Contrasting digestive strategies of fruit-eating birds

1. Sugary fruits dominate the annual diet of Cedar Waxwings (Bombycilla cedrorum), whereas lipid-rich foods dominate the diets of frugivorous thrushes. Nutrient utilization and preferences of Cedar Waxwings and thrushes fed sugary and lipid-rich natural fruits were assessed to ascertain the potential for nutrient-based specializations that could explain diet selection in nature. 2. Compared allometrically, Cedar Waxwings showed higher rates of sugar assimilation from sugary fruits than did thrushes, by virtue of higher intake rates, and achieved the highest energy assimilation rates when eating a sugary fruit. A trade-off to this digestive strategy appears to be less efficient utilization of lipids when eating exclusively lipid-rich fruits. 3. Thrushes digested lipids more efficiently than Cedar Waxwings as a function of intake rate or estimated retention time, and thrushes achieved the highest energy assimilation rates when eating a lipid-rich fruit. Specialization to a diet rich in lipids appears to limit the rate at which sugary fruits can be processed. 4. Within each bird species, sugar digestive efficiencies were high and were not reduced at higher intake rates. Lipids were consumed at lower rates than sugars, and lipid digestive efficiencies declined with increasing intake rates. Frugivorous birds modulated digestive processing of fruits according to the time needed for efficient digestion and/or absorption of sugars and lipids, respectively. 5. Rate of nitrogen intake, not sugar assimilation, positively influenced body mass changes of birds. Sugary fruits appear nutritionally rich in energy, but limited in protein for avian frugivores. Relatively high intake rates and low protein requirements of Cedar Waxwings suggest that specialization to sugary, low-protein diets involves traits that facilitate acquisition and conservation of protein/amino acids. 6. Cedar Waxwings and thrushes show digestive specialization to the utilization of sugars and lipids, respectively, as dominant dietary nutrients. These traits explain patterns of food selection by these birds in the laboratory and in nature. Variation in the value of particular fruit nutrients to different birds renders the dietary descriptor of ‘frugivore’ ambiguous in a nutritional context.