Although rare, cowbirds have been documented parasitizing barn swallow nests, leaving a cowbird egg to be raised by the barn swallow.
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 brown-headed cowbird (Molothrus ater) 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 one 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.