Bronzed Cowbird

Molothrus aeneus

  • Version: 2.0 — Published June 24, 2009
  • Kevin Ellison and Peter E. Lowther

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Figure 1. Distribution of the Bronzed Cowbird.

Only northernmost populations are migratory. Seasonal altitudinal movements occur at high elevations in the tropics; see Distribution and Nature of Migration.

Adult male Bronzed Cowbird, Laguna Atascosa NWR, Los Fresnos, TX, April

Note the black plumage with contrasting iridescent bluish winds. The following link is to this contributor's Flickr stream or website., Apr 11, 2008; photographer Bill Walker

Adult female Bronzed Cowbird, Laguna Atascosa NWR, Los Fresnos, TX, April

Females of the eastern subspecies M. a. aeneus. Females of the western subspecies M. a. loyei are grayer overall, lacking the dark blackish brown tones of eastern birds. The following link is to this contributor's Flickr stream or website., Apr 14, 2009; photographer Paul Kusmin

Adult male Bronzed Cowbird, South Padre Is., TX

Note black plumage with red eye and large bill.; photographer Brian E. Small

The Bronzed Cowbird is a brood parasite that ranges from the U.S. – Mexico border southward to northern Colombia, occurring in all Central American countries in between. Bronzed Cowbirds are slightly larger than the better known and related Brown-headed Cowbird (Molothrus ater); the “average” host of the Bronzed Cowbird is also larger than the “average” host of the Brown-headed Cowbird. Many aspects of the Bronzed Cowbird's breeding biology are probably similar to those of the Brown-headed Cowbird, although the former may be more specialized in its host selection.

Human settlement of the New World has led to habitat changes favorable for Molothrus cowbirds; range expansion by Bronzed Cowbirds, in particular, was first noted early this century and a more dynamic northward expansion began in the 1950s in the United States. As the overlap in distribution between Bronzed and Brown-headed Cowbirds increases, the resulting dynamics of community interactions between brood parasites and hosts will become more interesting and complex. For biologists that must manage threatened host species, the presence and intensity of cowbird parasitism can be a cause of concern. Too little is known about interactions between Bronzed Cowbirds and their hosts to make more than general descriptive statements.

With only limited distribution in the United States, this brood parasite has failed to capture the same amount of attention as the Brown-headed Cowbird (see Lowther 1993a). The disparity in knowledge of these two species is well illustrated in Herbert Friedmann's monograph (Friedmann 1929) on the six species of cowbirds: only 7% of the 359-page text is devoted to the Bronzed Cowbird; 48% refers to the Brown-headed Cowbird. This disparity in interest has only increased in the years since then.

Such neglect is unfortunate because, compared to other parasitic cowbirds, the Bronzed Cowbird seems intermediate in many aspects of its behavior and breeding biology. A better understanding of this species would help to advance our understanding of the evolution of brood parasitism.

Recommended Citation

Ellison, K. and P. E. Lowther (2009). Bronzed Cowbird (Molothrus aeneus), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.