Annual Diet of Cedar Waxwings Based on U.S. Biological Survey Records (1885-1950) Compared to Diet of American Robins: Contrasts in Dietary Patterns and Natural History

Abstract

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 Amer- ican 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. Received 24 February 1995, accepted 25 August 1995. ECOLOGISTS OFTEN equate caloric content of foods with nutritive value without considering the specific nutrient composition of foods and the digestive characteristics of consumers (e.g. Martin 1985). Such assessments assume that an- imals are similar in the ways that they process and assimilate foods and that digestion of var- ious nutrients does not vary among species. This perspective has probably resulted from the his- torical importance of energy in ecological the- ory (Elton 1927, Lindeman 1942, Schoener 1971, Krebs 1978, McKey 1975). Although knowledge of caloric content of foods can be informative in systems where animals have comparable di- gestive characteristics and foods are chemically similar (Pyke 1980), this approach ignores cru- cial information in comparisons between ani- mals with different digestive systems eating chemically diverse diets. Detailed comparisons of dietary habits can identify specializations to particular foods and corresponding limitations imposed by such adaptations (e.g. Walsberg