Fundamental to our understanding of the ecology of animal communities in the tropics is knowledge of the effect of seasonal changes in the abundance of food sources in consumer diets. We determined stable-isotope composition ( 13C/12C and 15N/14N) in whole blood of 14 resident avian species in a tropical dry forest to quantify the origin of their assimilated protein. We used a probabilistic approach (IsoSource) to estimate the relative contribution of C3 plants, CAM-C4 plants, C3 insects, and CAM-C4 insects during the dry and rainy seasons. IsoSource iteratively creates each possible combination of source contribution and produces a distribution of all feasible combinations that adequately predict the observed isotopic signature of the consumer. Granivore-frugivores and granivore-frugivore-insectivores were modeled as predominantly dependent upon plants whereas insectivorous birds were modeled to derive protein almost exclusively from insects. Between these extremes there were several species using mixed diets such as insectivore-frugivores or insectivore-granivores. In most species, virtually all assimilated food was of C3 origin with the exception of Ruddy Ground-Doves (Columbina talpacoti) in which CAM or C4 plants contributed significantly. Seasonal changes in relative food source contribution were followed in eight species of birds. Of these species, White-tipped Doves (Leptotila verreauxi), Grayish Saltators (Saltator coerulescens), and Social Flycatchers (Myiozetetes similis) increased their use of insects in the rainy season, in contrast to Great Kiskadees (Pitangus sulphuratus), which decreased their use of insects. Our study suggests that that diverse strategies are used by various avian species to obtain dietary proteins within seasonal habitats. © 2006 The Author(s).