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Summer 1994

Research Magazine > ARCHIVE > Summer 94 > Article

Birds of a Feather
by Cara Runsick

The birds and bees are getting easier to explain all the time. In fact, avian ecologists like Patty Gowaty are finding that Eastern bluebirds have some rather surprising mating habits. And those may shed light on how social behavior evolves among songbirds.

With the help of some new techniques and even newer questions, Gowaty is among the scientists who now have discovered, for example, that female bluebirds are not necessarily the monogamous mates they were once thought to be.

"Although females are socially monogamous, it turns out they are genetically polyandrous -- they mate with more than one male," said Gowaty, a UGA associate professor of ecology. "Sometimes they also have multiply sired broods."

Broods that are multiply sired result in healthier, more diverse offspring that could have increased chances of surviving. But it's another aspect of their social behavior that fascinates Gowaty and guides her research.

"I am interested in the evolution of social behavior, and this is an opportunity to test hypotheses about natural selection," she said.

Specifically, Gowaty is looking at how natural selection -- the principle that the proportion of individuals possessing characteristics advantageous to survival will increase in succeeding generations -- affects variation in social processes in bluebirds.

"I study bluebirds because they interest me," she said. "But if somebody wants to know whether there's any direct utility for understanding, say, human behavior from bluebird behavior, the answer is, 'No.' However, studies of bluebird behavior can suggest hypotheses about human behavior."

Aggression, for example. Gowaty speculates that social monogamy in bluebirds may, in part, be explained by the fact that females fight among themselves.

"While aggression is fairly common between two females, it's almost nonexistent between the females and males," she said. "So why are female bluebirds immune to something human females are so vulnerable to?"

Although scientists don't have hard and fast answers on that, they know that male bluebirds may have more than one mate and one nest. And until recently scientists thought all bluebird eggs laid in the same nest at least had the same father.

But DNA testing confirms that female bluebirds multiply mate. In fact, one study revealed that 19 percent of bluebird chicks were not genetically related to their "fathers" (the male socially paired with their mothers), and 30 to 60 percent of individual nests contained babies from more than one father.

"The DNA studies allow us to infer the genetic parentage of the chicks in each nest," Gowaty said. "The Eastern bluebird, Sialia sialis, is one of several North American bird species now known to be socially monogamous yet genetically polyandrous AND polygynous."

Many other species form lifelong pairs -- a male and a female who stay together through many mating seasons -- but the vast majority of social monogamy research in birds has been done on European songbirds. Based on a literature survey, Gowaty found that only about 20 percent of bird species studied so far actually are genetically monogamous for both sexes.

These and other recent findings, coupled with a subtle change in the way some scientists approach their research, have Gowaty and others pondering new possibilities.

There's been a shift in perspective, said Gowaty, who is also on the faculty of the UGA Women's Studies Program.

"Many new theories about avian behavior are distinguished by really incorporating the female perspective," she said. "Looking at social behavior from a female perspective -- either human or bluebird -- gives rise to new questions. Questions like: What motivates female behavior from the female bluebird's perspective? What are the constraints on female bluebird behavior? Why would she be constrained to mate with one male when another might be a better mate?"

Such questions are shaping up into what Gowaty calls a "paradigm riffle."

"It's not that the whole theoretical orientation of the world is changing, but there definitely are differences in the way newer theories are conceptualizing problems," she said. "We are basing questions on females' perspective."

For example, based on evidence that female bluebirds multiply mate, Gowaty is investigating the advantages of genetic polyandry.

"If we assume that there will be advantages for females that mate with males with good genes, then multiply mating is just a by-product," she said. "The question changes from, 'Why do females multiply mate?' to 'When should a female be genetically monogamous?'"

The answer to the latter seems to be "Almost never," since multiply mating is advantageous in many situations. After all, female bluebirds often can't see all available mates before pairing off. If a lady bird chooses a mate from a limited pool, a "better" one might fly in the next day.

Or perhaps her original choice was a result of what Gowaty calls coercion: The most "genetically attractive" male may not have the best territory or the best nest site. "And the male that displays the most helpful behavior may not have the best genes for, say, resisting parasites and pathogens," she said. "By expanding her mating horizons, the female may provide her offspring with better genes and a better environment."

In the bluebird world, much of this recent riffle was launched by the finding that female Sialia sialis multiply mate, and Gowaty is among those who have expanded its application. The unexplained findings from her previous field studies are a case in point. She found that bluebird males "guard" their mates, following the females around when the latter are fertile. Under the unriffled paradigm, it was believed that guarding males were simply keeping other males away. Logically, this means closely-guarded females should have fewer matings with males other than with their social mates. Closely guarding males, therefore, should be less likely to end up caring for other males' babies.

But several years ago Gowaty found just the opposite: Males who follow their mates most have MORE non-kin babies in their nests. In light of this surprising result, Gowaty re-evaluated the theory using the new perspective.

"What we're thinking now is that the males are trying to keep the females at home," she said. "They don't start guarding until they realize their females might be copulating with other males. The tip-off for males might be how frequently females stay on home territory or stay away from home territory."

Although the overall view is the same -- birds don't want to waste resources caring for someone else's babies -- the slight shift in viewpoint led to a new interpretation.

Gowaty also established that the male of the pair reacts differently to mixed broods. The male and female share feeding duties roughly equally when the nestlings are young. As the chicks grow, the female feeds all the chicks about the same, but the male brings less food to the nest if some of the babies aren't his.

"The big question for us is how does he know. We know he does not tell by looking at them," Gowaty said, "because experiments with nests that contain only the father's chicks have shown that when chicks are switched, he can't tell the difference; the male continues to feed with the same frequency as before.

"I think the male may adjust his behavior based on the behavior of his mate," she said.

In upcoming work, Gowaty will investigate a possible link between female reproductive behavior and male caretaking behavior by attempting to correlate stay-away behavior in females with reduced feeding activity by males.

Other upcoming work also will shed light on guarding behavior by tracking how much the males follow different types of females. Gowaty said she thinks the research team can predict how a male, once he observes that "his" female frequently strays from the territory, will react.

"What we expect," she said, "is that those stay-away females, once they get back into their territories, are going to be guarded like glue."

-For more information go to http://www.ecology.uga.edu/gowaty.html or e-mail gowaty@sparrow.ecology.uga.edu


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