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 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. It is a bird associated with open habitats, including disturbed land from agriculture and deforestation. and seeds, and they have been recorded foraging for grains in cattle troughs. Different host species show different responses to their nests being parasitised, with behaviours ranging from accepting and caring for the cowbird eggs, to rejecting the eggs from the nest.
Taxonomy
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. He placed it with the tanagers in the genus Tangara and coined the binomial name Tanagra bonariensis. The specific epithet is Modern Latin for Buenos Aires, the type location. Gmelin based his description on "le tangavio" from Buenos Aires that had been described in 1778 by the French polymath the Comte de Buffon. A hand-coloured engraving by François-Nicolas Martinet was published separately to accompany Buffon's text. The shiny cowbird is now one of six cowbirds placed in the genus Molothrus that was introduced in 1832 by William Swainson.
Seven subspecies are recognised: M. b. cabanisii males have plumage similar to M. b. bonariensis, while females are paler in colouration. This range shift into new regions allows the cowbird to exploit new naive host species.
Behaviour and ecology
Breeding
Sexual behaviour and courtship
Shiny cowbirds do not form monogamous pairs. They have a promiscuous mating system where individuals will copulate with many different mates.
During courtship, male shiny cowbirds perform a song while circling a female, and when the song is finished they bow to their prospective mate. This bow is a display used in both mating rituals and as a show of aggression toward other males. They are generalists, and have about 250 different host species.
Female shiny cowbirds do not build nests, as they rely on their hosts to care for their offspring, but they will preferentially select hosts that build enclosed nests such as nests built in cavities. In hosts such as the creamy-bellied thrush, where parasitism by shiny cowbirds does not have a large negative effect on the survival of their own chicks, the hosts do not exhibit egg-ejection behaviour. This acceptance of parasitic eggs may also be due to the fact that the eggs are similar in appearance, and the host would risk harming its own eggs in the process.
Effect on host species
thumb|225x225px|Fed by [[Masked water tyrant|Masked water Tyrant in Brazil]]
Brood parasitism from shiny cowbirds will have a negative effect on the reproductive success of their hosts through a variety of factors employed by the different life stages of the cowbird. Adult females can negatively affect the host by pecking and killing host eggs and removing the host eggs from the nest. Shiny cowbird eggs have a short incubation period of about 10–11 days. Laying their eggs before their host, as well as having a shorter incubation period, allows for the hatching of the parasitic chick to occur before the host eggs hatch. When the cowbirds hatch before the sparrows in the nest, sparrows usually do not gain much weight and die within about three days.