SPRING 2008/VOLUME 22, NUMBER 2
Extortion Rackets and Egg-Farming by Cowbirds
Brown-headed cowbirds are consummate nest parasites, laying their eggs in the nests of hundreds of other bird species. Cowbird nestlings grow fast and generally out-compete their nest mates, meaning that raising a cowbird chick often leads to the death of some or all of the host’s own offspring. Some host species have evolved the ability to recognize and discard cowbird eggs from their nests, but many other species that are commonly parasitized by cowbirds apparently lack this ability to reject the foreign cowbird egg. Because the costs of raising a cowbird are so high, and because cowbird eggs have conspicuously different sizes, colors, and markings than the eggs of most hosts, it has long been a mystery why the ability to recognize and reject cowbird eggs is not more common.
A new study by Florida-based researchers Jeff Hoover and Scott Robinson ("Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs," Proceedings of the National Academy of Sciences of the United States of America) may have resolved this cowbird egg-rejection paradox. These scientists studied a population of Prothonotary Warblers nesting in artificial boxes along the Cache River in Illinois. Because they had monitored the warblers’ interactions with cowbirds over many years, they had a suspicion that cowbirds have a way of forcing warblers to accept their eggs.
Hoover and Robinson tested this possibility through an elegant set of experiments, and determined that the cowbirds respond with retaliatory, “Mafia-like” destruction of the warblers’ eggs when the warblers reject the cowbirds’ eggs. In previous studies, cowbirds have occasionally been seen eating host eggs, but it had been impossible to separate egg predation by cowbirds from losses to the many other possible nest predators. In this study, however, the researchers first ensured that only warblers and cowbirds had access to their nest-boxes by putting the boxes high on thin greased poles to keep out mammalian and reptile predators.
Prothonotary Warblers are one of the host species that does not naturally reject cowbird eggs, so by manually removing just the cowbird egg from some parasitized warbler nest boxes, Hoover and Robinson explored what would happen if some warblers initiated this rejection behavior. In more than half of the nests where the cowbird egg was removed by the researchers, the remaining warbler eggs were quickly destroyed by retaliating cowbirds. In contrast, no such losses were seen in nests where the cowbird egg was left intact, or in nests from which the cowbird was kept away after laying her egg. Intermediate levels of cowbirds destroying eggs were seen in warbler nests that had never contained a cowbird egg.
These findings tell us two important things about how these cowbirds successfully manipulate their warbler hosts. First, cowbird retaliation can limit the practice of cowbird-egg rejection by a host. For example, in this study the Prothonotary Warbler pairs with the simulated egg-rejection behavior raised, on average, just one warbler offspring because most of their own eggs were destroyed by retaliatory cowbirds. In contrast, those warblers that accepted and raised a cowbird along with their own nestlings produced almost three warbler offspring per warbler pair. This is less than the four warbler offspring raised by non-parasitized pairs but, given the presence of a cowbird egg in the nest, the best available option for the warblers is simply to raise the cowbird along with their own chicks.
The second intriguing finding is that cowbirds may “farm” their hosts to increase the number of available warbler nests that suit their needs. Cowbirds destroyed clutches of Prothonotary Warbler eggs that were too close to hatching to be suitable for a newly-laid cowbird egg. Overall, cowbirds destroyed about one in five of such nests, thereby forcing the warblers to lay a new clutch of eggs that provided the cowbird a new opportunity to insert one of its own.
We’ve long known that raising a cowbird nestling is costly to most songbird hosts. We now know that the cowbirds use a sophisticated combination of retaliation and predation to punish hosts that reject cowbird eggs and to create new opportunities for parasitism.
Irby Lovette is director of the Cornell Lab of Ornithology’s Fuller Evolutionary Biology Program
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