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.

To assess how the high-sugar/low-protein content of fruit diets affects digestive function and nutrition of frugivorous birds, I compared intake, passage rate, sugar utilization, protein requirements, and mass changes of cedar waxwings (Bombycilla cedrorum), American robins (Turdus migratorius), and wood thrushes (Hylocichla mustelina) fed synthetic diets simulating the range of sugar (6.6%, 12.4%, and 22.0% solutes) and protein (4.5%, 3.0%, and 1.5% of dry matter) content of bird-dispersed fruits. The dietary emphasis on sugary fruits by cedar waxwings suggests the potential for digestive and physiological specializations to this food type. All birds increased volumetric food intake and passage rates as sugar concentration declined. Birds completely (22.0%-12.4% sugar solute concentration) or incompletely (12.4%-6.6% sugar solute concentration) compensated for dietary dilution. Cedar waxwings consumed each diet at higher rates than did thrushes, as they do when eating sugary fruits, demonstrating that interspecific differences in ingestion rates of sugary fruits are a consequence of nutrient composition, rather than seed bulk or secondary compounds of fruits. Passage rate was not responsible for interspecific differences in short-term food intake rate, implicating gut morphology as the key functional feature limiting intake. Most sugary fruits are nutritionally deficient in apparent protein for thrushes but are nutritionally adequate in protein for cedar waxwings because of this species' relatively high intake rates and low protein requirements. The digestive systems of frugivorous birds respond flexibly to dietary sugar concentration, but protein content of fruits can present a nutritional limitation, potentially influencing the proportions of fruit and animal foods in birds' diets.

The diet of Cedar Waxwings (Bombycilla cedrorum) is described using records of gut contents collected by the U.S. Biological Survey. Cedar Waxwings eat more fruit than most other Temperate Zone birds, including one of the most frugivorous thrushes, the American Robin (Turdus migratorius; 84 vs. 57% fruit in their annual diets, respectively). Cedar Waxwings are almost exclusively frugivorous in the winter and early spring. During the spring period of fruit scarcity, flowers comprise a large portion of the diet of waxwings (44% of May diet). Cedar Waxwings eat aerial and vegetation-borne animal prey, whereas American Robins eat vegetation-borne and terrestrial prey. The fruits eaten by Cedar Waxwings are characterized by high sugar and low lipid content. American Robins, like other North American thrushes, eat sugary and lipid-rich fruits, suggesting contrasting digestive strategies in waxwings and thrushes. This perspective is reinforced by the correspondence between these birds' diets, the timing of breeding in relation to availability of preferred foods, and flocking patterns.

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