Why Some Birds Flock Together

Aug. 22, 2007 — -- Scientific research is rarely easy, but Dustin Rubenstein may have raised the bar considerably during the years he spent in Africa trying to answer a basic biological question. At one point he was chased by rhinos, and one night he watched a huge cat make a kill just 25 feet from his tent, but he still managed to come up with the answer.

Rubenstein wanted to know why some species help each other raise their young while other species that are closely related go it alone. Why, for example, does the "superb starling" maintain complex social relationships with its relatives, letting them help with all the nesting chores, while other starlings seem virtually antisocial?

For decades other scientists have asked the same question, not only about birds, but all sorts of animals that choose "cooperative breeding" over the familiar mom-and-pop lifestyle. Some have suggested that it might have something to do with where they live, or their immediate environment, but that wouldn't explain why some similar species that share the same environment choose such different courses.

Rubenstein's research, published in the Aug. 21 issue of the journal Current Biology, strongly suggests that the answer does indeed lie in the environment, or more specifically, the predictability of seasonal weather patterns. He has collected DNA samples of starlings around the world, and he has studied rainfall patterns across Africa covering nearly 150 years, and all of it points to one conclusion. If you can't count on the rain coming when it's supposed to, thus producing the food you will need for yourself and your young, you're going to need a lot of help from other members of your family.

Rubenstein is an evolutionary biologist at the University of California, Berkeley, but the research was carried out while he was working on his doctorate at Cornell. His partner on the multiyear project was Irby Lovette, director of Cornell's Fuller Evolutionary Biology Program.

There are 117 species of starlings around the world, but there are 45 species in Africa alone.

Rubenstein roamed across much of Africa, especially Kenya, where there are 26 species -- more than in any other country in the world -- but he paid particular attention to the superb starling, a colorful bird with a very complex social network.

Using DNA analysis, Rubenstein determined that when a new superb starling hatches, all sorts of kinfolk show up to help out. Most are siblings, or at least "first cousins," he said.

The researchers found that one third of all African starlings are cooperative breeders, but here's the key. They all live in the savannas, the semi-arid grasslands scattered across much of Africa. And nearly all the noncooperative species live in the forests.

Long-term rainfall data collected by the National Oceanic and Atmospheric Administration from more than 2,000 sites across Africa filled in the most important blank. The records showed that rainfall in the savannas is very unpredictable from one year to the next.

"Savannas have a pronounced dry season and a pronounced wet season," Rubenstein said. "But what was surprising was that the variability in each of the seasons is much higher in the savannas than it is in the forests or deserts.

"For a particular bird, you can be pretty sure that the rainy season is going to come at some point, usually when it's supposed to come, but it may be late, or it may be early. The amount of rain may be much higher, or much lower. So there's really no way to predict how much rain, or how much food, you're going to get during a given breeding season."

So for birds that chose to live in the savanna, he added, "it pays to live in family groups. That gives them a better chance to feed their babies."

OK, but what's in it for the cousins, and the siblings, who hang back to help out around the nest? Is this really just altruistic behavior?

Not exactly. Whether starling or human or whatever, what drives the mating instinct is the desire to pass on one's own genes, guaranteeing at least a partial hold on immortality. In an ideal world, Rubenstein said, it's best to go it alone, because your offspring will get half your genes. But if the chances of success are slim, it's better to hang out with the family because you share some genes, so at least some of your genes will be passed on down the line.

"Everyone's looking out for their own best interest," he said. "If you breed on your own you will be producing offspring that are more related to you than if you are helping someone else. But if you can't go it alone, you can pass on at least a share of your genes by helping to raise relatives."

So in the savanna, where rain and food is less predictable, a smart starling chooses to maintain a close relationship with its kinfolk to ensure propagation of the family lineage.

Rubenstein is comfortable with his conclusions, and he said the results were worth all those long and hazardous trips across Africa, but he knows there are still a lot of questions concerning "cooperative breeding." Environmental factors are probably part of the reason why some species work together, while others remain aloof, but the study was based on only 45 species of starlings.

"That's a good group," he said. But there are many birds that don't conform to the patterns seen in Africa, he added.

"It's still not clear why some that live in the same area do (engage in cooperative breeding) and some don't," he said. "There are still a lot of questions."

He expects to expand the research to other animals. Hopefully, he will always be able to outrun the rhinos.

Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.