Species Interactions

Commenalistic Relationships

Brown-headed cowbirds are native to the Great Plains in North America and followed nomadic bison, pronghorns (Antilocapra americana) and elk (Cervus canadensis) from place to place. The ungulates would kick up insects and invertebrates and allow the cowbirds easy access to their food. Occasionally the cowbirds would eat invertebrates directly off of the ungulates as well, but the ungulates were not significantly influenced by this relationship. Today, cowbirds feed around cattle and livestock for the same reasons, as well as for the abundance of waste grain in agricultural areas.

Mutualistic Relationships

During the non-breeding seasons cowbirds form mutualistic relationships with other bird species. In winter they join European starlings, red-winged blackbirds, common grackles, and American robins in large flocks for protection from predators. All birds within the flock benefit, because with larger flocks the birds receive more protection. These flocks may reach sizes larger than a million birds.

Parasitic Relationships

As mentioned on the About page, cowbirds have a parasitic relationship with over 200 different bird species.

In the United States, the top 17 parasitized species include:

Yellow Warbler, Song Sparrow, Red-eyed Vireo, Chipping Sparrow, Eastern Phoebe, Rufous-sided Towhee, Ovenbird, Common Yellowthroat, American Redstart, Indigo Bunting, Yellow-breasted Chat, Red-winged Blackbird, Kentucky Warbler, Willow Flycatcher, Bell's Vireo, Yellow-throated Vireo, and Field Sparrow.

Majority of the parasitized species do not recognize the cowbird eggs as foreign or do not wish to have their nests destroyed and accept the eggs anyway. Species such as the American robins, the yellow warbler, yellow-breasted chats, and gray catbirds will often reject the parasitic eggs and as a result female cowbirds will destroy the nest and kill host chicks.

Competition

Nestling competition occurs between the brown-headed cowbird chicks and the host chicks, with the cowbird chicks receiving the most food and attention because of their larger size and more intense begging in comparison to the host chicks. No significant species competition for resources has been recorded for adults, but minor competition for food may exist between cowbirds and other bird species that also eat seeds and invertebrates.

Predators

Adult brown-headed cowbird predators include red-tailed hawks, sharp-shinned hawks, barred owls, great horned owls, raccoons, eastern grey squirrels, and short-eared owls. Fledgling brown-headed cowbird predators include southern black racers, black rat snakes, and blue jays.

Additional Relationships

It has been found that brown-headed cowbirds can become infected with ectoparasites, helminth parasites, and infectious bacteria. Enteric pathogenic bacteria such as Salmonella enterica and Escherichia coli within the intestinal tract of brown-headed cowbirds can be spread to humans and animals.