COWBIRDS, like cuckoos, are brood parasites—that is, they lay their eggs in the nests of other birds and leave those others to do the hard work of raising their changeling young. But there is a difference. A cuckoo chick usually pushes the original nestlings out, so that it can monopolise the food brought by its unwitting adoptive parents. Cowbird chicks, by contrast, seem to tolerate their nestmates.
The victims of the racket are prothonotary warblers. These birds do not reject cowbird eggs even though they look quite different from their own.
Dr Hoover and Dr Robinson demonstrated what was going on by erecting 182 warbler nestboxes at the top of narrow, greasy poles. (The grease stopped any ground-dwelling predators getting close to the nests.)
The first phase of their study was observational. Over the course of six years, they watched 472 nests in which warblers had laid their eggs. Almost half of these were parasitised by cowbirds. But, parasitised or not, almost all—protected as they were from ground-based predators—successfully produced fledgling warblers.
Then the experiment began. In the following seasons Dr Hoover and Dr Robinson removed cowbird eggs from some of the parasitised 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 thus attacked yielded, on average, but a single fledgling, whereas those with a cowbird egg in them yielded three warbler fledglings. Paying protection money in the form of food for the cowbird nestling thus looks 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 prothonotaries.
The cowbirds' dastardly tricks do not stop at this protection racket, either, for a fifth of those warbler nests that had never had cowbird eggs in them also got destroyed. Dr Hoover and Dr Robinson ascribe this behaviour to a strategy they call “farming”. If warblers lose a clutch, they will often produce a second. 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 favour by killing the first clutch. Even the Mafia never thought of that one.