Aug 17 2010

How do cowbirds know they are cowbirds?

Published by at 7:34 am under Bird Behavior,Songbirds

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)

4 responses so far

4 Responses to “How do cowbirds know they are cowbirds?”

  1. faith cornellon 17 Aug 2010 at 9:33 am

    Such a curiosity. Haven’t thought about cowbirds for a long time. You wonder what nature was up to when this bird originally showed up. Why do they do this, & therefore what is their purpose. I cannot imagine a fox leaving its young around in a bear’s cave or a squirrel putting its babies into a groundhog den. Strange. Thanks for the interesting bit of nature story today.

  2. Marianneon 17 Aug 2010 at 10:00 am

    Thanks for the info, Kate. I didn’t know about teenage cowbird gangs. Very interesting.

    But, I will state it bluntly: I don’t like cowbirds, due to their brood parasitism way of reproducing.

    Cowbirds earned their common name from the habit of following herds of buffalo (and cattle) in search of the insect prey that were flushed up by the large grazing mammals. Since they couldn’t stay in once place long enough to build a nest and raise young, they learned to let other species do that job for them.

    I have seen cowbirds feeding near our horses. Cowbirds will follow the horses around the pasture within a foot of their powerful hooves. The horses don’t mind and tolerate the birds.

    A female cowbird can lay 30-40 eggs over the 2-3 month breeding period (May-July). Because female cowbirds usually lay only one egg in a host nest, this translates into 30-40 nests parasitized (usually of at least several different species) per female in one season.

    The “foster parents”, called hosts, usually raise cowbird young at the expense of their own eggs or young.

    The historical range of the cowbird prior to European settlement was restricted to the open short-grass prairies of the Midwest. Cowbirds have now spread throughout southern Canada and the lower 48 states.

    Almost everything humans do in manipulating our environment is beneficial to cowbirds.

    Some of the above info came from an excellent article by the Smithsonian National Zoological Park. You can read more here:

  3. Kate St. Johnon 18 Aug 2010 at 10:40 am

    Marcy, I think it’s possible to get a permit to trap them. I knew someone who said they had such a permit. When they caught them they killed only the females.

  4. Marcy Con 18 Aug 2010 at 10:26 am

    I watched several cowbirds being fed by a pair of Cardinals. Once they are big enough to fly and know where to get the food, the cowbird parents are nearby and like Kate said, they make their calls sounds. I watched this happen and the young went to the cowbirds and stayed with them…now the Cardinals were able to start a new nest and hopefully full of cardinals and not cowbirds. I really get upset seeing warblers and sparrows doing their frantic feeding of cowbird babies and no time for their own baby species…This is one bird that should be trapped, but they are “protected.” They do trap them where the Kirtland’s Warbler nests…lots of them in big traps. ☺

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