Age-related differences in the stopover of fall landbird migrants on the coast of Alabama

-We estimated fat load, length of stopover, and rate of mass change for six Neotropical migrant landbird species at a site along the northern coast of the Gulf of Mexico. Swainson's Thrushes (Catharus ustulatus), Gray Catbirds (Dumetella carolinensis), White-eyed Vireos (Vireo griseus), Red-eyed Vireos (V olivaceus), Magnolia Warblers (Dendroica magnolia), and American Redstarts (Setophaga ruticilla) were captured on Fort Morgan Peninsula in coastal Alabama during fall migration, 1990 to 1992. In Swainson's Thrushes, White-eyed Vireos, and American Redstarts, adults carried significantly higher fat loads than young birds, whereas no age-related differences in fat loads were evident in the other species. The likelihood of staying beyond the day of capture and the rate of change in body mass did not differ between age classes. One consequence of differences in fat load is reflected in flight range in relation to the Gulf of Mexico. On average, adult Swainson's Thrushes, White-eyed Vireos, American Redstarts, and both age classes of Magnolia Warblers carried sufficient energy stores to complete a trans-Gulf flight, whereas young Swainson's Thrushes, Whiteeyed Vireos, American Redstarts and both age classes of Gray Catbirds carried insufficient stores for such a flight, based on flight-performance simulations. Received 17 October 1996, accepted 13 May 1997. A SUCCESSFUL MIGRATION is dependent on how effectively migrants satisfy energy demands and meet en-route contingencies. Problems that arise during passage undoubtedly are magnified for hatching-year birds on their first migration (e.g. Ralph 1978, Gauthreaux 1982, DeSante 1983, Moore 1984, Lindstrom and Alerstam 1986). If the high cost of migration (i.e. reduced fitness, increased mortality) is absorbed largely by inexperienced, hatchingyear birds (Nisbet and Medway 1972; Johnson 1973; Alerstam 1978; Greenberg 1980; GossCustard et al. 1983; Ketterson and Nolan 1982, 1983, 1985; DeSante 1983), differential costs should be reflected in age-dependent differences in stopover biology. Moreover, individuals with different levels of migratory experience can be expected to respond differently to the exigencies of migration (e.g. Metcalfe and Furness 1984; Ketterson and Nolan 1985, 1988; Terrill 1988; Ellegren 1991). An important constraint during migration is the acquisition of adequate fat stores. Individuals in migratory disposition (sensu Berthold I Present address: Mississippi Museum of Natural Science, Mississippi Department of Wildlife, Fisheries, and Parks, 111 North Jefferson Street, Jackson, Mississippi 39202, USA. E-mail: woodrey@mmns.state.ms.us 1975) become hyperphagic and deposit fat, increasing their body mass by 30 to 50% (Berthold 1975, Blem 1980). If young migrants satisfy the energy requirements of migration and meet en-route contingencies with less proficiency than adults, they may carry less fat than adults when encountered at migratory stopovers. Alternatively, young migrants may deposit more fat than adults to increase their "/margin of safety" if they anticipate greater difficulty solving en-route problems by virtue of their inexperience or subordinate social status. Finally, fat loads may be independent of age, but young migrants may take longer to achieve the same level of preparation (i.e. they deposit fat at a slower rate than adults). Agedependent differences may arise not only because of constraints on the performance of young migrants, but also because young birds invest less time and energy in migration than adults because of disproportionate costs (see Wiley 1981, Curio 1983). We estimated fat stores, length of stay, and rate of mass change during stopover for young and adult landbirds at a stopover site along the northern coast of the Gulf of Mexico. We also examined the consequences of differences in fat load in relation to a simulated trans-Gulf flight. The movement of landbird migrants in relation to the Gulf of Mexico is a prominent feature of