How Cowbirds Run Protection Rackets

The Economist magazine recently published an article (subscriber only) on some interesting findings published in the Proceedings of the National Academy of Sciences concerning Cowbirds (pictured right).

Cowbirds, like Cuckoos, are brood parasites. This means that they lay their eggs in the nests of other birds and leave those other birds to raise their young for them. But there is a difference between cowbirds and cuckoos. A cuckoo chick usually pushes the original chicks out, so that it can monopolize the food brought by its adoptive parents. Cowbirds, however, seem to tolerate their nestmates.

Scientists thought this was odd, and decided to look into the matter. What they found is that the host bird's real chicks are pawns in a protection racket of a sort the Mafia would be proud to have invented. The victims of the racket are prothonotary warblers (pictured right). These birds do not reject cowbird eggs even though they look quite different from their own. That in itself is intriguing, for cuckoos, again in contrast to cowbirds, lay eggs that mimic those of their hosts.

The scientists proceeded to conduct a multi-year study to determine what was going on between the warblers and the cowbirds. The first phase was observational, where for six years they watched 472 nests in which warblers had laid their eggs. Almost half of these were parasitized by cowbirds. Then the real experiment began. In the following seasons, the scientists removed cowbird eggs from some of the parasitized nests. At the same time, they reduced the diameter of the entrances to some of the nest boxes, in order to deny admission to cowbirds (which are larger than warblers).

Warblers whose nests were thus protected did well, raising an average of four chicks to maturity in the absence of a cowbird parasite. Nests from which cowbird eggs had been removed, but which lacked protection, did badly. In fact, more than half of them were attacked! The eggs were pecked open and the nests themselves torn to pieces. Nests that were attacked yielded, on average, one chick, whereas those with a cowbird egg in them yielded three warbler chicks. Paying "protection money" in the form of food for the cowbird chick thus looks like a good deal from the warbler's point of view, and explains why cowbirds do not need to disguise their eggs to look like those of warblers!

The cowbirds' tricks did not stop at this protection racket, either. A fifth of the warbler nests that had never had cowbird eggs in them also got destroyed. The scientists call this strategic behavior "farming." If warblers lose their eggs, they will often produce more. If a cowbird female fails to lay in a warbler nest in time for her egg to hatch with those of the host, she can reset the clock in her favor by killing the first group of eggs. Even the Mafia never thought of that one!

An example of brood parasitism is shown in this picture of a common cuckoo being raised by a reed warbler: