Rather than striking out to raise their family, members of some bird species cooperate to help raise their siblings, nephews, nieces, cousins -- or even unrelated young. Researchers have long noted which factors lead to these seemingly altruistic decisions, but now for the first time, Cornell researchers have linked a specific environmental factor to the evolution of cooperative family life in numerous bird species: unpredictable rainfall.

In the Aug. 21 issue of Current Biology, authors Dustin Rubenstein and Irby Lovette report that among African starlings, cooperative breeding is most common among species that live in savannas, where the rainfall varies greatly from one year to the next.

"When you don't know what conditions you will be facing in the next breeding season, it pays -- in an evolutionary sense -- to live and breed in family groups because more chicks survive over the long haul," said lead author Rubenstein, Cornell Ph.D. '06, now at the University of California-Berkeley, who started the study as a graduate student in the Fuller Evolutionary Biology Program at the Cornell Lab of Ornithology.

"It's similar to the way humans group together in the face of uncertainty and buy mutual funds: We're pooling our risk and working together to mitigate an uncertain future," added Lovette, director of the Fuller Evolutionary Biology Program. "Birds that breed in groups buffer the effects of an uncertain environment."

To look for patterns in the breeding behaviors of starlings and the environments in which they live -- including savannas, deserts and tropical forests -- Rubenstein and Lovette examined rainfall patterns at thousands of African sites over more than a century.

To consider the evolutionary relationships among dozens of starling species, they also collected starling DNA samples from museums and from expeditions to East Africa, where they dodged lions, water buffalo, rhinos and other wildlife to capture the birds. Using the DNA, they constructed an evolutionary tree that showed how breeding behavior has evolved over millions of years.

"What's important here is how many times behavior changed," said Lovette. "If you find the same pattern consistently repeated, you can be confident of cause and effect. In this case, we found cooperative breeding evolved when different starling species moved to the savanna, where rainfall varies enormously from year to year. And we found that it's not the amount of rainfall that really matters, but rather whether the rainfall pattern is predictable in advance."

Other factors must also play a role in the evolution of cooperative breeding, and future studies may be able to isolate more of them. Rubenstein wonders if more birds will start cooperating if weather extremes predicted as a result of global climate change materialize. "By studying how animals have already adapted to unpredictable environments," he said, "we may get an idea how behavior could change in the future -- including our own."