The Yellow-shouldered Blackbird, Agelaius xanthomus, is a diurnal blackbird prevalent to the archipelago of Puerto Rico and one of the eleven species belonging to the Agelaius genus of the Icteridae family. As a result of forest loss due to sugar cane farming and housing development, this bird is now limited to only three areas of Puerto Rico: Mona Island and Monito Island, the Roosevelt Roads Naval Station area in eastern Puerto Rico, and the southern Puerto Rican dry forests and mangroves.
The Yellow-shouldered Blackbird is a glossy black bird with a small yellow shoulder patch outlined by a white margin. Immature individuals possess a duller coloration and a brown abdomen. Although plumage coloration is identical between the sexes, sexual dimorphism is present in this species with males being larger than females. Adult individuals measure from 7.8 to 9 inches and weigh 1.2 to 1.44 ounces.
Yellow-shouldered Blackbirds are omnivorous, but are considered to be arboreal insectivores since the majority of their diet consists of insects. Aside from natural material, the species also consumes processed food such as cattle ration, human food (cooked rice and sugar), dog food and monkey chow. Plant matter was acquired from processed foods while insects are gleaned from the canopy and sub-canopy layers of trees.
The Yellow-shouldered Blackbird breeding season commonly spans from
April to August but breeding activity has been observed from February to November.
The species is believed to be monogamous with a single attempt at nesting per year and with nesting being performed in loose colonies. Nests contain from 1 to 4 eggs with an average of 3 eggs. Eggs are blue-green with brown spots and are incubated for 13 days by the female. Nestlings leave the nest 13 to 16 days after hatching and reach sexual maturity at 1 year of age.
Yellow-shouldered Blackbirds engage in anting, a rare behavior only observed in the Puerto Rican Tanager among West Indian birds. Individuals were observed applying ants to its body and feathers for a short period of time.
The species also engages in mobbing, a behavior in which a pack of birds, from one or more species, attack a known predator (usually to defend eggs or hatchlings).