Brown-headed Cowbird

Appearance

The Brown-headed Cowbird is typical for an icterid in general shape but is distinguished by a finch-like head and beak and is smaller than most icterids. The adult male is iridescent black in color with a brown head. The adult female is slightly smaller and is dull grey with a pale throat and very fine streaking on the underparts. The total length is 16–22 cm and the average wingspan is 36 cm. Body mass can range from 30–60 g, with females averaging 38.8 g against the males' average of 49 g.

Behavior

Social behaviors of cowbird males include aggressive, competitive singing bouts with other males and pair-bonding and monogamy with females. By manipulating demographics so juveniles only had access to females, juvenile males developed atypical social behavior; they did not engage in the typical social singing bouts with other males, did not pair bond with females, and were promiscuous. This demonstrates that there is great flexibility in the behavior of cowbirds, and that the social environment is extremely important in structuring their behavior. Adult males housed with juvenile males were shown to have greater reproductive success compared to adult males housed with other adult males. Being housed with juvenile males honed the reproductive skills of the adult males by providing them with a more complex social environment. This finding was further studied by comparing the behaviors and reproductive success of males exposed to a dynamic flock, consisting of changing individuals, with males exposed to a static group of individuals. The individuals that stayed with the same group had a stable, predictable relationship between social behavior and reproductive success; the males that sang high amounts to females experienced the greatest reproductive success. The adult males that were exposed to a rotating roster of new individuals had an unpredictable relationship between social variables and reproductive success; these males were able to copulate using a much greater variety of social strategies. The males who lived in static flocks had high levels of consistency in their behaviors and reproductive success across multiple years. The males in dynamic flocks experienced varying levels of dominance with other males, differing levels of singing to females, and differing levels of reproductive success.Brown-headed cowbirds do not raise their own young, instead laying their eggs in the nests of other bird species. Because of this, cowbirds are not exposed to species-typical visual and auditory information unlike other birds. Despite this, cowbirds are able to develop species-typical singing, social, and breeding behaviors.

Habitat

They occur in open or semi-open country and often travel in flocks, sometimes mixed with Red-winged Blackbirds and Bobolinks, as well as Common Grackles or European Starlings. These birds forage on the ground, often following grazing animals such as horses and cows to catch insects stirred up by the larger animals. They mainly eat seeds and insects.

Before European settlement, the Brown-headed Cowbird followed bison herds across the prairies. Their parasitic nesting behaviour complemented this nomadic lifestyle. Their numbers expanded with the clearing of forested areas and the introduction of new grazing animals by settlers across North America. Brown-headed Cowbirds are now commonly seen at suburban birdfeeders.

Reproduction

This bird is a brood parasite: it lays its eggs in the nests of other small passerines, particularly those that build cup-like nests. The Brown-headed Cowbird eggs have been documented in nests of at least 220 host species, including hummingbirds and raptors. The young cowbird is fed by the host parents at the expense of their own young. Brown-headed Cowbird females can lay 36 eggs in a season. More than 140 different species of birds are known to have raised young cowbirds.

Unlike the Common Cuckoo, it has no gentes whose eggs imitate those of a particular host.

Some species, such as the House Finch, feed their young a vegetarian diet. This is unsuitable for young Brown-headed Cowbirds, meaning almost none survive to fledge.Social behaviors of cowbird males include aggressive, competitive singing bouts with other males and pair-bonding and monogamy with females. By manipulating demographics so juveniles only had access to females, juvenile males developed atypical social behavior; they did not engage in the typical social singing bouts with other males, did not pair bond with females, and were promiscuous. This demonstrates that there is great flexibility in the behavior of cowbirds, and that the social environment is extremely important in structuring their behavior. Adult males housed with juvenile males were shown to have greater reproductive success compared to adult males housed with other adult males. Being housed with juvenile males honed the reproductive skills of the adult males by providing them with a more complex social environment. This finding was further studied by comparing the behaviors and reproductive success of males exposed to a dynamic flock, consisting of changing individuals, with males exposed to a static group of individuals. The individuals that stayed with the same group had a stable, predictable relationship between social behavior and reproductive success; the males that sang high amounts to females experienced the greatest reproductive success. The adult males that were exposed to a rotating roster of new individuals had an unpredictable relationship between social variables and reproductive success; these males were able to copulate using a much greater variety of social strategies. The males who lived in static flocks had high levels of consistency in their behaviors and reproductive success across multiple years. The males in dynamic flocks experienced varying levels of dominance with other males, differing levels of singing to females, and differing levels of reproductive success.