Migratory disposition and choice of diet by the yellow-rumped warbler (Dendroica coronata)

Al•stl•^ct.--Because migration is a period of exceptional energy demand and surely exerts strong selective pressure for efficient foraging, we expected Yellow-rumped Warblers (Dendroica coronata) to be especially efficient foragers when in migratory disposition and to select a diet that more nearly achieves energy maximization relative to control individuals not in migratory disposition. We induced migratory disposition in experimental birds by artificially prolonging daylength, then provided both controls and experimentMs with a choice among large, medium, and small larvae (Tenebrio molitor) that varied in calories per unit handling time. Diet choice was studied during the premigratory fattening period, as experimentMs gained weight and deposited subcutaneous fat, and later as experimentals attained maximum weight and began to show nocturnal restlessness. Warblers in migratory disposition consumed more food items per feeding bout; handled items more rapidly, thereby increasing the net energetic value of the food they consumed; and fed selectively on more profitable items than control individuals. As a result of these adjustments in their food intake, Yellowrumped Warblers in migratory disposition foraged more efficiently and came closer to achieving energy maximization during the premigratory fattening period and the subsequent migratory period. Received 19 November 1984, accepted 8 May 1985.