Brown-headed cowbirds (Molothrus ater) are perhaps the best-known brood parasite bird in North America, with females laying up to 40 eggs in a season in the nests of other unsuspecting breeding birds. Females and males are unalike in appearance, and are not easily confused. But are there other traits and behaviors that male and female brown-headed cowbirds have in common? Let’s take a closer look.
Unlike male brown-headed cowbirds, which, as their name suggests, have brown heads, females of the species are brownish-gray all over. Their brazen egg-dumping habits set them apart from many other bird species seen in the United States.
As brood parasites, brown-headed cowbirds demonstrate no traditional ‘nest construction’, incubation or maternal brooding behavior as seen in a vast majority of other bird species. Females have no association with their own offspring and play no part in caring for or raising any young.
To get more insight into the fascinating and unusual behavior and habits of these elusive nest invaders, please keep reading.
Close up of a perched Brown-headed Cowbird female
Male Brown-headed Cowbirds are black with brown heads, and a conical gray bill. Females do have the same thick gray bill, but instead of sharing the same iridescent black plumage of males, are gray-brown all over with streaky markings on their throat and belly.
Male and female brown-headed cowbirds don’t look anything alike, and are also unalike in behavior too.
Female Brown-headed Cowbird
Male Brown-headed Cowbird
Female brown-headed cowbirds are roughly the same size as red-winged blackbirds. Their plumage is gray-brown, with slightly darker upper parts, and lighter streaky markings on their belly and breast.
They are stocky birds, with short tails and wings, and have a gray conical bill, dark brown eyes and black legs and feet.
Close up of a female Brown-headed Cowbird perched on a mossy log
Female brown-headed cowbirds are marginally smaller than males, and there are significant differences in the recorded average weights of the different sexes. Females are at the lower end of the size range, which is 16 cm to 22 cm (6.3 in to 8.7 in) across the species.
The average weight for female brown-headed cowbirds is 38.8 g (1.37 oz) compared to 49 g (1.7 oz) for males.
In winter, male and female brown-headed cowbirds are regularly seen foraging with other species on farmland and parklands, but in spring, their behavior changes considerably.
Males become more vocal in an attempt to attract a mate, and females are harder to spot, as they remain out of sight in vegetation, scoping out potential host nests in which they can successfully lay their own eggs.
Male left, and female right, Brown-headed Cowbirds
Only male brown-headed cowbirds sing, although females do respond to male song with a series of whistling notes known as a chattering call.
This call is frequently heard during courtship, with male brown-headed cowbirds attracted to the flowing notes. Females also make a short click sound when feeding.
Unlike the majority of bird species, brown-headed cowbirds do not build nests or raise their own young. Instead, they lay their eggs in the nests built by other breeding birds and their offspring are then reared by unwitting host birds.
Female brown-headed cowbirds spend large periods of time during the breeding season in search of viable nests in which to lay their eggs. They will seek out birds that are actively laying, and seize the chance to approach their nest when it is briefly left unattended.
The cowbird will then usually destroy, damage or remove one or more of the eggs before laying one or more of their own in their place, and then leave the site without the resident bird having any idea that the eggs they are incubating are no longer their own.
This process is repeated up to 40 times in a season by the female brown-headed cowbird, who acts alone rather than being accompanied in her deceitful deeds by the male.
Female Brown-headed Cowbird foraging on the ground
Females may return to nests in which they have laid their eggs in as incubation progresses. While some birds may continue to incubate the cowbird’s egg or eggs without any idea that it is not their own, others may instinctively realize that their nest has been infiltrated by an opportunistic cowbird, and eject the egg that isn’t theirs from the nest.
In such cases, ‘mafia behavior’ may be observed, with the female brown-headed cowbird taking revenge and destroying the entire nest from which her own egg has been ejected.
Male and female brown-headed cowbirds survive on a diet of grains, seeds, and bugs, including beetles, grasshoppers and flying insects.
During the breeding season, females are observed to increase their intake of insects and also eat more snail shells and the shells of eggs from the host nests in order to produce enough calcium to meet their own intense egg-laying demands.
Female brown-headed cowbirds are very similar in size and appearance to the female shiny cowbird, but a close inspection shows that the shiny cowbird’s bill is longer and wider. Female bronzed cowbirds are also very easily confused with female brown-headed cowbirds, but the former has a red-brown eye, a wider bill, and has a slightly browner than the gray-brown of the female brown-headed cowbird.
Close up of a pair of Brown-headed Cowbirds, perched on a branch together
Female brown-headed cowbirds are brood parasites and do not play any active role in raising their own young, or the young of other birds. Their involvement with their own offspring ends when they lay their eggs in a host bird’s nest. There are no existing records of brown-headed cowbirds building nests or raising young.
As cowbirds do not build nests or establish their own territories, they are not classed as traditionally territorial birds, and are frequently seen foraging with other species in mixed grazing flocks.
Males will defend their mate from the advances of other males, using song to assert their own position among competing males, and females may display slight aggression on hearing chatter calls made by nearby females.
Close up of a female Brown-headed Cowbird perched in a tree
Female brown-headed cowbirds are brown-gray all over, with darker brown upperparts and streaky gray-brown breast, belly and underparts.
Female brown-headed cowbirds are heard making a rapid-fire chattering call in response to the song of males. Chattering calls are heard less in male brown-headed cowbirds. Females can also be heard making a short clucking call while feeding.
Only male brown-headed cowbirds sing; females are vocal, but their vocalizations are classed as calls rather than songs.
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