Brood parasitism poses an identity problem. A songbird normally learns its species song, and often its species identity, from its parents. A cowbird has none, so imprinting on the foster parents would be a fatal error, leading it to court warblers.
The resolution appears to involve an innate recognition trigger. Experiments identified a chatter call given by cowbirds that, when heard by naive young birds, increases their attention to and learning from cowbird stimuli. The parasite is not simply born knowing its song; it is born knowing what to pay attention to, and then learns the rest from the birds that signal in that way.
The consequences for hosts are substantial and uneven. Cowbird nestlings hatch early and grow fast, reducing the survival of host young. Some hosts, such as catbirds and robins, reject the egg. Others do not, and species with small populations and no evolved defence, notably the Kirtland's warbler, have required active cowbird control as part of their recovery programmes.