The protein requirement for maintenance in the White-crowned Sparrow, Zonotrichia leucophrys gambelii

Abstract

Assumptions about potential mismatches between protein demand and protein availability are implicit in many hypotheses pertaining to the ecology of wild birds. However, few direct measures of the protein requirements of wild birds have been made. I evaluated the requirement for protein for maintenance of a wild granivorous passerine, the White-crowned Sparrow (Zonotrichia leucophrys gambelii). I estimated protein requirement from measures of endogenous nitrogen (N) loss, N balance, and changes in body mass in wintering adult birds. The experimental birds were kept indoors at 23 °C (8.5 h light: 15.5 h dark) and fed semisynthetic diets that differed only in their concentrations of protein (0–17.3%) and starch, which were reciprocally adjusted. The amino acid profiles of the protein in the diets were identical and were formulated to match the average amino acid profile of the mixed proteins of a diet containing grains, seeds, insects, and fruits. In White-crowned Sparrows, the protein requirements for maintenance of body mass (366 mg/d) and a positive N balance equal to that of birds ingesting the acclimation diets (436 mg/d) were 7.3 and 8.7% protein, respectively, in a diet containing 12.4 kJ apparent metabolizable energy per gram dry mass. The estimated minimum maintenance requirement for protein for N equilibrium, derived from regressing N excretion on N intake, equalled 264 mg protein/d, or ca. 5.3% dietary protein. Measures of endogenous N loss (endogenous urinary N loss plus metabolic fecal N loss) equalled 14.7 mg N/d, indicating that White-crowned Sparrows need to replace at least 92 mg body protein/d. Dietary protein content had little influence on the birds' utilization of dietary energy and daily metabolized energy. The apparent contradiction in the requirement for a positive N balance to maintain body mass is discussed.