THE JOURNAL OF RAPTOR RESEARCH A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC. VoI. 29 JuNw 1995 No. 2 j RaptorRes. 29(2):85-92 ¸ 1995 The Raptor ResearchFoundation, Inc. FLIGHT STRATEGIES OF MIGRATING OSPREY: FASTING VS. FORAGING GRADY L. CANDLER • AND PATRICIA L. KENNEDY 2 Departmentof Fisheryand WildlifeBiology, Colorado State University, Fort Collins, CO 80523 U.S.A. ABSTRACT.--Wedeveloped energetics models to predictmigration times and fat consumptionrates of osprey (Pandionhaliaetus) migrating south from their breedinggrounds in the IntermountainWest of North America. In thesemodels we simulatedthree migration strategies: fasting, foraging at severalmid-migration stopovers (jump strategy)and frequentforaging at stopovers(hop strategy).Because these piscivores appear to migrate predominantlyover land and are rarely seen along ridges used by othermigrant raptors that exploitdeflection updrafts,we assumedthey primarily used thermal soaring during migration. Our modelpredicts a 1.68-kg ospreywould take 12 d and 0.25 kg of fat (a fat densityof 15% of lean bodymass), to completea fasting migrationof 3780 km (mean of migrationdistances estimated from 21 band recoveriesof ospreynesting in northernIdaho and easternWashington) when wind speedis 0 m s-•. A sensitivityanalysis of this model showedthat changesin wind speed(+5 m s-l) had the greatestinfluence on migrationtime (8-21 d) and fat consumption(0.16-0.45 kg). In the foragingmodel, maximum fat depositionrate was predicted to be 0.04 kg d-•. Giventhis predictionand assumingosprey settle for 1 d at eachstopover, migrations with one,three (jumpstrategies), five or 11 (hopstrategies) stopovers were predicted to take17, 21, 25, or 34 d, respectively. With no settlingtime at stopoversthe predictedforaging migration times only range from 15-17 d. The model predictionsfor boththe foraging(without settling costs) and fasting migrations are consistent with the limited data availableon fall migrationstrategies of ospreybreeding in the IntermountainWest. Our resultsalso suggestthat, under certain assumptions, nonstop migration may be energeticallypossible for westerninterior osprey. KEYWORDS: aerodynamicmodel;fat deposition; migration strategies; osprey; Pandion haliaetus; sensitivity analysis. Estrategiasde vuelode Pandionhaliaetus migrantes; rapidez vs. forrajeo Rv_,SUMV,N.--Desarrollamos un modeloenerg&ico para predecirlos tiempos de migraci6ny tasasde consumo degrasas de Pandion haliaetus migrando hacia el surdesde sus fireas de reproducci6n en el oestede Norteamgrica. En estosmodelos simulamos tres estrategias de migraci6n:rapidez, forrajeo en variossitios de descansoen el transcursode la migraci6n("jump strategy") y forrajeofrecuente en sitiosde descanso ("hop strategy"). Debido a que estasaves piscivoras parecen migrar predominantemente sobre el campoy raramenteson observadas a lo largode cordones montafiosos, usados por otrasrapaces migratorias que explotan la deflexi6nde corrientes de aire,presumimos que ellas se remontan usando corrientes de aire ascendentes.Nuestro modelo predice que un individuode estaespecie de 1.68 kg podrlatomar 12 diasy 0.25 kg de grasaen recorrerrapidamente 3780 km (distanciamigratoria media, estimada de 21 individuosmarcados), cuando la velocidaddel vientoes de 0 m/s. Un anfilisisde sensibilidaddel modelomostr6 que cambiosen la veloddaddel viento(+5 m/s) tiene graninfluencia sobre el tiempode migraci6n(ocho a 21 dias)y sobreel consumode grasa(0.16-0.45 kg). En el modelode forrajeo, la tasamfixima de depositaci6n degrasa, se estim6 en 0.04 kg/dla. Dada esta predicci6n y presumiendoque P. haliaetusse detiene un dla en cadaparada, se presume que migracionescon una, tres ("jumpstrategies"), dnco o onceparadas ("hop strategies") toman 17, 21, 25 o 34 dias,respectivamente. Sin establecerseen cadaparada, el modelode forrajeopredice tiempos de migrad6ndel rangode 15 a 17 dias. Current address:New York BotanicalGardens, Bronx, NY 10458 U.S.A. PLK is the correspondingauthor. 85 86 GRADYL. CANDLERAND PATRICIAL. KENNEDY VOL. 29, NO. 2 Las prediccionesdel modelotanto para la migraci6ncon forrajeo (sin costosde establecimiento)como para migracionesrfipidas son consistentes con los limitados datos disponibles sobre las estrategias migracionales de otofiode P. haliaetusnidificantes en el oesteintermontafioso. Nuestros resultados sugieren que, bajo ciertas presunciones,la migraci6n sin detenciones puede ser energ•ticamente posible para esta rapaz proveniente del oeste interior. [Traducci6nde Ivan Lazo] The migrationstrategies of long-distancemigrants soaringover land but probablyresort to flappingflight have been the focusof extensiveresearch (see Baker when crossingthese water barriers(Kerlinger 1989, 1982, Kerlinger 1989, Alerstam 1990 and Berthold Alerstam1990). Osprey breeding in the Intermountain 1993 for reviews),but it is still unclearwhether many West seemto migrateinland over the semi-aridsouth- speciesof long-distancemigrants make stopoversto westernUnited Statesand CentralAmerica (Henny feed during migration,rely totally on pre-migratory and Van Velzer 1972,Melquist et al. 1978, Pooleand tissuedeposition to fuel their migration,or usea com- Agler 1987) and probablyuse thermal soaring for the binationof both strategies.Even the type of tissue majorityof their migration(Kerlinger 1989). We de- accumulatedfor energyreserves during migration has cidedto modelthe migrationstrategies of ospreyfrom come into questionin recent years (fat vs. protein the Intermountain West because the model would re- [Piersma1990, Lindstr6m and Piersma1993]). quire fewer variablesthan a migrationmodel of Eu- Using a bioenergeticsmodel, Smith et al. (1986) ropean ospreywhich would have to incorporatepa- predictedthat the broad-wingedhawk (Buteoplatyp- rmeters (and assumptions)describing both thermal terus)and the Swainson'shawk (Buteoswainsoni) could soaringand flappingflight. migrate from southernNorth America to northern In this paper we describemodels that predict fall SouthAmerica (4300 km) withouteating if they de- migrationtimes and fat consumptionrates for osprey positedfat amountingto 20-25% of lean bodymass breedingin the IntermountainWest usingthree mi- prior to migration.However, Smith et al. (1986) only gration strategies:(1) fasting;(2) foragingat several evaluatedone migrationstrategy (fasting). Similar to mid-migrationstopovers (,jump strategy [Piersma migrationmodels developed for passerinesand waders 1987]);and (3) frequentforaging at stopovers(hop (Castroand Myers 1989, Gudmundssonet al. 1991, strategy[Piersma 1987]). We presentresults of a sen- LindstriSmand Alerstam1992), they did not conduct sitivityanalysis of the fastingmodel in which the pa- a sensitivityanalysis of their model to evaluatethe rameter estimates were varied in a manner that reflects extentto whichtheir modelpredictions vary with un- their empiricalvariation. This analysisprovides in- certaintiesin parametervalues (see Kirkley 1991 and sightsinto the key factors influencing model predictions Goldstein and Smith 1991 for additional evaluations andindicates the extentto whichthe modelpredictions of the Smith et al. model). varywith the uncertaintiesin the parameterestimates. Furtherinsight could be providedon possibleraptor A preliminaryevaluation of the modelis alsopresented migrationstrategies by creatingmodels that compare in which we comparemodel predictions to empirical fastingand foragingmigration. However, the species estimatesof (1) ospreymigration times from lookout modeledshould be well-studiedsince empirical data observationsand bandingdata and (2) raptorfat den- are necessaryfor parameterestimation and for a pre- sities. liminaryevaluation of the realismof the model'spre- dictions.In the United States,the osprey(Pandion METHODS haliaetus)is oneof the few speciesof largeraptors for Fasting Migration Model. To determineif thesebirds which there are empiricaldata on its flight behavior coulddeposit sufficient fat to fuel a fastingmigration, an (Kerlinger 1989), and migrationdistance and time energeticsmodel was constructedbased on morphometrics, flight characteristics,diurnal activity patterns, energy cost of (Henny and Van Velzer 1972, Melquist et al. 1978, soaringflight, energycost of roosting,and wind speed.This Melquist and Johnson1984, Pooleand Agler 1987). modelassumes that fat depositsare the main sourceof energy Althoughthere is an abundanceof bandingdata on duringmigration since there are no data availableto estimate Europeanosprey migrations (•)ster16ff 1977, Alerstam the role of other fuel resourcesused by ospreyduring hU- gration.Values used as input parametersare shownas nom- 1990), thesemigrations occur across large water bar- inal valuesin Table 1. We assumeda migrationdistance of rierssuch as the Mediterraneanand Baltic seas 3780 km. This distanceis the mean of migrationdistances terl/Sff1977, Alerstam 1990). These osprey use thermal estimatedfrom 21 bandrecoveries of ospreynesting in north- JUNE 1995 OSPREYMIGRATION STRATEGIES 87 ern Idaho and easternWashington (Melquist et al. 1978, still allowingthem to glidefarther and take advantageof Melquist and Johnson1984, Johnsonand Melquist 1991). strongerthermals than if they choseVopt. Therefore, cross- Morphometricvalues were basedon averagemeasure- countryair speedof the osprey[Vxc, (m s-l)] wascalculated mentsof body mass,wing span and wing area for osprey usingan interthermalvelocity that was 2 m s-l lessthan Vopt breedingin New York (Kerlinger 1989) becauseno com- estimatedby Pennycuick'smodel. parabledata