The Brown-headed Cowbird ( Molothrus ater) is a familiar sight across much of North America. These birds are known for their unique feeding habits and their role as brood parasites. While often associated with seeds and grains, the Brown-headed Cowbird exhibits a diverse diet that adapts to its surroundings and the changing seasons.
Habitat and Distribution
Brown-headed Cowbirds thrive in grasslands dotted with sparse trees, woodland edges, brushy thickets, prairies, fields, pastures, orchards, and even residential areas. They generally avoid dense forests. The development and fragmentation of forests in the eastern United States have facilitated the eastward expansion of the Brown-headed Cowbird's range. Originally a bird that followed bison of the Great Plains, the Brown-headed Cowbird spread eastward in the 1800s as forests were cleared.
General Feeding Habits
Brown-headed Cowbirds are opportunistic feeders. They forage mostly by walking on the ground. Often, they associate with cattle or horses in pastures, catching the insects flushed from the grass by the grazing animals. Centuries ago, the Brown-headed Cowbird probably followed bison herds on the Great Plains, feeding on insects flushed from the grass by the grazers. Today it follows cattle, and occurs abundantly from coast to coast. In winter, Brown-headed Cowbirds often concentrate in farmland, pastures, or cattle feedlots, where foraging is easy.
Dietary Components
The Brown-headed Cowbird's diet varies depending on the season and the availability of food sources.
Seeds and Grains
Seeds form a significant portion of the Brown-headed Cowbird's diet, especially during the winter months. They feed mostly on seeds from grasses and weeds, with some crop grains. Seeds (including those of grasses, weeds, and waste grain) make up about half of the diet in summer and more than 90% in winter.
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Insects
Insects constitute a crucial part of the cowbird's diet, particularly during the breeding season. Insects such as grasshoppers and beetles, often caught as cows and horses stir them into movement, make up about a quarter of a cowbird’s diet. The rest of diet is mostly insects, especially grasshoppers, beetles, and caterpillars, plus many others, also spiders and millipedes. Originally, was closely associated with bison herds on the Great Plains
Calcium Sources
Female cowbirds have a large calcium requirement from laying so many eggs. To satisfy it, they eat snail shells and sometimes eggs taken from nests they’ve visited.
Foraging Behavior
Brown-headed Cowbirds usually forage on the ground in mixed flocks of blackbirds, grackles, and starlings. They get their name from their close association with grazing livestock (and formerly bison), which flush up insects for the birds to eat.
Brood Parasitism and its Impact
The Brown-headed Cowbird is a brood parasite, its eggs and young being cared for by other bird species. Female Brown-headed Cowbirds don’t build a nest or rear young. They find nests by watching quietly for signs of other birds building nests, or they flutter through vegetation trying to flush birds from their nests. Nest: No nest built; eggs laid in nests of other birds.
Nest Selection
Experiments done with artificial nests in an aviary suggest that Brown-headed Cowbirds tend to choose nests containing eggs of smaller volume than their own.
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Host Species
Brown-headed Cowbird lay eggs in the nests of more than 220 species of birds, and over 140 of those are known to have raised young cowbirds. Known to have laid eggs in nests of over 220 species of birds, and over 140 of those are known to have raised young cowbirds. Cowbirds lay eggs in a great variety of nests, including Red-winged Blackbird nests in marshes, dome-shaped Ovenbird nests on the forest floor, cup nests in shrubs and treetops, and even occasionally in nests in tree cavities. Over 140 host species of the Brown-headed Cowbird have been described, from birds as small as kinglets to as large as meadowlarks. Common hosts include the Yellow Warbler, Song and Chipping sparrows, Red-eyed Vireo, Eastern and Spotted towhees, and Red-winged Blackbird.
Nesting Facts
Clutch Size: 1-7 eggsEgg Length: 0.7-1.0 in (1.8-2.5 cm)Egg Width: 0.6-0.7 in (1.5-1.8 cm)Incubation Period: 10-12 daysNestling Period: 8-13 daysEgg Description: White to grayish-white with brown or gray spots.Condition at Hatching: Naked except for sparse tufts of down, eyes closed, clumsy. Cowbird chicks tend to grow faster than their nestmates, allowing them to get more attention and food from their foster parents.
Impact on Host Species
Heavy parasitism by cowbirds has pushed some species to the status of 'endangered' and has probably hurt populations of some others. Undoubtedly far more abundant and widespread today than it was originally, and having a negative impact on other species. When young cowbirds hatch, they may roll the other eggs out of the nest.
Identification
Brown-headed Cowbirds are smallish blackbirds, with a shorter tail and thicker head than most other blackbirds. The bill has a distinctive shape: it’s much shorter and thicker-based than other blackbirds', almost finch-like at first glance.
Male Cowbirds
Male Brown-headed Cowbirds have glossy black plumage and a rich brown head that often looks black in poor lighting or at distance. The brown head of male Brown-headed Cowbirds can be difficult to see in poor light, so body shape and bill shape are the best clues. The male Brown-headed Cowbird is shiny black all over, save for his head, which is (as might be expected) chocolate brown. In breeding season, male displays by fluffing up body feathers, partly spreading wings and tail, and bowing deeply while singing. Groups of males sometimes perch together, singing and displaying. When males sing, they often raise their back and chest feathers, lift their wings and spread their tail feathers, and then bow forward. Groups of males may do this together.
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Male - Length: 7.5-8.7 in (19-22 cm); wingspan: 14.2 in (36 cm); weight: 1.4-1.8 oz (40-50 g). The male Brown-headed Cowbird is glossy black, with contrasting brown head.
Female Cowbirds
Female cowbirds are much less streaky than female Red-winged Blackbirds. Female Brown-headed Cowbirds are drab gray-brown all over, with faint or no streaking.
Female - Length: 6-8 in (15-20 cm); wingspan: 12-15 in (32-38 cm); weight: 1.1-1.6 oz (32-45 g). Female confusing, plain dusty brown with dark, sparrowlike bill.
Juvenile Cowbirds
Juvenile streaked at first.
Vocalizations
Squeaky gurgle. Call is check or a rattle. Brown-headed cowbirds are members of the blackbird family with short, conical bills and long, pointed wings. Their song is a bubbly glug-glug-glee, their call is a chuck, and their flight note is a high whistle.
Conservation Status
Brown-headed Cowbirds are common across most of North America, although populations slightly declined between 1966 and 2019, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 130 million and rates them 7 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Surveys suggest slight declines in Brown-headed Cowbird total numbers in recent decades.