Age-Related Differences in the Stopover of Fall Landbird Migrants on the Coast of Alabama

Abstract

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, White- eyed 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 de- mands and meet en-route contingencies. Prob- lems 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, Lindstr6m