Although incidents of cowbirds parasitizing barn swallow nests are rare, they have been documented. A 1994 observation of 67 Barn Swallow nests found two of these nests to contain cowbird eggs, which were laid by the parent cowbird and left in the barn swallow nest in a parasitic fashion for the barn swallows to raise. Each of these nests contained 1 cowbird egg and both eggs were incubated by the barn swallows along with their own eggs. However, only one of the cowbird eggs hatched. The single cowbird hatchling fledged normally, thus demonstrating that barn swallows are capable of acting as cowbird hosts.
Barn swallows frequently engage in a symbiotic relationship with ospreys, coexisting in a single nesting area to the mutual benefit of both species. Barn swallows will nest either below a much larger osprey nest or in a portion of an abandoned osprey nest. By nesting near an osprey population, the barn swallows receive protection from birds of prey, which are driven away from the nests by the much larger ospreys. In return, ospreys are alerted to the presence of these predators by the barn swallows which give alarm calls when predators are nearby.
Barn swallows eat an enormous amount of insects and are very important in the control of their populations. Barn swallows are also a useful food source for many predators.