Main foods taken. Arthropods (insects, spiders, crustacea, centipedes, scorpions), mollusks, fruits, nectar, seeds (sorghum, millet, maize, sunflower), food of domestic animals (dogs monkeys, cattle). At bird feeders, takes granulated cane sugar, cooked rice, bread, mangos, and bananas.
Major food items. In southwestern Puerto Rico (Post 1981), food delivered to nestlings (163 items) consisted of immature Lepidoptera, 56% (92 of 163 items); Araneae, 18%; Homoptera, 11%; Orthoptera, 10%; Coleoptera, 3%; Hymenoptera and unidentified Arachnida, <1 %.
Larval moths (Olethreutidae) occurred most often: 14 of 24 samples (56%), followed by spiders (Anyphaenidae, (40%). Vegetable matter (bread, sugar, fruit, rice, monkey chow) was found in 60% of samples, broken into small pieces, but composed a relatively small part of the nestling diet.
Order of importance, based on cumulative sizes in diet: larvae of Olethreutidae, 78%; pupae of Noctuidae, 8%; Gryllidae, 5%; larvae of Noctuidae, 3%.
Birds probe for nectar in January and February when aloe (Aloe vulgaris) is blooming (fig,). Yellow-shoulders also probe yucca flowers. Wetmore (1916) saw Yellow-shoulders probe bucare (Erythrina sp.) blossoms, and Danforth (1926) reported them taking nectar from guama (Inga laurina), as well as fruits of several species of cacti, e.g. Selenicerus sp. and Cephalocereus royenii.
On Mona Island, the most frequently utilized foods were nectar (47% 0f 273 observations), fruits (38%), and seeds (14%). Plants from which fruits and seeds were extracted were Cissus trifoliata (32% of 154 observations), Harrisia portoricenxis (12%), Reynosia uncinata (10%), Paspalum rupestre (7%), Panicum maximum (6%), and Bursera simarumba (6%). Plants utilized for nectar and sap were primarily Aloe vera (87% of 133 observations). Invertebrate foods were insects (62% of 273 records), spiders (28%), and mollusks (3%) (Hernandez-Prieto and Cruz 1989).
Feeding Microhabitats. In breeding season in southwestern Puerto Rico, 18% of 648 maneuvers were in outer canopy, 29% in outer subcanopy; 18% each in middle and inner subcanopy. Birds rarely foraged on ground (9%) or in herb layer (7%). In Salinas habitats, did not forage on the open mud in the nest vicinity, but occasionally fed on trunks and branches of nearby mangroves (Post 1981).
In the post-breeding period in southwestern Puerto Rico, foraging zones (n=133 observations) were: canopy, 63.9%; subcanopy, 20.3%; ground, 15.8%. The foraging substrates were mesquite, 51%; ground and forbs, 20%; ucar, 8%; gumbo-limbo, 4%; Pithecellobium, 4%; tamarind, 3%; eight other plant species, (each < 2%), 10% (T. K. Nakamura, unpublished report).
On Mona Island, the most important foraging zones were: the shrub layer (28% of 599 observations), herb layer (23%), lianas and vines (13%), understory of trees (12%), and upperstory of cacti (11%) (Hernandez-Prieto and Cruz 1989).
Foraging tactics. In southwestern Puerto Rico (Post 1981), breeding season foraging tactics, in order of frequency (648 observations) were: probing, 45%; gleaning, 39%; flycatching and hovering, 8%; pecking, 5%; and chasing, 2%. Arboreal foraging composed 91% of the maneuvers. The remaining 9%, pecking, chasing and fruit-probing, occurred on the ground.
When foraging above ground, blackbirds glean leaves and twigs as they walk on branches; also probe crevices while clinging to sides of trunks or undersides of branches. Probe leaf clusters or clumps of epiphytes Tillandsia recurvata while standing on top or hanging below. Also gape inside holes, fruit, buds, and cocoons, Most of arthropods brought to young are either always hidden (Buprestidae and immature Olethreutidae) or are nocturnal and hide during the day in crevices, leaf clusters, or epiphytes (Noctuidae, Gryllidae, and Anyphaenidae).
Breeding season foraging tactics on Mona Island: probing flowers, 23% of 599 observations; gleaning fruit clusters, 22%; gleaning live wood, 14%; gleaning leaves, 11%; probing wood 4%; gleaning spider webs, 4% (Hernandez-Prieto and Cruz 1989).
Ground foraging is common in nonbreeding season. Yellow-shoulders walk, occasionally scratching with one foot; double-scratch not seen. They gape under large items to turn them over, pushing the food away from the body; also insert bill under such items (e.g., monkey biscuits) and overturn them toward body. Yellow-shoulders use sides as well as tip of the beak to break off material from large pieces of food, sides of the beak being moved back and forth in a shearing motion. Blackbirds attending monkey feeding stations carried biscuits to water troughs, dipping them before eating. Yellow-shoulders probe flowers by fully inserting beaks, gaping to enlarge entrances; carry foods, or vegetation containing food, to branches, where held down with one foot and probed or sheared. To obtain food, also enters enclosed spaces, such as monkey feeders and garbage cans.