Shiny Cowbird Molothrus bonariensis Scientific name definitions

A native of South America, and with a broad and expanding distribution throughout South America and the the Caribbean, the Shiny Cowbird reached North America (Florida) in 1985 by natural expansion through the Antilles island chain. A single male was seen on Lower Matecumbe Key, Monroe Co., Florida, on 14 June 1985; the next year, 3 males were seen in July at nearby Islamorada on Plantation Key, Florida; and in 1987, a pair was seen in Everglades National Park in May, and up to 5 males and 2 females were seen there in June (Smith and Sprunt IV 1987 Smith, P. W. and A. Sprunt IV. (1987). The Shiny Cowbird reaches the United States. American Birds 41:370-371. Close ). By 1991, Shiny Cowbirds were considered permanent residents in southern Florida (Post et al. 1993 Post, W., A. Cruz and D. B. McNair. (1993). The North American invasion pattern of the Shiny Cowbird. Journal of Field Ornithology 64:32-41. Close , Stevenson and Anderson 1994b Stevenson, H. M., and B. H. Anderson (1994). The Birdlife of Florida. University Press of Florida, Gainesville, Florida, USA. Close ). A definite breeding record has not yet been established there, but a fledgling cowbird fed by Red-winged Blackbirds (Agelaius phoeniceus) in Dade Co., FL, in 1991, was claimed to be a Shiny Cowbird (Stevenson and Anderson 1994b Stevenson, H. M., and B. H. Anderson (1994). The Birdlife of Florida. University Press of Florida, Gainesville, Florida, USA. Close ). Since the 1980s, Brown-headed Cowbirds (Molothrus ater) have also expanded their breeding range to include most of Florida, so specific identification of Molothrus young in southern Florida is problematic.

Shiny Cowbirds are generalist brood parasites. Their arrival in North America raises concerns for potential host species. In the West Indies, this concern has fostered studies of the endangered Yellow-shouldered Blackbird (Agelaius xanthomus) and island endemic Puerto Rican Vireo (Vireo latimeri) and their interaction with Shiny Cowbirds.

Since publication of Herbert Friedmann's monograph on the cowbirds (Friedmann 1929 Friedmann, H. (1929). The Cowbirds: A Study in the Biology of Social Parasitism. Charles C Thomas Publisher, Springfield, IL, USA. Close ), the number of known victims (species receiving cowbird eggs) and hosts (victims that have reared cowbird young) of the Shiny Cowbird has increased steadily. In 1929, the count was 82 victims and 14 hosts.

Since then, the counts have increased to 146 victims, including 26 hosts (Friedmann 1963 Friedmann, H. (1963). Host relations of the parasitic cowbirds. Volume 233. U.S. National Museum Bulletin, Washington, DC, USA. Close ); 176 victims, including 36 hosts (Friedmann et al. 1977 Friedmann, H., L. F. Kiff, and S. I. Rothstein (1977). A further contribution to knowledge of the host relations of the parasitic cowbirds. Smithsonian Contributions to Zoology 235:1–75. Close ); 201 victims, including 53 hosts (Friedmann and Kiff 1985 Friedmann, H., and L. F. Kiff (1985). The parasitic cowbirds and their hosts. Proceedings of the Western Foundation of Vertebrate Zoology 2:226–302. Close ); and 232 victims, including 74 hosts (see the Appendix 1). With entry of the Shiny Cowbird into North America, and continued studies of Shiny Cowbirds in the West Indies and in South America, the numbers of victims and hosts will no doubt continue to increase.

Opportunistic in its feeding behavior and choice of hosts, the Shiny Cowbird is in many respects the South American counterpart of the Brown-headed Cowbird. Shiny Cowbirds usually are associated with open, disturbed areas such as agricultural regions, and often ride on the backs of cattle, horses, and sheep while feeding. It appears to differ from the Brown-headed Cowbirds in being more insectivorous, and in feeding higher off the ground. In some regions, it may specialize in parasitizing marsh-nesting blackbirds, particularly Agelaius species. It also more frequently parasitizes cavity-nesting hosts. What little is known about the mating systems of the Brown-headed and Shiny cowbirds suggests that both species are labile, varying from monogamy to polygamy-polygyny, depending perhaps on host availability.