Bronzed Cowbird, Molothrus aeneus

Bronzed Cowbird, Molothrus aeneus loyei, Male. Photograph taken within a residential community in Alamos, Sonora, March 2018. Photograph and identification courtesy of David F. Smith, Alamos, Sonora.

Bronzed Cowbird, Molothrus aeneus loyei, Female. Photograph taken within a residential community in Alamos, Sonora, April 2018. Photograph and identification courtesy of David F Smith, Alamos, Sonora.

Bronzed Cowbird, Molothrus aeneus loyei, Female, Expressing Interest in a Streak-backed Oriole Nest for Parasitism. Photograph taken within a residential community in Alamos, Sonora, May 2018. Photograph and identification courtesy of David F Smith, Alamos, Sonora.

Bronzed Cowbird, Molothrus aeneus loyei, Males. Birds photograph taken in the Parque National Huatulco, Huatulco, Oaxaca, March 2021. Photographs and identification courtesy of Marina Sutormina, Stockholm, Sweden.

Background and Identification

The Bronzed Cowbird, Molothrus aeneus assimilis and Molothrus aeneus loyei, are two of four subspecies of Bronzed Cowbird. Three of these four subspecies are found in Mexico. They are a member of the Icteriidae Family of Troupials and Allies which includes Grackles, New World Blackbirds, and Orioles. The Icteriidae Family has one hundered and five individual species that have been placed into thirty genera. They are one of the six global species of the Molothrus Genus. The Bronzed Cowbird is a brood parasite, meaning that it does not make nests and lays its eggs in the nests of other species. No parental care is provided to its offspring by the male or female parent. They are known in Mexico as Tordo Ojirojo.

The Bronzed Cowbird is small in stature, with an average mass of 55 g (1.94 oz) to 70 g (2.47 oz) and have an average length of 20 cm (7.9 inches). They are a sexually dimorphic species, and the sexes are easily differentiated. The adult males are blackish with a bronzy iridescent sheen on their back and bronzy or violaceous iridescence on the rump, a conspicuous erectile ruff on the back and sides of the neck, and a red iris. The females are slightly smaller than the males and are an overall dull black or gray-brown and do not have the neck ruff found in the males. They have slightly rounded tails that are about two-thirds the length of the wings, a stout black bill that is shorter than their head, and black legs and feet.

Habitat and Geographical Range

The Bronzed Cowbird is found in open fields, pastures, and scrubby areas. They forage mostly on the ground, consuming grass seeds, insects, grain crops, and weeds. They have life spans of up to six years. A portion of their population are year-round residents while others are winter visitors, and they can be found at elevations up to 2,200 m (7,200 feet). Research on social behavior and migratory patterns has shown that they normally prefer lower elevations closer to sea level. Biologically, the Bronzed Cowbird has been poorly studied and not much is known about this species besides their brood parasitism behavior.

The Bronzed Cowbird is found throughout Mexico, but absent from most of Baja California, Baja California Sur, and northern Sonora. In Mexico, the assimilis subspecies is found from Jalisco south to Chiapas. The loyei subspecies is found in northwest Mexico, Baja California on a limited basis, and from southern Sonora to Nayarit.

Common Misidentifications

The Bronzed Cowbird is very similar to the Brown-headed Cowbird, Molothrus ater, which has a slightly smaller, shorter bill, smaller head, longer tail, and red iris.

Conservation Status

From a conservation perspective the Bronzed Cowbird has currently considered to be of Least Concern with either stable stable or rapidly expanding populations, attributed primarily to agricultural development and human urbanization in some regions, with a wide distribution . In regions where they overlap they have been a factor in the decline of populations of Altamira Oriole, Icterus gularis, due of their brood parasite behavior. Juvenile Orioles are often thrown out of the nest by the much larger young cowbirds, or die of starvation because the cowbird offspring requires a much greater volume of food.