Brown-headed Cowbird

Brown-headed Cowbird

BY DAVE ZITTIN

GeneralIy I find it unsettling to watch a tiny Dark-eyed Junco feeding a much larger immature Brown-headed Cowbird. The junco in our backyard worked overtime feeding the voracious, constantly begging juvenile cowbird.

Brown-headed Cowbirds are brood parasites. Brood parasites lay their eggs in the nests of other birds of the same or a different species. Brown-headed Cowbirds do not build their own nests, so their eggs are always laid in nests of a different species. Bird biologists call this obligate nest parasitism. The bird receiving the nest parasite’s eggs is called a host.

Brown-headed Cowbird by Teresa Cheng.

The Brown-headed Cowbird is a member of the genus Molothrus which combines two ancient Greek words: “to struggle” and “to sire”. The Brown-head Cowbird is a member of the family Icteridae which includes, among others, blackbirds, orioles, and meadowlarks.

Female Brown-headed Cowbirds are stealthy and spy on the nests of host species. When the time is right, they sneak into the nest and lay their eggs. They sometimes destroy the host’s eggs. The cowbird eggshell is strong, and their eggs have an incubation period that is usually shorter than that of the host eggs. The young cowbird grows faster than the young of the host bird. Young cowbirds often push eggs and the young of the host species out of the nest to reduce competition for food.

Dark-eyed Junco feeding juvenile Brown-headed Cowbird by Sushanta Bhandakar

Before Europeans arrived in the Americas, Brown-headed Cowbirds were limited to the grasslands of central North America where they followed buffalo herds that stirred up insects. The diet of Brown-headed Cowbirds is about 70% grain and 30% insects. They parasitize nests on the edges of forests. Early settlers cleared trees which led to forest fragmentation that allowed them to expand their range by providing more open areas, more forest edges, and more potential host species. By the 1980s, the Brown-headed Cowbird had spread over the entire United States, northern Mexico, and southern Canada. They took advantage of new host species that had no experience with their aggressive nest parasitism, and today they are nest parasites of over 240 bird species.

Dark-eyed Junco feeding two juvenile Brown-headed Cowbirds by Tom Grey

Some host species eject the eggs of nest parasites, bury them by constructing a new nest floor or damage the eggs by breaking their shells. However, some species accept the Brown-headed Cowbird eggs.  For some hosts, the situation is dire. A well-studied case is that of the endangered Kirtland's Warbler. Before 1900, Brown-headed Cowbirds had not occurred in the breeding areas of this warbler in Northern Michigan. In 1971, only 200 male Kirtland’s Warblers were known to exist and about 70% of the nests of this warbler were parasitized by cowbirds. Cowbird traps were employed and trapped cowbirds were killed. In 2018, the program was deemed successful, and the traps were removed. There are now estimated to be 2300 breeding Kirtland’s Warbler males and less than 1% of nests are estimated to be parasitized. Today the Cowbird population is greatly reduced in areas where this warbler nests. This is not so much because of trapping, but because forests have been managed to regrow, increasing the ratio of forest area to forest edge. It’s hard to fault the cowbird. It did what it does best and human activities opened the way for it to expand its range. Today it’s recognized that trapping cowbirds and habit restoration have to go hand in hand to rescue an endangered species impacted by nest parasitism from the Brown-headed Cowbird.

Attracting Brown-headed Cowbirds to Backyards

If there are nesting birds in and around your backyard, Brown-headed Cowbirds will likely appear during the breeding season, especially if you have grain feeders. In our backyard, the Brown-headed Cowbird seems to appear a few days per year during the onset of nesting.

Description

Brown-headed Cowbirds are stout blackbirds with a thick, conical bill. Adult males have brown heads and black bodies. Under poor light conditions, the head appears black. Females are light brown with a slightly lighter-colored head, a white throat, and fine streaking on the belly. Juveniles have heavy streaking on their underparts and their backs appear scaly.

Female Brown-headed Cowbird by Tom Grey

Distribution

Brown-headed Cowbirds occur over most of Mexico, the United States, and southern Canada. They are in Santa Clara County throughout the year.

Similar Species

The female Brewer’s Blackbird resembles a female Brown-headed Cowbird. The cowbird has a lighter-colored throat and a shorter, thicker conical bill compared to the longer, thinner bill of the female Brewer’s Blackbird. In general, the thick, conical bill separates the Brown-headed Cowbird from other similarly colored local icterid species. The eye color of the Brown-headed Cowbird is dark, never red or yellow.

Explore

Biology, ecology, and evolution of nest parasitism

Brown-headed Cowbirds

More Backyard Bird Information

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Banner Photo: Brown-headed Cowbird by Teresa Cheng