Brown-headed Cowbirds (Molothrus ater) are brood parasites that lay their eggs in the nests of many species of birds. Most birds that are parasitized by cowbirds accept the egg(s) and raise the chick as their own. However, some species of birds, like American Robins, recognize cowbird eggs and readily eject them from their nest. To do this, birds typically use their beak to toss the foreign egg from their nest. Birds whose beaks are too small to grasp the cowbird egg must use a different strategy to get rid of it.
Yellow Warblers (Dendroica petechia) are one such species. As an alternative to ejecting eggs, females often build a second nest directly on top of their existing nest, burying the cowbird egg along with some of their own eggs. In these pictures, you can see a Yellow Warbler nest that was parasitized by a cowbird and the subsequent nest constructed in response to the cowbird egg. This nest was found in 2008 in a Hawthorn bush surrounded by scrubby open fields and scattered farms, which provide excellent habitat for cowbirds. In an extreme case, a single Yellow Warbler built five consecutive nests, one on top of the other, all in response cowbird parasitism.
– Post by Vanya Rohwer –