Most birds learn the art of being a bird from their parents. Since behavior is species-specific, this usually means lessons in how to be a duck, eagle or condor. Cowbirds, however, are reared by different species entirely, because their parents force them on others rather than doing the hard work themselves. This raises the question if they are more hardwired than other birds. A paper in Animal Behavior provides a glimpse of an answer.
The researchers found that juvenile delinquent brown-headed cowbirds sneak out of their homes at sunset to spend the night away from their nestmates and the birds that raise them. As birds reared by the adults of other species, this prevents the cowbirds from learning the ways of their adopted parents or becoming attracted to their nestmates.
Although unrelated to the common cuckoo, brown-headed cowbirds (Molothrus ater) are sometimes referred to as cuckoos because they demonstrate similar brood parasitic behavior. Both species lay their eggs in another bird's nest, leaving it to incubate the egg and feed the young, leaving the parasite free to spend more time feeding.
However, where many other brood parasites rely on making their eggs blend in, so that the host birds don't realize they have been conned, brown-headed cowbirds intimidate their victims into doing what they want. They use mafia tactics, letting the foster birds know that if they don't play ball, their own eggs will be sacrificed.
While the parent birds forced into raising the young cowbirds provide warmth and food, they can hardly be expected to teach cowbirds how to behave. Dr. Matthew Louder, now at East Carolina University, explored how juveniles learn necessary cowbird skills while he was a Ph.D. student at the University of Illinois at Urbana-Champaign.
"If I took a chickadee and I put it in a titmouse nest, the chickadee would start learning the song of the titmouse and it would actually learn the titmouse behaviors," Louder said in a statement. "And then, when it was old enough, the chickadee would prefer to mate with the titmouse, which would be an evolutionary dead end." Louder wanted to know how cowbirds avoid the same trap, particularly since they have been shown to become attracted to other species when left too long in a cage with them while young.
Louder anticipated that maternal cowbirds would pay visits to the nests in which their offspring are maturing to encourage them to leave their hosts and learn to seek an appropriate cowbird mate. However, after radiotagging adult female cowbirds, Louder and co-authors wrote: “Contrary to our predictions, we found no support for the facilitation of juvenile cowbird dispersal by adult female cowbirds.”
Instead, young cowbirds leave the nest, and indeed the forest, at sunset. “Our results suggest that the solitary nocturnal roosting behavior of juvenile cowbirds may facilitate independence from their hosts,” the paper notes. Nights away from home probably reduce the danger of cowbirds becoming sexually attracted to their host species. The cowbirds have yet to be observed spending time together during these nighttime escapades, although the authors think this may occur, allowing them to interact with, and learn from, other cowbirds.
Just what the forest needs – gangs of teenage mafiosa hanging out together and learning to bully.
Image in text: Traditional radiotracking methods failed, forcing researchers to establish radio telemetory towers to track the birds. Credit: Matthew McKim Louder.