Age-related differences in foraging efficiency, molt, and fat deposition of Gray Catbirds prior to autumn migration

The ability to satisfy the energy requirements of migration and to cope with problems that arise en route is likely to be age dependent. We studied hatch-year (HY) and after-hatch-year (AHY) Gray Catbirds (Dumetella carolinensis) to determine if differences in foraging efficiency and molt affected fat deposition prior to autumn migration. Hatch-year catbirds were less efficient foragers than AHY catbirds; however, efficiency improved with age to the extent that many HY catbirds are probably as efficient as AHY birds by the time they commence migration. We discovered age-dependent patterns of fat deposition, which probably resulted from differences in foraging efficiency and molt timing. Hatch-year catbirds began to deposit fat sooner than AHY catbirds but did so more slowly throughout the postbreeding and premigratory phases of the annual cycle. Adult catbirds did not become hyperphagic until they neared the completion of their molt, then deposited fat rapidly.