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Where did cooperation come from?

Speckled warblers are cooperative breeders. Here, a speckled warbler feeds a fledgling black-eared cuckoo.
[Image courtesy of David Cook]

Why do people (and animals) help each other? What's in it for them? It's a fundamental question that scientists have been asking for years—and a new study in birds is helping to explain how the idea of cooperation first evolved in the animal kingdom.

William Feeney and colleagues studied birds known as superb fairy-wrens, which sometimes team up and work in groups of three or more to raise baby birds in their nests. This type of behavior is known as cooperative breeding, and it's been associated with slow-paced lifestyles; monogamous relationships, in which couples partner up for life; and unpredictable, changing environments. But, the researchers say that something else might inspire cooperation among these fairy-wrens too: the presence of brood parasites, or "cheating" birds like cuckoos that sneak their eggs into fairy-wren nests.

Feeney and his team noticed that two regions of the world—sub-Saharan Africa and Australasia, which includes Australia, New Zealand, New Guinea and neighboring Pacific islands—have become hotspots for both cooperative breeding and brood parasites over the years. They discovered that cooperatively breeding birds were more likely to be targeted by brood parasites in those regions, and that parasites usually grew bigger and survived longer when they associated themselves with the cooperating birds.

But, the researchers also found that larger groups of cooperating birds could resist the parasites better than smaller groups of the birds. And taken together, their results suggest that the two different behaviors—cooperative breeding and brood parasitism—actually reinforce each other. Parasites receive better care from cooperatively breeding birds, they say, but cooperating birds have better chances of defending themselves.

In light of their findings, the researchers suggest that brood parasitism—cheating birds, like cuckoos—may have been a factor that helped cooperation take root in bird populations long ago.