a populationor frost-freetemperatures in Fairbanks,Alaska (Fig. 1). speciesof migratorypasserine. In comparison,a low- Nearctic-Neotropic migrants rely primarily on in- er-latitudestudy estimated73 daysbetween median sectsfor food, and are likely to be presentas that re- dates of spring and autumn passageof Alder Fly- source waxes, becomes abundant, and then wanes. catchers(adults only; Hussel 1991a,b). Conversely,most of the Nearctic-Nearcticspecies we We estimatedthat someAlaska populations of Yel- studied rely on a seed-or fruit-dominant diet in win- low Warblersoccupied their breeding range for 84 ter, accountingfor their ability to be presentduring days, which suggestsdecided compressionwhen periods of frost and indeed frozen conditions.Fur- comparedwith the 104days estimated to berequired ther, Nearctic-Nearctic migrants may be better to accomplishbreeding-season activities in Manito- adapted to adverseclimatic conditions,which they ba, at 50øl'N (Busbyand Sealy 1979).Other studies may also experienceon wintering areas. In north- of YellowWarblers also indicate that breedingactiv- westernNorth America,the Nearctic-Neotropicmi- ities at high latitudesare indeedcompressed. Studies grants we studied time their breeding-rangeoccu- by Briskie (1995) and Rimmer (1988) provide an es- pancy to occur within the summer frost-free period timate of 79-115 days required by YellowWarblers and complete their annual reproductivecycle in a for breedingrange activitiesat other high-latitude brief temporal window. sites, and that is concordant with our estimate of 84 Acknowledgements.--Wegratefully thank Thomas daysfor completionof breedingseason activities for H. Pogson,the founder of the Alaska Bird Observa- this speciesin interior Alaska. tory, for his vision and expertisein developinga Breeding-seasonactivities begin later in Alaska long-termmist-netting program. We thank the many comparedwith other latitudes.For example,in Pal- volunteers, interns, and staff who have made contri- omarin County, California, juvenile Swainson's butions to the data collectionprocess at ABO; N. D. Thrushes have been observed as early as 15 May DeWitt, T. H. Pogson,and S. K. Springerin particular (Johnsonand Geupel 1996), and juvenile Wilson's banded thousands of birds here. Ted Fathauer (Na- Warblershave been observed as early as 15 April tional WeatherService) provided insight into the use (Chaseet al. 1997).The mediandate of springarrival of weather data. Members of the Alaska Bird Obser- of adults of thosetwo speciesin Fairbanks,Alaska, vatory funded this researchtogether with large con- does not occur until 15 May. Further, in Orange- tributions from ABR, Inc. Environmental Research crowned Warblers, nest construction has been ob- and Services,Alaska Departmentof Fish and Game, servedin early March on Channel Island, California ARCO Alaska, the Bureau of Land Management, (Sogge et al. 1994). In contrast, Orange-crowned Warblers do not arrive in central Alaska until -75 Exxon Company,USA, the SkaggsFoundation, and the U.S. Fish and Wildlife Service. A.M. Benson also days later. thanksthe WilsonOrnithological Society for the Paul The latest date on which freezing temperatures A. StewartAward. N. D. DeWitt, E. C. Murphy,L. T. have occurredin spring in Fairbankswas 13 June Quakenbush,and E. A. Rexstadimproved the man- (1922),and the earliestdate of freezingtemperatures uscript with their insightful comments. in autumn was 3 August (1944; based on a 95-year National Weather Service record). At the extreme, therefore,that regionhas the possibilityof delivering LITERATURE CITED only 51 consecutivedays of temperaturesgreater than 0øC. In that region, Alder Flycatchersarrive BENSON,A.M. 2000.Temporal patterns of migration, near the recordlast date of frost in spring and leave molt, and fat storage among high-latitude pas- before the record first date of frost in autumn (Fig. serine migrants. M.S. thesis,University of Alas- 1). That is probably not a coincidence.It is well ka Fairbanks, Fairbanks. known that flying insectavailability is negativelyaf- BRISKIE,J. V. 1995.Nesting biology of the YellowWar- fected by frosts, and that would seem to provide bler at the northernlimit of its range.Journal of strong selectionagainst Alder Flycatchers'occupa- Field Ornithology66:531-543. tion of their breeding grounds during times of po- BUSBY,D. G., ANDS. G. SEALY.1979. Feeding ecology tential frost. Other insectivorousbirds probablyfor- of a populationof nestingYellow Warblers.Ca- age on a more diverseprey basewhen flying insects nadian Journalof Zoology57:1670-1681. April 2001] ShortCommunications 519

CHASE, M. K., N. NUR, AND G. R. GEUPEL. 1997. Sur- RAPPOLE,J. H., E. S. MORTON, T. E. LOVEJOYIII, AND vival, productivity, and abundancein a Wilson's J. L. RUOS.1983. Nearctic Avian Migrants in the Warblerpopulation. Auk 114:354-366. Neotropics.U.S. Department of the Interior, Fish COOPER,B. A., AND R. J. RITCHIE. 1995. The altitude and Wildlife Service,Washington, D.C. of bird migration in east-centralAlaska: A radar RIMMER,C. C. 1988.Timing of the definitiveprebasic and visual study. Journalof Field Ornithology molt in Yellow Warblers at JamesBay, Ontario. 66:590-608. Condor 90:141-156. DWIGHT,J., JR. 1900. The sequenceof plumagesand SOGGE, M. K., W. M. GILBERT, AND C. VAN RIPER III moults of passerinebirds of New York. Annals 1994. Orange-crownedWarbler (Vermivoracela- of the New York Academyof Sciences13:73-360. ta). In The Birds of North America, no. 101 (A. HUSSEL,D. J. T. 1991a. Fall migrations of Alder and Pooleand E Gill, Eds.).Academy of Natural Sci- Willow flycatchersin southernOntario. Journal ences, Philadelphia, and American Ornitholo- of Field Ornithology 62:260-270. gists' Union, Washington,D.C. HUSSEL,D. J. T. 1991b. Spring migrations of Alder WINKER, K., D. W. WARNER, AND A. R. WEISBROD. and Willow flycatchers in southern Ontario. 1992. Migration of woodlandbirds at fragment- Journalof Field Ornithology62:69-77. JOHNSON,M.D., AND G. R. GEUPEL.1996. The im- ed inland stopoversite. Wilson Bulletin 104:580- 598. portance of productivity to the dynamics of a Swainson'sThrush population.Condor 98:133- WOODREY,M. S., AND C. RAYCHANDLER. 1997. Age- 141. relatedtiming of migration:Geographic and in- KESSEL,B. 1984. Migration of Sandhill Cranes,Grus terspecificpatterns. Wilson Bulletin 109:52-67. canadensis, in east-central Alaska, with routes Received31 January2000, accepted27 November2000. through Alaska and westernCanada. Canadian Associate Editor: E Moore Field-Naturalist 98:279-282. MCINTYRE,C. L., AND R. E. AMBROSE.1999. Raptor migrationin autumn throughthe Upper Tanana River Valley,Alaska. WesternBirds 30:33-38.

The Auk 118(2):519-524, 2001

Growth Rate and Energeticsof Arabian Babbler (Turdoidessquamiceps) Nestlings

AVNER ANAVA, 1'2MICHAEL KAM, 2 AMIRAM $HKOLNIK, 3 AND A. ALLAN DEGENTM lDepartmentof Life Sciences,Ben-Gurion University of theNegev, Beer-Sheva 84105, Israel; 2DesertAnimal Adaptations and Husbandry, Wyler Department of DrylandAgriculture, JacobBlaustein Institute for DesertResearch, Ben-Gurion University of theNegev, Beer-Sheva 84105, Israel; and 3Departmentof Zoology,George S. WiseFaculty of Life Sciences,Tel Aviv University,Tel-Aviv 69978, Israel

ABSTRACT.--ArabianBabblers (Turdoidessquami- lings was 0.450,which was 18%higher than that pre- ceps)are territorial, cooperativebreeding passerines dicted for a passerineof its body mass.Asymptotic that inhabit extreme deserts and live in groups all body mass of fledglings was 46 g, which was only year round. All membersof the group feed nestlings 63% of adult body mass,a low percentagecompared in a singlenest, and all group membersprovision at to other passerines.Energy intake retained as energy similar rates. Nestlings are altricial and fledge at accumulatedin tissuedecreased with age in babbler about 12 to 14 days,which is shortfor a passerineof nestlingsand amountedto 0.29 of the total metabo- its body mass. Becauseparents and helpers feed lizable energyintake over the nestlingperiod. How- nestlings,we hypothesizedthat the growth rate of ever, energy content per gram of body mass in- nestlingsis fast and that they fledge at a body mass creasedwith age and averaged4.48 kJ/g body mass. similar to other passerinefledglings. Using a logistic We concludedthat our hypothesiswas partially con- growth curve, the growth rate constant(k) of nest- firmed. Growth rate of babbler nestlingswas rela- tively fast comparedto other passerinespecies, but fledgling masswas relatively low. Address correspondenceto this author. E-mail: Desertsare characterizedby unpredictablerainfall [email protected] and unpredictable, often sparse, food availability