Cowbirds are brood parasites, the American equivalent of cuckoos, laying their eggs in the nests of other birds and leaving the rest to their hosts. This has led to a lot of fascinating behavior to ensure the hosts do the hard work of incubating the eggs and feeding the young, but another aspect is much less understood: how does a cowbird learn to be a cowbird when raised by another species?
Birds usually learn their parents’ calls by spending so much time with them when young. They may even develop ideas of what to look for in a mate this way. Since cowbirds could no more mate with members of their foster species than a human could with a wolf, this poses something of a problem. A male cowbird that sang the mating song of its foster parents, or a female attracted to such a song, would have no offspring.
Dr Mark Hauber of the University of Illinois is a leader in the study of cowbird behavior, previously bringing to light the way juvenile cowbirds sneak out at sunset to hang with others of their species. Now he has revealed in Current Biology the existence of a secret “password” that enables cowbirds to learn from older members of their species.
Besides their songs, cowbirds make what ornithologists refer to as a “chatter call”, which is consistent through the year and across their geographic range. Hauber and co-authors suspected this call might serve as an alert to young cowbirds, telling them to tune in because they were about to learn something important.
He tested the theory by playing adult songs from canaries to young brown-headed cowbirds with and without cowbird chatter calls and observed their reactions. Association with a chatter call induced young males to learn the canary song much more than hearing it paired with sounds from doves. They also showed more familiarity with canary songs paired with cowbird chatter calls than unpaired songs. Similarly, young female cowbirds’ brains were more affected by songs from other birds if they were exposed to them together with their own species’ chatter calls.
For this to work, an affinity with their species’ chatter call must be hard-wired into brown-headed cowbirds, and it is likely other species have their own version. However, it is far simpler for birds to have developed a predisposition to listen out for a particular call than to know the entire songs, and possibly other behaviors, of adult cowbirds. The term password is apt; the chatter calls act as a simple method to access rich stores of information that would be harder to remember or biologically hardwire.
Besides being an example of the ingenuity evolution can produce, brood parasites can sometimes teach us something about more common methods for raising young, in this case, the delicate dance between learned and innate behavior.