Research on brood parasites usually focuses on the evolutionary arms race between host and parasite, but findings published this week in Nature highlight the importance of a broader perspective. The paper presents seemingly paradoxical evidence that even when the parasite imposes a minimal cost, it still isn't tolerated by the host. The reason lies in an intraspecific battle being fought within the host species: the parasite is simply caught in the crossfire.

The black-headed duck Heteronetta atricapilla, a little-known South American species, is the only obligate brood parasite whose chicks require no more parental care than incubation as eggs: within a day of hatching, they leave the host nest and set off for independent life. Despite the apparently minimal costs of parasitism—limited to incubating an extra egg—the two coot species that play host to over 80% of the duck's eggs frequently reject them from their nests.

However, Bruce Lyon...

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