The Diet of American Robins: An Analysis of U.S. Biological Survey Records

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Abstract
I present a quantitative description of the diet of American Robins (Turdus migratorius) and consider how food habits (particularly the proportion of fruit eaten and the diversity of individual meals) are influenced by season, habitat, sex, and time of day. The study is based on an analysis of records of stomach contents compiled by the U.S. Biological Survey and the U.S. Fish and Wildlife Service. Across their entire range, robins ate fruits representing over 50 genera and invertebrates representing over 100 families. Diets were diverse even within local regions, and there was no obvious single feeding niche. The major food classes, consumed in every combination, were soft-bodied invertebrates, hard-bodied invertebrates, and fruits. The same taxa (especially fruits of the family Rosaceae and invertebrates of the orders Coleoptera and Lepidoptera) predominated in robins' diets, irrespective of habitat or geographical location, which presumably reflects both selective foraging and the availability of these widespread taxa. The proportion of fruit (by volume) in the diet was much higher in the fall and winter (median values >90%) than in the spring (<10%); summer values were intermediate. The transition from a diet dominated by invertebrates to a diet dominated by fruits occurred over a 1-2-month period. The number of distinct food items in stomachs, a measure of the species diversity of individual meals, was positively correlated with the fraction of invertebrates in the diet. Thus, at the time of year when robins were dependent on fruits for food, the diversity of their meals was also lowest. The degree of fullness of the stomach showed few consistent trends with season or habitat. Despite different sex roles and nutritional requirements, male and female robins did not differ in the proportion of fruit in the diet in any month or in any region. Nor did their stomachs contain different numbers of distinct food items, different amounts of food, or a different range or distribution of prey taxa. Habitat was an important variable explaining dietary differences. The Biological Survey records have unavoidable shortcomings, most notably problems of sampling biases and the inability to correct for differential digestion of food items. Nonetheless, they are a valuable and underused data base for testing hypotheses, generating new questions of ecological interest, and describing in detail the diets of North American bird species.