Essential amino acid requirements of granivorous and omnivorous songbirds and the provision of natural foods

Wild birds must consume certain amounts of protein and an appropriate balance of amino acids while inhabiting environments where foods often differ in the quantity and quality of available protein. The requirements for amino acids are well documented for domestic bird species but are largely unknown for wild birds, which makes it impossible to reliably assess the nutritional adequacy of foods eaten by wild birds. We measured the maintenance requirements for three essential amino acids (lysine, methionine, and arginine) in two species of songbird, the omnivorous Hermit Thrush (Catharus guttatus) and granivorous White-throated Sparrow (Zonotrichia albicollis). Hermit Thrushes and White-throated Sparrows had similar requirements for lysine (20.02 and 19.95 mg/day, respectively) and methionine (12.3 and 10.85 mg/day, respectively), whereas thrushes had lower requirements for arginine (18.07 mg/day) compared to sparrows (34.5 mg/day). Consistent with previous studies, most birds fed diets with inadequate essential amino acid concentrations reduced food intake and fecal output, lost body mass, and had lower, but not negative nitrogen balance. However, we provide the first evidence that songbirds overcompensate when they consume diets very deficient in lysine. Available data on amino acid concentrations in natural foods suggests that most insects contain relatively high concentrations of all essential amino acids, seeds likely satisfy requirements of lysine and arginine but not methionine for Hermit Thrushes and White-throated Sparrows, whereas fruits generally contain inadequate amounts of all essential amino acids. Therefore, birds that eat mostly fruit may consume enough protein but likely must eat other types of foods to satisfy their essential amino acid requirements.