Aug 17 2010
Earlier this month I visited Cape Cod and enjoyed sitting on my sister-in-law’s porch watching the birds go by. One morning I saw a gang of two dozen birds land in her yard and poke through the grass looking for food. They were the same size and color as juvenile starlings but they had black feather patches visible among the brown.
What could they be? With binoculars I was able to identify them as teenaged brown-headed cowbirds, molting into adult plumage.
If you think about how cowbirds grow up, it’s a wonder this gang existed at all.
Cowbird mothers lay their eggs in the nests of smaller birds. Each cowbird chick is raised, not by its own mother, but by foster parents of another species. Yet instead of flocking with their foster families these young cowbirds had found each other.
How did they do it?
Juvenile brown-headed cowbirds, even while in the nest, are attracted to the sounds of their own species, especially the chatter call. As they grow up they pay attention to what they themselves look and sound like. Occasionally adult cowbirds, possibly their parents, visit near the foster nest and show them cowbird behavioral tips.
When the juvenile cowbirds become independent of their foster parents they use these visual and audio cues to find others of their own kind.
Their first winter is crucial. Studies have shown that if they’re forced to hang out with another species all winter, they think they’re a member of the other species and are confused for life.
So these teenage cowbird gangs serve a purpose. Without them, cowbirds wouldn’t know they are cowbirds.
Maybe that would be OK.
(Read Marianne’s comment below to see why.)
(photo in the public domain from Wikimedia Commons. Click the image to see the original)