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Parasitic Behavior of Cowbirds

Last week I wrote about the ruthless strategy of the Cuckoo bird to raise its young. To recap, their strategy is to plant their own eggs in the nest of other birds and have those birds raise their young instead of the Cuckoo having to do it. But the Cuckoo hatching nearly pushes the other hatchlings out of the nest and takes all the food for itself.

This week I want to touch on the American Cowbird and its less heartless strategy for getting raising its young. Instead of hogging all the food for itself, this bird joins with its “brothers and sisters” in making lots of chirping. This, in turn, signals to the parents that it is still hungry and therefore gets the hatchlings more and more food. So, by using this strategy the Cowbird actually gets more food because its chirping along with its nestmates makes more noise than it could by itself.

This strategy has proved to be pretty successful have Cowbirds who used to be only in the Midwest have taken over the entire United States. They have used one of the most common hosts, the Eastern phoebe to gain a presence throughout the country. Other hosts are warblers, tanagers and vireos

Now before, you go off thinking how much nicer these birds are than the Cuckoo bird from England, let me share some other information about Cowbirds. Sometimes, the imposter egg will be rejected from the nest. In retaliation for this, the female Cowbird will actually ransack the nest of the host.

In a study recently done of Warblers and Cowbirds of this parasitic behavior, it showed that when a parasitic egg is removed from the nest, it much more likely to be ruined than when left alone. Not only that, but the female will also plan her cycle of egg laying around when the Warbler rebuilds their nest. One hypothesis is that this had conditioned Warblers to see that when they raise the imposter eggs, they are more likely to be able to keep their young alive as well without their nest being destroyed.

It would be interesting to find out if, two different cowbirds placed their eggs in a host nest if one would edge the other one out of the nest for more attention.

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