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.

The Bronzed Cowbird, Molothrus aeneus assimilis and Molothrus aeneus loyei, are two of four subspecies of Bronzed Cowbird, of which three are found in Mexico. They are a member of the Icteriidae Family of Troupials and Allies that includes Grackles, New World Blackbirds and Orioles, that has one hundred five individual species that have been placed into thirty genera, and is one of six global species of the Molothrus Genus. They are known Mexico as tordo ojirojo.

The Bronzed Cowbird is small in stature. They are sexually dimorphic with the sexes easily separated. 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 their eyes have 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.

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. They can be found at elevations up to 2,200 m (7,200 feet) but normally at lower levels. The Bronzed Cowbird has been poorly studied and very limited information about their behavioral patterns and biology has been documented.

The Bronzed Cowbird is very similar to the Brown-headed Cowbird, Molothrus ater (slightly smaller, shorter bill, smaller head, longer tail and red iris) and the Shiny Cowbird, Molothrus bonariensis (longer bill, males entirely glossy black; females grayish-brown).

The Bronzed Cowbird is found throughout Mexico with the exception that they are 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, in Baja California on a limited basis and from southern Sonora to Nayarit.

From a conservation perspective the Bronzed Cowbird has not been formally evaluated, however their populations are stable or rapidly expanding, attributed primarily to agriculture development and human urbanization. In regions where they overlap they have been implicated in the decline of populations of Altamire Oriole, Icterus gularis and Audubon’s Oriole, Icterus graduacauda.