The Brown-headed cowbird is a sturdy blackbird with an unusual approach to parenthood. Females do not build nests but use all their energy for producing eggs, sometimes over three dozen per summer. They lay their eggs in other birds’ nests, who become their chicks’ foster parents, with usually at least some of their foster parents’ chicks being victims in the process. Heavy parasitism by this species has pushed some birds to being "endangered" and has affected other populations as well.
Brown-headed cowbirds live throughout the United States, most of Canada and in Mexico. Being partially migratory, they abandon the northern stretches of their range to winter in the south east of the United States, Arizona, southern California, southern Mexico and Florida’s tip. They breed from the south east of Alaska, through lower Canada, and throughout the United States. This species prefers habitat with scattered or low trees among grassland vegetation, like brushy thickets and woodland edges, as well as meadows, pastures, fields, orchards and residential areas.
These birds are social but are usually more solitary during the mornings, which is when mating occurs. Females also seek host nests in the mornings. During the afternoons, birds gather in flocks to forage. They walk on the ground to forage, feeding away from vegetation. In winter, they congregate in very big flocks with European starlings and other icterids. Roosting populations may number up to 38 million individuals. A hierarchy is established in social settings based on how many displacements, flight whistles and song spreads there are in the repertoire of a male. To determine hierarchy, cowbirds participate in "triangle and quadrangle ceremonies", during which males stand in a circle and perform song spreads. After social status is determined, it is usually maintained by nonverbal means. Singing in flocks is mostly performed by the dominant male and this may be enforced, if inferior males try to sing, by attacking them.
Brown-headed cowbirds are herbivores (frugivores and granivores), they eat mostly fruits and seeds. Also in their diet are eggs, spiders and arthropods, like grasshoppers, leafhoppers and beetles. Females eat mollusk shells during the breeding season.
Almost all populations of Brown-headed cowbirds have fewer females than males, so the females may be choosy. Males are usually monogamous for the breeding season and attempt to maintain this by guarding their mate from other males. However, females tend to be polygynandrous or promiscuous. When outside her mate's home range, a female is no longer guarded, so may copulate with a different male. Males not in a pair-bond sometimes mate with females that are not guarded, often while the female's mate is away foraging. Copulations amongst non-mates are more frequent in those populations where the females have more extensive home ranges than the males. The breeding season will vary with the region, but eggs are laid in May and June. One female lays up to 40 eggs each year in the nests of other species. They find nests by quietly watching for signs of nest building, or they flutter through the vegetation, aiming to flush the nest builders out. The incubation time is only 10 to 12 days. After 8 to 13 days the chicks leave the nest, and are fully independent from their hosts at around 25 to 29 days old.
The Brown-headed cowbird is widespread and does not face any major threats at present; however, it has some predators, such as hawks, owls, raccoons, squirrels and snakes. Other reasons for its populations decline are unknown.
According to the What Bird resource, the total population size of the Brown-headed cowbird is around 56 million individual birds. The All About Birds resource records the species’ total breeding population size as 120 million individuals, 77% of which spends some of the year in America, 31% in Mexico and 14% in Canada. Overall, currently Brown-headed cowbirds are classified as Least Concern (LC) on the IUCN Red List, but their numbers today are decreasing.