The yellow-shouldered blackbird (Agelaius xanthomus), known in Puerto Rican Spanish as mariquita de Puerto Rico or capitán, is a species of blackbird endemic to Puerto Rico. It has black plumage with a prominent yellow patch on the wing. Adult males and females are of similar appearance. The species is predominantly insectivorous.
Taxonomy
The nominate form of the yellow-shouldered blackbird (A. x. xanthomus) was first described from Puerto Rico and Vieques in 1862 by Philip Sclater as Icterus xanthomus. The recognized subspecies A. x. monensis, or Mona yellow-shouldered blackbird, was described by Barnes in 1945 from the islands of Mona and Monito.
The species is closely related to, and possibly derived from, the red-winged blackbird (Agelaius phoeniceus). The tawny-shouldered blackbird (Agelaius humeralis), a species from Cuba and Hispaniola, is morphologically intermediate between A. xanthomus and A. phoeniceus. Until recently, some authors considered A. xanthomus as a subspecies of A. humeralis. The 1983 American Ornithologists' Union edition considered A. xanthomus, together with A. humeralis, a superspecies. The main physical difference between A. xanthomus and A. humeralis resides in their bills, with that of A. humeralis being broader toward the base.
Description
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. Immature individuals possess a duller coloration and a brown abdomen. Although plumage coloration is indistinguishable between the sexes, sexual dimorphism is present in this species with males being larger than females. Plumage abnormality is rare in this species. Adult individuals measure from ; on average, males weigh and females weigh . Sexual categorization may also be made by measurement of the wings, with males' being 1.1 times larger and having a mean length of , while females' wings have an average length of .
Conservation and habitat
The species was once commonly found in the coastal forests of the archipelago of Puerto Rico, but during the early 20th century, these forests were cut down to allow for the development of sugarcane plantations. After the decline of the sugar industry after the 1930s the coastal areas were developed for housing. As a result, the species is now limited to three areas: the islands of Mona and Monito, where a subspecies developed (A. g. xmonensis); the Roosevelt Roads Naval Station area in eastern Puerto Rico; and the southern Puerto Rican dry forests and mangroves. Although all three locations are considered coastal subtropical dry forests, the species has been observed as far inland as the mountain town of Lares and in subtropical wet forests during the non-breeding season. The species has also been observed at the Cabo Rojo National Wildlife Refuge. 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. In 1976, the population of the nominate form was estimated at 2000 individuals, but in 1982, the population estimate dropped to 300 individuals. Conservation efforts have increased the population to 1250 pairs.
In 1976 Post and Wiley estimated the Mona subspecies population at 200 individuals. Subsequent roost counts and studies from 1981 to 1995 estimated the population at anywhere between 220 and 400 individuals. Studies performed in the island of Monito, located northwest of Mona, revealed an average of 25 individuals with breeding being observed. The studies also revealed that birds traveled from the west coast of Mona to Monito.
Yellow-shouldered blackbirds are non-migratory birds, but a portion of the population of the nominate form moves from coastal areas to inland areas during the non-breeding season to feed.