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.