Many songbirds are seasonally frugivorous and eat primarily fruit during migration and insects or seeds during nonmigratory periods. Previous work has suggested that most wild fruits may have inadequate protein for birds. Assessing the nutritional adequacy of fruit requires knowing the protein requirements of birds in relation to the composition of available fruits. We tested predictions of two hypotheses: (1) interspecific differences in protein requirements of birds are related to their foraging strategy; and (2) protein requirements of birds increase with demand, for example during migratory periods of the annual cycle. We measured the protein requirements of the omnivorous Hermit Thrush (Catharus guttatus) and the granivorous White-throated Sparrow (Zonotrichia albicollis) during nonmigratory and migratory stages of the annual cycle and compared the results with published estimates for other songbird species. In the nonmigratory state both species ate less, lost body mass, and had more negative nitrogen balance as dietary protein decreased. In the migratory state Hermit Thrushes lost body mass and had lower nitrogen balance but did not reduce food intake as dietary protein decreased, whereas White-throated Sparrows did not change body mass, food intake, or nitrogen balance as dietary protein decreased. Both species had lower protein requirements during migration (9.3 mg N day-1 and 15.8 mg N day1, respectively) than during nonmigratory periods (53.1 mg N day-1 and 46.0 mg N day-1, respectively) when fed a diet containing 15.9 kJ g-1. These findings may partially explain how birds can adequately refuel on protein-limited foods such as fruits during migration.
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Vol. 127 • No. 4