Bronzed Cowbirds in the eastern part of the U.S. range prefer open areas with few trees: agricultural fields, pastures, native coastal prairie (Texas), mesquite woodlands, shrublands, and golf courses. In the western part of the U.S. range, especially in Arizona and New Mexico, Bronzed Cowbirds frequent open habitats but also wooded canyons and riparian corridors. Like Brown-headed Cowbirds, Bronzed Cowbirds also venture into suburbs, especially where there are backyard feeding stations. South of the U.S. border, the species prefers scrub habitats and tropical evergreen and semideciduous forests, up to 8,200 feet in elevation. At night, Bronzed Cowbirds roost communally in tall grasses or reeds, often in marshes.Back to top
Bronzed Cowbirds eat seeds of forbs and grasses, along with some insects and other arthropods. They also consume grains such as milo, oats, corn, and rice. Bronzed Cowbirds forage on the ground, walking rapidly and stopping to take food with the bill, but also strip grain from the stalk, especially milo. Like Brown-headed Cowbirds, Bronzed often associate with cattle and horses, both in pastures and feedlots, taking cattle’s grain feed and capturing insects disturbed by the animals. There are also reports of this species taking ticks off mules, cows, and white-tailed deer. Well adapted to human environments, Bronzed Cowbirds come to backyard feeding stations, where they eat just about any offering, including watermelon and chicken scratch.Back to top
As brood parasites, Bronzed Cowbirds build no nest but rather lay eggs in nests of other birds.
|Clutch Size:||1 egg|
|Egg Length:||0.9-1.0 in (2.29-2.63 cm)|
|Egg Width:||0.6-0.7 in (1.62-1.78 cm)|
|Nestling Period:||10-12 days|
|Egg Description:||Unmarked bluish green.|
|Condition at Hatching:||Helpless with sparse gray down.|
Bronzed Cowbirds are gregarious year-round, foraging, resting, and roosting in small flocks up to 100 birds or more. In most such flocks, males outnumber females. From the few studies available, their mating system is promiscuous, with both males and females mating with multiple partners. Males set up display areas, which females regularly visit. Displaying males raise the nape feathers (into a “ruff”), then the back feathers, then the body feathers, lower the head and tail, and hold open the wings, singing as they do so. When a female is present, males also perform a lively hovering display in flight. They usually start from a tree perch and fly toward the female, fluttering the wings and fluffing out the nape, back, and body plumage. This display usually occurs in the morning. During displays, males are territorial and intolerant of close approach by other males. The territorial male may point the bill upward in threat posture against rivals. When confronting other blackbirds, cowbirds, or grackles, male Bronzed Cowbirds bow and distend the ruff. During the nonbreeding season, Bronzed Cowbirds often join mixed-species flocks of blackbirds.Back to top
Populations of Bronzed Cowbird have been roughly stable between 1966 and 2015, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 6.6 million and rates the species a 6 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Declining populations of Altamira Oriole and Audubon's Oriole in Texas appear to be associated with increased Bronzed Cowbird parasitism.Back to top
Ellison, K., and P. E. Lowther (2020). Bronzed Cowbird (Molothrus aeneus), version 1.0. In Birds of the World (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.
Lutmerding, J. A., and A. S. Love (2017). Longevity records of North American birds. Version 2017.1. Patuxent Wildlife Research Center, Bird Banding Laboratory, Laurel, MD, USA.
Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.
Partners in Flight (2017). Avian Conservation Assessment Database. 2017.
Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski, K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.
Sauer, J. R., J. E. Hines, Fallon J. E., K. L. Pardieck, D. J. Ziolkowski Jr. and W. A. Link. 2016c. The North American breeding bird survey, results and analysis 1966-2014a. Version 12.07.2015. USGS Patuxent Wildlife Research Center 20142016].
Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.