The tawny-shouldered blackbird (Agelaius humeralis), a species from Cuba and Hispaniola, is morphologically intermediate between A. xanthomus and A. phoeniceus.

[3] The yellow-shouldered blackbird, as its name implies, is a glossy black bird with a small yellow humeral patch around its "shoulders" outlined by a white margin.

Although plumage coloration is indistinguishable between the sexes, sexual dimorphism is present in this species with males being larger than females.

The destruction of habitat and brood parasitism by the shiny cowbird (Molothrus bonariensis) led to a drastic population decline from the mid-1970s to early 1980s.

Studies performed in the island of Monito, located 5 km (3.1 mi) northwest of Mona, revealed an average of 25 individuals with breeding being observed.

The studies analyzed samples of food parents brought to nestlings, and found evidence of consumption of insects belonging to the orders Lepidoptera, Orthoptera, Hemiptera, Coleoptera, Diptera, Dermaptera and Hymenoptera, arachnid material of the order Araneae, unidentified molluscs, and plant matter.

Plant matter was acquired from processed foods while insects are gleaned from the canopy and sub-canopy layers of trees.

[6] Yellow-shouldered blackbirds engage in anting, a rare behavior only observed in the Puerto Rican tanager among West Indian birds.

[7] Individuals were observed applying Pheidole species ants to their body and feathers for a short period of time (8 minutes).

[8] Critical habitat was subsequently established for the entire Mona and Monito islands, the Roosevelt Roads Naval Station in Ceiba, an area expanding from Cabo Rojo to Guánica in the southwest region of Puerto Rico, and a small area in San Germán.

The mites Ornithonyssus bursa and Androlaelaps casalis have also been observed in nests, but the information presently available does not indicate they are a threat.

The majority of the activity coincided with the recession of water from July to August which exposed the mangrove nesting area to dry land.

Since the middle 1980s artificial PVC nesting structures have been created in mudflats surrounding mangrove forests to reduce rat predation.

The loss of feeding and breeding habitat and brood parasitism by the shiny cowbird are among other threats that limit and endanger the yellow-shouldered blackbird populations.