The Bay-headed Tanager is an omnivore; it consumes a diet of seeds, fruits, and insects. Although this species is omnivorous, it favors fruit over arthropods. Isler and Isler (1999) summarize stomach content information from 12 individuals: 10 had vegetable matter, and two had both animal and vegetable matter. Specific items found in these stomachs included fruit, large seeds, and insects. At a site in Costa Rica, Naoki (2003) found that 82.4% of foraging observations involved fruit, while 17.6% of observations involved arthropods (n=336). Although arthropod foraging increased in the breeding season at this site, the species still preferred fruit (Naoki 2003). In Bolivia, 72.3% of foraging observations involved fruit, while 27.7% of foraging observations involved arthropods (n=83) (Naoki 2003). Birds never were observed feeding on müllerian bodies, flower buds, or nectar in either Bolivia or Costa Rica. In Trinidad, fruit made up 70% of 564 foraging observations (Snow and Snow 1971) and in Colombia it consisted of 60% of 188 observations (data from Steven Hilty, cited by Isler and Isler 1999).
In Trinidad and Colombia, 43 different species of fruing plants were used (Isler and Isler 1999). Prefereed fruits include Melastomaceae (including Miconia), Cecropia, and Ficus (Isler and Isler 1999). Other species taken include the epiphytic vine Souroubea guianensis and other arillate fruits, flower buds of Topobea brachyura, berries of melastomes and the epiphytic shrub Lycianthes synanthera, and Coussapoa fruits (Skutch 1980, Stiles and Skutch 1989, Isler and Isler 1999). In addition to taking Cercropia fruit, the species has been observed eating protein corpuscles from the bases of leaf petioles in Costa Rica (Skutch 1954). However, this behavior was not observed in Trinidad (Snow and Snow 1971). In Costa Rica, fruit taken include: Ficus (37.3%), Miconia (28.0%), Cecropia (15.8%), Conostegia (8.4%), Coussapoa (7.4%), Cavendishia (1.0%), Hedyosmum (0.6%), Satyria (0.6%), Rubus (0.3%), Sabicea (0.3%), and Tetrochidium (0.3%) (Naoki 2003). In Bolivia, fruit taken include: Miconia (47.2%), Guettarda (20.8%), Cestrum (17.0%), Cecropia (8.5%), Tournefortia (3.8%), and Ficus (1.9%) (Naoki 2003).
Little data is available on specific arthropod species in their diet; however, the following have been observed: coleoptera, small moth, and small winged insects (Snow and Snow 1971, Isler and Isler 1999).