Shiny Cowbird

(Molothrus bonariensis)

by Vanina Fiorini

The Shiny Cowbird (Molothrus bonariensis) is a generalist brood parasite whose eggs have been found in nests of more than 200 hosts across South America, the Caribbean and southeastern United States.
Neither shiny cowbird eggs nor chicks mimic those of any particular hosts, but they exhibit a variety of traits that enhance their success in a wide range of host species. Shiny Cowbird chicks do not evict or kill their nestmates but they are often reared together in mixed broods.

Yellow Cardinal, Chalk-browed Mockingbird and House Wren nests with one, four and two Shiny Cowbird eggs.
Photos by M. Domínguez, V. Fiorini and A. López.

Shiny Cowbird eggs have certain characteristics that provide them resistance against puncturing, and their chicks are very efficient at manipulating host’s parental behaviors through their begging displays. Shiny Cowbird females also show adaptations that facilitate parasitism and increase the survival of the parasitic offspring. For example, during their visits to host nests and prior to lay down its own egg, they puncture one or more hosts' eggs, effectively reducing future competition for food, for the parasitic offspring.


Our field work is being carried out at Reserva El Destino - Elsa Shaw de Pearson Foundation, located in the Province of Buenos Aires, Argentina. From October to February, Shiny Cowbirds and two of their main hosts, the Chalk-browed Mockingbird and the House Wren reproduce in this place, allowing us to perform field experiments and to work with individuals in captivity.

Sunrise in the Reserva El Destino (A. López, 2014)

Latest related publications  (


Scardamaglia RC and Reboreda JC. Ranging behavior of female and male Shiny and Screaming Cowbirds while searching for host nests. Auk 131: 610-618 (2014).

Fiorini VD, Gloag R, Kacelnik A and Reboreda JC. Strategic egg destruction by brood parasitic cowbirds?. Animal Behaviour 93: 229-235 (2014).

Gloag R, Fiorini VD, Reboreda JC and Kacelnik A. Shiny cowbirds share nests, but not mothers, in multiply parasitized mockingbird broods. Behavioral Ecology and Sociobiology 68: 681-689 (2014).

Gloag R, Fiorini VD, Reboreda JC and Kacelnik A. The wages of violence: mobbing by mockingbirds as a frontline defence against brood parasitic cowbirds. Animal Behaviour 86: 1023-1029 (2013).

Tuero DT, Fiorini VD, Mahler B and Reboreda JC. Do sex ratio and development differ in sexually size-dimorphic shiny cowbirds (Molothrus bonariensis) parasitizing smaller and larger hosts?. Biological Journal of the Linnean Society 110: 442-448 (2013).

Reboreda JC, Fiorini VD and De Mársico MC. Antiparasitic defences in hosts of South American cowbirds. Chinese Birds 4: 57-70 (2013).

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