Shiny Cowbird (Molothrus bonariensis)

Shiny Cowbird


The shiny cowbird (Molothrus bonariensis) is a passerine bird in the New World family Icteridae. It breeds in most of South America except for dense forests and areas of high altitude such as mountains. Since 1900 the shiny cowbirds' range has shifted northward, and it was recorded in the Caribbean islands as well as the United States, where it is found breeding in southern Florida. It is a bird associated with open habitats, including disturbed land from agriculture and deforestation. Physical appearance of the shiny cowbird adult depends on subspecies. Sizes range from 31-40 grams in mass and 18 cm in length (M. b. minimus), to 55-65 grams in mass and 22 cm in length (M. b. cabanisii).

Distribution And Habitat

The Shiny Cowbird is a common resident species native to South America but has expanded its distribution range north throughout the Lesser and Greater Antilles, The Bahamas, and the Southern United States (Cruz and others 1985, Post and Wiley 1977a, Raffaele and others 1998). The cowbird is believed to have arrived in Puerto Rico before 1955 (Post and Wiley 1977a), where it is now established (Bond 1961, Raffaele 1989a) and has also colonized the island of Culebra (Sorrié 1975). In Vieques, the cowbird is an uncommon resident in spring, summer, and fall, and rare in winter (Gemmill 2015). This species occurs mostly near livestock in open or brushy fields, but can also be found in gardens, open forests (Oberle 2018), and agricultural areas with available grains (Raffaele and others 1998), as well as into the mountains where suitable habitat is available (Raffaele 1989b, Raffaele and others 1998). The atlas fieldwork yielded a total of 502 records within 317 hexagons or 66 percent of the 479 total hexagons (see map). Of the 317 hexagons where this species was found, breeding met the atlas definition of confirmed in 9 percent (27) of the hexagons, probable in 32 percent (101), and possible in 60 percent (189) (see map).

Shiny Cowbird Distribution

Breeding Habits

Previously published reports indicate that the Shiny Cowbird breeds primarily from March through July (Raffaele and others 1998). Cowbirds are brood parasites in which the females do not build nests, but rather lay their eggs in the nests of other species, which serve as foster parents, incubate the cowbird eggs and brood, and feed the nestling and fl edgling cowbirds. Cowbird nest parasitism has reduced the nesting success of several host species including the endemic Yellow-shouldered Blackbird Shiny Cowbird 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. 269Shiny Cowbird/Tordo Lustroso(Agelaius xanthomus) (Post and Wiley 1977b) and Puerto Rican Vireo (Vireo latimeri) (Woodworth 1997). The cowbird is a generalist in terms of host selection as it has been found to parasitize the nests of 27 bird species on Puerto Rico, although these host species differ in their abilities to successfully fl edge cowbird young (Wiley 1985). Cowbird breeding tends to coincide with the breeding periods of “high-quality” hosts (i.e., those that fl edge >55 percent of cowbird eggs hatched) and not with breeding periods of “low- quality” hosts (i.e., species that fl edge <55 percent of cowbird chicks that hatch) (Wiley 1985). Atlas results show that this species breeding season extends throughout the year with the most breeding activity from March to June (see chart). The breeding activity peaks in May during the onset of the rainy season, and the overall breeding pattern mostly takes place in the subtropical moist forest life zone (see chart). Results show that this species breeds mostly in lowlands within the subtropical moist forest life zone (58 percent of the hexagons) (see table and map). However, results indicate that it also breeds at higher elevations within the subtropical wet forest life zones (22 percent of the hexagons), as well as in the coastal plains of the subtropical dry forest life zones (20 percent of the hexagons) (see table and map).


The current population trend for the Shiny Cowbird is described as increasing. This species is currently listed as a species of least concern by the IUCN (BirdLife International 2016). Locally, this species is not listed in any of the threatened categories of PRDNER and USFWS. In Puerto Rico, the Shiny Cowbird has a protected habitat in land of about 12 percent or 929 km2 of the total area covered by the hexagons where evidence of breeding was found for this species (7582 km2).