Brown-headed cowbirds are brood parasites;...Following the constantly roaming herds, cowbirds couldn’t expend the time for nesting, but rather laid their eggs in host nests as they moved along, leaving host parents to raise their eggs to adulthood. As people began to disperse across North America, clearing forests and expanding domesticated livestock herds and agricultural production, the cowbird range expanded also. Nest parasitism enabled brown-headed cowbirds to quickly establish new populations and expand their distribution across the country.
Impacts
Brown-headed cowbirds parasitize the nests of more than 220 bird species in their range. Each cowbird can lay up to 30 eggs per season and usually lay 1 or 2 (or occasionally more) eggs in each host nest. When parasitizing nests, they often remove the egg(s) of the host bird. Brown-headed cowbird chicks usually hatch sooner than the host chicks, are larger, and develop faster. Their larger size and persistent behavior gains them more care from the host parents. Nest parasitism lowers the reproductive success of host birds and has led to population declines in several bird species. In California, the riparian songbirds least Bell’s vireo (Vireo bellii pusillus) and willow flycatcher (Empidonax traillii) are listed as endangered due to loss of riparian habitat and nest parasitism by brown-headed cowbirds.
Actions Taken if Found
If you observe this species in California, please report your sighting to the CDFW Invasive Species Program, by email to Invasives@wildlife.ca.gov, or by calling (866) 440-9530.
https://wildlife.ca.gov/Conservation/Invasives/Species/Cowbird
