Patterns of Stopover by Warblers during Spring and Fall Migration on Appledore Island, Maine

-Migrant warblers mist netted on Appledore Island, Maine, during spring and fall migration in 1990 and 199 1 allowed analysis of seasonal differences in stopover patterns, including recapture rates, stopover lengths, and changes in mass and fat during stopover. Northern Waterthrushes (Seiurus noveborucensis) and American Redstarts (Serophaga ruticilla) had higher recapture rates in the fall than in the spring on Appledore Island. The Ovenbird (Seiurus aurocapillus) and the Canada Warbler (Wilsonia canadensis) had significantly longer stopovers in the fall than in the spring. During fall migration, young Northern Waterthrushes were more likely to be recaptured than adults, and young American Redstarts had longer stopovers than adults. During spring migration, female Magnolia Warblers (Dendroica magnolia) were more likely to be recaptured than males. Higher numbers of both male and female American Redstarts were recaptured in the fall than in the spring. American Redstarts, Northern Waterthrushes, and Ovenbirds showed no significant increase in fat class in the spring, while exhibiting significant increases in fat class in the fall. These three species also exhibited significant increases in mass during fall migration, while only the Ovenbird significantly increased mass in the spring. These data suggest that migrants show species, age, and sex specific seasonal differences in stopover pattern as well as differences in mass and fat accumulation. Such differences are likely affected by hormone titer, risk of predation, seasonal/temporal changes in weather patterns, and/or differences in food availability. Moreover, stopover sites may be used differently as a result of seasonal differences in relation to ecological barriers as well as distances to either breeding or wintering grounds. Received 28 April 1993, accepted 20 April 1994. Few researchers have investigated differences in the stopover biology of migratory passerines between spring and fall migration (e.g., Rappole and Warner 1976; Cherry 1982; Safriel and Lavee 1988; Winker et al. 1992a, b, c; Weisbrod et al. 1993) and none of these studies was conducted along the western North Atlantic Ocean. Moreover, studies comparing age and sex differences in the stopover biology within species are few in number (e.g., Ellegren 1991, Lavee et al. 1991). Because patterns of passerine migration differ between seasons (Rappole and Warner 1976, Yom-Tov 1984) and timing of migration differs between age and sex groups (Francis and Cooke 1986, Ellegren 1991), patterns of stopover behavior may also differ among these groups and seasons. In the spring, migrants presumably must arrive at breeding sites early to establish quality territories, but not so early as to risk mortality, from ’ New York Cooperative Fish and Wildlife Research Unit, Fernow Hall, Cornell University, Ithaca, New York 14853. z U.S. Fish and Wildlife Service. 3 5643-A Harper’s Farm Rd., Columbia, Maryland 21044.