Cowbirds and bison have a commensal relationship in which the cowbirds benefit from the activities of the bison and the bison is neither helped nor harmed by the presence of the birds. Historically, cowbirds evolved to travel alongside bison herds as they moved through prairies and meadows. The movements and activity of the bison disturb many insects, which the cowbird can then eat.
This relationship between cowbirds and bison was instrumental in the development of the cowbird’s parasitic nesting habits. As they follow the bison while they migrate, cowbirds are unable to stop, build a nest and care for eggs. Instead, cowbirds deposit eggs in the nests of other species. When the young cowbird hatches, the surrogate parents feed it until it is old enough to fledge.
Many other birds take residence near large herbivores, such as cows, oxen and rhinoceroses. In addition to benefitting from the disturbed insects, many of these animals also eat the ticks and other parasites that feed on the larger animals. In such cases, the relationship is better described as mutualistic than commensal, as the herbivore benefits from having fewer parasites. Additionally, the large herbivores tend to discourage the presence of the small birds’ predators.