Brown-headed Cowbird, Molothrus ater

Brown-headed Cowbird, Molothrus ater artemisiae. Photograph taken within a residential community in Alamos, Sonora, March 2018. Photograph and identification courtesy of David F Smith, Alamos, Sonora.

Brown-headed Cowbird, Molothrus ater obscurus, Female. Found within the bush at Bahía Santa Rosalillita, Baja California, August 2018. Photograph and identification courtesy of Barry Mastro, Escondido, California.

The Brown-headed Cowbird, Molothrus ater artemisiae and Molothrus ater obscurus, are two of four subspecies of Brown-headed Cowbird, and all four 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 genus and one of six global species of the Molothrus Genus. They are North America’s best known brood parasite, laying its eggs in the nests of many different species. They are known Mexico as tordo cabecipardo.

The Brown-headed Cowbird is They are small in stature and they are sexually dimorphic with the females being slightly smaller than the males. The adult male is a uniform glossy black color with a unique brown head and neck. The adult females are a plain grayish or brownish. The bill is black in males and light gray in females with a dusky tip, their iris is brown, and their legs and feet are black. They have a short conical compressed bill with a straight profile, long pointed wings, and a slightly rounded tail. They can be found at elevations up to 2,400 m (7,900 feet) but normally at lower levels. They are found within low scattered trees in grasslands, brushy thickets, prairies, fields, pastures, orchards, and residential areas They are ground foragers that consume arthropods and seeds including crop grains, grass seeds and weed seeds. The Brown-headed Cowbird has generated significant interest from the scientific community due to their brood parasitic behavior where one female will deposit one egg into the nests of up to 40 different birds in one year. They are not selective in host selection and will select almost any passerine. They have life spans of up to six years. The Brown-headed Cowbird has been difficult to study because their breeding activities occur among many host nests. They have life spans of up to sixteen years.

In Mexico the Brown-headed Cowbird is found throughout the country with the exception that they are absent from the extreme southern states south of Oaxaca and Veracruz. The artemisiae subspecies is found in western Mexico from the United States border south to Michocán; the obscurus subspecies is found in northwest Mexico south to Guerrero, northern Tamaulipas and Oaxaca including Baja California and Baja California Sur.

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

From a conservation perspective the Brown-headed Cowbird is currently considered to be of Least Concern with stable, widely distributed populations. Their North American populations have rapidly expanded, and now estimated to be between 20 million and 40 million individuals, which is attributed primarily to agriculture development and human urbanization generating a serious negative impact on native populations due to brood parasitism. They date to the Late Pleistocene period, 500,000 years ago.