"Across North America this spring, female brown-headed cowbirds will wait in the pre-dawn light for a songbird next to be left unattended. In those moments of opportunity, the cowbirds will swoop down and lay an egg in the nest of an unsuspecting mother.
Cowbirds are obligate brood parasites, which means that they lay their eggs in the nests of other birds and leave them to be hatched and reared by foster parents. Completely shirking the labor-intensive duties of nest-building, egg incubation, and raising young, they pawn the work off onto another species. Over one year, a female may lay 40 eggs in the nests of other species. Cowbirds often sneak their eggs into the nests of the yellow warblers, song sparrows, and red-winged blackbirds.
The cowbird chicks take full advantage of this system: they grow faster and beg louder than the songbird chicks in the nest with them. Often, they become comically larger than their host parents. The nestlings can drain the foster parents’ energy, and meanwhile, the cowbird parents don’t have to endure fussy babies or late nights. It’s a neat trick.
Brood parasitism creates an interesting problem, though. Most animals learn their species type by observing those physically closest to them in the first days of life. However, when a cowbird chick hatches, it sees, hears and smells a songbird. An adult songbird brings it food, and it lives in a nest full of baby songbirds. Everything in a cowbird chick’s surroundings would lead it to believe that it, too, is a songbird. Given this confusing foster care system, how does it learn its cowbird identity?
In addition to their normal calls, adult cowbirds make special vocalizations termed “chatter.” Chatter is only used and understood by cowbirds – it is essentially a secret language. Nestling cowbirds have selective hearing for this chatter. When they hear it, they raise their heads, open their eyes, and look for the source of the sound. This genetically-endowed selective hearing for “chatter” creates an auditory connection between the nestling and its future flock, which may play a large role in resolving the chick’s early identity crisis.
For cowbirds, it is an easy transition from the nest to the real world. Juveniles recognize adult cowbirds as soon as they leave the nest and begin to associate with them, completely neglecting the adults of their foster species. Chatter plays an important part in this phase of development, too, providing a way for cowbirds to identify one another.
Adolescent cowbirds have a steep learning curve, and they quickly learn the necessary “real world” cowbird social behaviors: calls, feeding strategies, mating signals and stealthy egg-laying practices. They make no mistake about which species they are to mate with come springtime, and they are quick to carry on the egg-dumping tradition.
Brown-headed cowbirds deploy an underhanded method of raising their young, as well as a fascinating password system to influence chicks while they’re being raised in another bird’s nest. Depending on your perspective, either cowbirds have developed an ingenious system, or they’re just ungrateful avian foster children."
(Broadcast: "Fieldnotes," 5/17/15 & 5/22/15. Listen weekly on the radio, Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., or Fridays at 4:54 p.m., or via podcast.)