Article July 7, 1986 Issue

Don't nurture the cowbird's egg

ALICE WILT STRAUSS

A tiny warbler was building her nest. In the brambles nearby a cowbird waited and watched. Now, the cowbird is a nest parasite that depends upon other birds to hatch and nurture its young. So when the warbler had finished her nest building and left the nest unattended, the cowbird hastened to deposit her own larger egg among the other bird's smaller ones.

The unsuspecting warbler returned, and not recognizing the larger egg as an imposter, brooded it with her own. Being larger, the intruder egg received more warmth, and so the cowbird hatched first and was the first to be fed. As the chick grew, it became more demanding, and the warbler, still deceived, catered to its voracious demands. Her rightful heirs were neglected and finally perished from lack of nourishment or else were pushed out of the nest by the cowbird chick.

In some ways it's a grim story, but even in the world of nature one finds examples of protection against such imposition. Some birds, like the robin and the gray catbird, throw out the egg. And there is one kind of warbler that has devised an ingenious way of combating the parasite. Through her alertness the yellow warbler recognizes the cowbird's egg, and she will build a second nest over the first, sometimes even a third one, in her determination to destroy the intruder.

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