Measurements
Male
- Length
- 7.5–8.7 in
19–22 cm - Wingspan
- 14.2 in
36 cm - Weight
- 1.5–1.8 oz
42–50 g
Female
- Length
- 6.3–7.9 in
16–20 cm - Wingspan
- 15–12.6 in
38–32 cm - Weight
- 1.3–1.6 oz
38–45 g
Relative Size
- Slightly smaller than a Red-winged Blackbird
Other Names
- Vacher à tête brune (French)
- Tordo negro (Spanish)
Cool Facts
- The Brown-headed Cowbird is North America’s most common “brood parasite.” A female cowbird makes no nest of her own, but instead lays her eggs in the nests of other bird species, who then raise the young cowbirds.
- Brown-headed Cowbird lay eggs in the nests of more than 220 species of birds. Recent genetic analyses have shown that most individual females specialize on one particular host species.
- Social relationships are difficult to figure out in birds that do not build nests, but male and female Brown-headed Cowbirds are not monogamous. Genetic analyses show that males and females have several different mates within a single season.
- Some birds, such as the Yellow Warbler, can recognize cowbird eggs but are too small to get the eggs out of their nests. Instead, they build a new nest over the top of the old one and hope cowbirds don’t come back. Some larger species puncture or grab cowbird eggs and throw them out of the nest. But the majority of hosts don’t recognize cowbird eggs at all.
- Cowbird eggs hatch faster than other species eggs, giving cowbird nestlings a head start in getting food from the parents. Young cowbirds also develop at a faster pace than their nest mates, and they sometimes toss out eggs and young nestlings or smother them in the bottom of the nest.
- In winter, Brown-headed Cowbirds may join huge roosts with several blackbird species. One such mixed roost in Kentucky contained more than five million birds.
- The oldest recorded Brown-headed Cowbird was 16 years 10 months old.
Habitat
Grassland
Brown-headed Cowbirds occur in grasslands with low and scattered trees as well as woodland edges, brushy thickets, prairies, fields, pastures, orchards, and residential areas. Brown-headed Cowbirds generally avoid forests. Development and fragmentation of forests in the eastern United States have allowed Brown-headed Cowbirds to greatly expand their range eastward. In winter, Brown-headed Cowbirds roost along with several species of blackbirds in flocks numbering more than 100,000 birds.
Food
Seeds
Brown-headed Cowbirds feed mostly on seeds from grasses and weeds, with some crop grains. 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. As you might imagine, 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.
Nesting
Nesting Facts
- Clutch Size
- 1–7 eggs
- Egg Length
- 0.7–1 in
1.8–2.5 cm - Egg Width
- 0.6–0.7 in
1.5–1.8 cm - Incubation Period
- 10–12 days
- Nestling Period
- 8–13 days
- Egg 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.
Nest Description
Doesn’t build its own nest. 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.
Nest Placement
Tree
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.
Behavior
Ground Forager
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. Cowbirds fly directly, with constant wingbeats. 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. 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. When young cowbirds hatch, they may roll the other eggs out of the nest.
Conservation
Least Concern
Originally a bison-following bird of the Great Plains, the Brown-headed Cowbird spread eastward in the 1800s as forests were cleared. It is now common across most of North America. Its habit of nest parasitism can threaten species with small populations, such as the endangered Kirtland's Warbler and Black-capped Vireo.
Credits
- Lowther, Peter E. 1993. Brown-headed Cowbird (Molothrus ater). In The Birds of North America, No. 47 (A. Poole, Ed.). The Birds of North America Online, Ithaca, New York.
- Dunne, P. 2006. Pete Dunne’s essential field guide companion. Houghton Mifflin, Boston.
- Ehrlich, P. R., D. S. Dobkin, and D. Wheye. 1988. The birder’s handbook. Simon & Schuster Inc., New York.
- USGS Patuxent Wildlife Research Center. 2011. Longevity records of North American Birds.
Migration
Short distance migrant. Brown-headed Cowbirds move as far as 530 miles between breeding and wintering grounds.
Backyard Tips
Even though Brown-headed Cowbirds are native to North America, many people consider them a nuisance bird, since they destroy the eggs and young of smaller songbirds and have been implicated in the decline of several endangered species, including Kirtland's Warbler and Black-capped Vireo. Cowbirds often flock with other species of blackbirds, and they may come to your yard if it contains open ground or lawn, or if you scatter grain for ground birds. If your yard is large enough to keep livestock, there's a good chance you will find cowbirds there.
Find This Bird
Look for Brown-headed Cowbirds in fields, meadows, and lawns. During winter and migration, search through mixed-species blackbird flocks and look for the glossy black plumage and subtle brown head in males and the short, stout bill and unmarked brown of females. Learn the male’s gurgling song and the female’s chatter call, and you’ll hear them often.
Get Involved
Brown-headed Cowbirds are a focal bird species for the Celebrate Urban Birds! project. Conduct a 10-minute count and record whether or not you see cowbirds.
Visit NestWatch to learn how to observe and report activity at bird nests