The yellow-shouldered blackbird (Agelaius xanthomus), also known as in Puerto Rican Spanish as la mariquita de Puerto Rico or capitán, is a diurnal blackbird endemic to the archipelago of Puerto Rico and belongs to the genus Agelaius of the family Icteridae. It has black plumage with a prominent yellow shoulder on its wing. Adult males and females are of similar appearance. The species is predominantly insectivorous. 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 20–23 cm (7.9–9.1 in); on average, males weigh 41 g (1.4 oz) and females weigh 35 g (1.2 oz). Sexual categorization may also be made by measurement of the wings, with males' being 1.1 times larger and having a mean length of 102 cm (40 in), while females' wings have an average length of 93.3 cm (36.7 in).
The Yellow-shouldered Blackbird is an endemic and endangered species in Puerto Rico that can be found primarily in mangroves and arid scrubland (Raffaele and others 1998) (see map). The species was once commonly found in the coastal forests of the archipelago of Puerto Rico (Wetmore 1916), but during the early 20th century, Puerto Ricos coastal forests were destroyed for sugar cane plantations and later for urban development (Post 1981, USFWS 2011). According to Oberle (2018), the Yellow-shouldered Blackbird was formerly found in lowlands around Puerto Rico. At present, the species is primarily limited to four areas: Mona and Monito islands, and three populations in eastern, southern, and southwestern Puerto Rico, where the largest population is found (in the municipalities of Cabo Rojo and Lajas) (USFWS 2011). The species has been observed as far inland as the mountain towns of Lares and Ciales (USFWS 2011). The atlas fieldwork yielded a total of 60 records within 29 hexagons or 6 percent of the 479 hexagons (see map). Of the 29 hexagons where this species was found, breeding met the atlas definition of confirmed in 24 percent (7) of the hexagons, probable in 21 percent (6), and possible in 48 percent (14), while the species was observed in 7 percent (2) of the hexagons but without evidence of breeding (see map). Atlas results support the current distribution described in the literature for the Yellow-shouldered Blackbird. Yellow-shouldered Blackbird distribution. The map shows the highest breeding code by hexagon and overlaying the ecological life zones in Puerto Rico. Note: percentages may not total 100 due to rounding.
The Yellow-shouldered Blackbird nests in a variety of sites, including cliff crevices or cavities in trees and in palms at the base of palm fronds (Biaggi 1997, Oberle 2018, Post 1981, Raffaele and others 1998). Previously published reports indicate that breeding activity occurs from February to November, 267Yellow-shouldered Blackbird/Mariquita but the beginning of the species breeding season coincides with the onset of the rainy season, which explains the fl uctuation in the start and end of the breeding period (USFWS 2011). Atlas results indicate breeding activity occurs year-round, except for the months of August and September, when no breeding data are available (see chart). Most breeding evidence is clustered in the subtropical dry forest life zone (81 percent of the hexagons), followed by the subtropical moist forest life zone (15 percent of the hexagons) and one hexagon in the subtropical wet forest life zone (4 percent of the hexagons) (see table and map).
The Yellow-shouldered Blackbird is listed as Endangered by the IUCN (BirdLife International 2017) and by local and Federal laws (PRDNER 2016; USFWS 1973, 2011). Nest parasitism by the Shiny Cowbird (Molothrus bonariensis) is the single most important factor reducing blackbird reproductive output on Puerto Rico (reviewed in Cruz and others ), but other factors including habitat loss, predation, and low effective population size are also affecting the species population viability (Liu 2016). Trapping of cowbirds and providing blackbirds with nest boxes, which the cowbirds do not enter, has resulted in fewer parasitized blackbird nests, and parasitism declined from 95 percent (from 19731983) to <3 percent (from 20002003) in the southwestern portion of the island (Cruz and others 2005). The Mona population of blackbirds is less threatened by cowbird parasitism as the female cowbirds will not enter the cavities in cliffs used for nesting by the blackbirds on Mona. The USFWS considers the Yellow- shouldered Blackbird population to be improving because the population has a high recovery potential with a population size increasing since 1995 and because some of the major threats to the species have been greatly reduced (USFWS 2011). In Puerto Rico, the Yellow-shouldered Blackbird has a protected habitat in land of 16 percent (103 km2) of the total area covered by the hexagons where this species is known to breed (648 km2). In addition, the PRDNER proposed to designate natural critical habitat in locations within the municipalities of Cabo Rojo, Ceiba, Guánica, Guayama, Lajas, Mayagüez (including Mona and Monito), Salinas, and Santa Isabel (PRDNER 2009).