Fruit preference by birds is a complex process based upon the morphology and spatial arrangement of fruits and on the physiological needs and capabilities of birds. In North America, most fruits can be divided into two groups based on nutritional content: those rich in sugars relative to lipids, and those rich in lipids relative to sugars. To investigate how fruit preference may change seasonally and to determine if it is correlated with physiological state, we designed a simple laboratory experiment using American Robins (Turdus migratorius) and artificial fruits. During summer and autumn, we offered eight robins a choice between synthetic sugar-rich and lipid-rich fruits of equal caloric value and then measured food intake and assimilation efficiency for each fruit type. Overall, robins preferred sugar-rich to lipid-rich fruits during both seasons. Robins had a higher assimilation efficiency for sugars than for lipids during both seasons, although assimilation efficiency of lipids increased significantly from summer to autumn. During experiments, robins consumed significantly more sugar-rich than lipid-rich fruits in summer but not in autumn. Coupling fruit intake with assimilation efficiency indicates that in summer, robins had a higher rate of energy gain from sugars than from lipids, but by autumn the rate of energy gain from lipids increased to nearly the same level as that from sugars. Our results suggest that robins prefer sugar-rich fruits because of their simple and fast rate of digestion, enabling higher rates of energy gain, but that lipid-rich fruits become important with the onset of autumn.