Ecological and evolutionary implications of energy and protein requirements of avian frugivores eating sugary diets

To assess how the high-sugar/low-protein content of fruit diets affects dig estive function and nutrition of frugivorous birds, I compared intake, pass age rate, sugar utilization, protein requirements, and mass changes of ceda r waxwings (Bombycilla cedrorum), American robins (Turdus migratorius), and wood thrushes (Hylocichla mustelina) fed synthetic diets simulating the ra nge of sugar (6.6%, 12.4%, and 22.0% solutes) and protein (45%, 3.0%, and 1 .5% of dry matter) content of bird-dispersed fruits. The dietary emphasis o n sugary fruits by cedar waxwings suggests the potential for digestive and physiological specializations to this food type. All birds increased volume tric food intake and passage rates as sugar concentration declined. Birds c ompletely (22.0%-12.4% sugar solute concentration) or incompletely (12.4%-6 .6% sugar solute concentration)compensated for dietary dilution. Cedar waxw ings consumed each diet at higher rates than did thrushes, as they do when eating sugary fruits, demonstrating that interspecific differences in inges tion rates of sugar fruits are a consequence of nutrient composition, rathe r than seed bulk or secondary compounds of fruits. Passage rate was not res ponsible for interspecific differences in shortterm food intake rate, impli cating gut morphology as the key functional feature limiting intake. Most s ugary fruits are nutritionally deficient in apparent protein for thrushes b ut are nutritionally adequate in protein for cedar waxwings because of this species' relatively high intake rates and low protein requirements. The di gestive systems of frugivorous birds respond flexibly to dietary sugar conc entration, but protein content of fruits can present a nutritional limitati on, potentially influencing the Proportions of fruit and animal foods in bi rds' diets.