[2] Since 1900 the shiny cowbird's 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.

[2] The shiny cowbird's diet consists mainly of insects, other arthropods[4] and seeds, and they have been recorded foraging for grains in cattle troughs.

[2] Like most other cowbirds, it is an obligate brood parasite, laying its eggs in the nests of many other bird species such as the rufous-collared sparrow.

[6] The shiny cowbird was formally described in 1789 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae.

[8] Gmelin based his description on "le tangavio" from Buenos Aires that had been described in 1778 by the French polymath the Comte de Buffon.

[2] Basic adult plumage for M. b. bonariensis is black with purple-blue iridescence for males, and dusty gray-brown for females.

[2] Within the last century, the range of the species has shifted northward, and birds have been recorded in the West Indies and southern Florida.

[3] This shift in range is due to increased human conversion of forests into open cultivated and agricultural land, habitats which are preferred by the shiny cowbird.

[13] The species spread from South America to mainland Puerto Rico in 1955, and subsequently reached the Dominican Republic in 1973, and Cuba in 1982.

[12] The shiny cowbird is an obligate brood parasite, meaning that adults will lay their eggs in the nests of other species and their offspring rely entirely on their hosts for parental care.

[16] In regions of South America including Argentina, Uruguay, Brazil, and Venezuela, the main host species of the shiny cowbird is the rufous-collared sparrow.

[17] In one study, nestling mortality almost doubled when comparing a non-parasitised nest to one that had been parasitised by a shiny cowbird.

[17] Shiny cowbirds can have a large negative effect on critically endangered species, such as the pale-headed brush finch.

Human modification of their restricted geographic range led to habitat loss in the case of the finches but also introduced more cowbirds into the now open area.

Parasitism by shiny cowbirds is thought to be an important factor in the population decline of the pale-headed brush finch.