Parasites avoided by controlled hatching

Biologists have found that female birds can modify the hatching order and growth of their nestlings in order to protect them against seasonal nest parasites, illustrating another example of the continual race between host and parasite.

Alexander Badyaev and colleagues studied the breeding of house finches in the Sonoran desert of Arizona. Desert finches breed twice a year, in winter, when nests are parasite free, and in late spring, when most nests are heavily parasitized by mites. In response to selection pressure from the mites, finches have evolved two distinct egg-laying strategies.

The team found that mite-free winter conditions favour longer growth of nestlings, while in the spring, when mites are present, faster nestling growth is favoured to evade the mites. In spring, but not in winter, finches laid female eggs first and male eggs last to minimize the exposure of slower-growing and more vulnerable males to mite infestation. The team says their results show how evolution can produce precise and complex adaptations.