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Crows and cuckoos: A unique relationship

A great spotted cuckoo chick is raised alongside a carrion crow chick.
[Image courtesy of Vittorio Baglione]

The great spotted cuckoo is known to be a nuisance. This parasitic bird sneaks its eggs into other birds' nests and tricks the other birds into caring for their young. However, a new study of these cuckoos and of carrion crows shows that the cuckoos aren't all bad: In addition to crowding the crows' nests, they seem to protect the birds from predators.

Daniela Canestrari and colleagues studied these parasites (the cuckoos) and their hosts (the carrion crows) for 16 years in northern Spain. Some of the crows' nests they looked at were parasitized by cuckoos, and others were not. Surprisingly, the researchers found that the nests that cuckoos had snuck into were actually more successful—meaning they were more likely to produce at least one crow fledgling—than the cuckoo-free nests.

The parasitic cuckoos did limit the crows, by competing with their hosts for food, according to the researchers. But, the cuckoo chicks also protected the crows from mammalian predators and other birds, they say. Canestrari and her team performed experiments and discovered that the cuckoos secrete a noxious substance that can drive away cats.

Their findings suggest that the lines between parasitism, commensalism and mutualism—terms that define how organisms interact with one another—are not as black-and-white as researchers have thought. They also show how dependent species' interactions can be upon their environment.