Cowbirds are famous, or rather infamous, for a particular breeding habit: they don't raise their own chicks. Instead, they wait until other songbirds aren't looking and lay an egg in the nest, letting another bird family do the heavy lifting of parenting.
But if cowbird chicks are raised by other species, how do they know they're cowbirds? Brown-headed cowbirds are the most widespread brood parasite in North America. They are known to lay their eggs in the nests of as many as 220 different species. (Though only 144 of those species have been able to raise the cowbird chick to fledging. Hawks and hummingbirds apparently are lousy foster parents to cowbirds. Go figure.) That leaves a lot of room for mistaken identity.
The secret, it turns out, lies in a vocal clue. It's a chatter call from adult cowbirds that, once heard, fires off neural responses that helps very young cowbird chicks recognize members of their own species. Despite being widespread across the continent, the call doesn't vary or have dialects — meaning it's innate and not learned.
How the cowbirds chatter call works
The finding is described in a study by Mark Hauber, a professor in the psychology department at Hunter College and the interim vice provost for research at The City University of New York.
The cowbird chicks must hear the chatter call at a young age for it to help them recognize they're cowbirds and not a member of whatever species is raising them. They also need to find and socialize with other cowbirds as they grow up to avoid mis-imprinting with other species.
According to GrrlScientist reporting on Forbes, "This study revealed that the cowbirds' brains change after the birds hear the chatter call by rapidly increasing production of a protein known as ZENK. This protein is ephemeral; it is produced in neurons after exposure to a new stimuli, but disappears only a few hours later, and it is not produced again if the same stimuli is encountered. The production of ZENK occurs in the neurons in the auditory forebrain, which are regions in the songbird brain that respond to learned vocalizations, such as songs, and also to specific unlearned calls."
The study, published in Journal of Experimental Biology, also notes that understanding how cowbirds respond to the sound of this chatter call can inform us about how brood parasite species like the cowbird evolved.
And as GrrlScientist points out, "Cowbirds provide fundamental information about the molecular mechanisms occurring in the brain for guiding how social recognition and learning actually occurs in birds as well as in other organisms — such as people."