Barn swallows have a symbiotic relationship with ospreys and will nest near them for protection while alerting the osprey of predators with alarm calls.
Barn swallows frequently engage in a symbiotic relationship with ospreys (Pandion haliaetus), 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.