Mc. Witmer, Ecological and evolutionary implications of energy and protein requirements of avian frugivores eating sugary diets, PHYSL ZOOL, 71(6), 1998, pp. 599-610
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.