Research Stories

by Skip Derra

Kevin McGraw holds three barn swallows.

In the world of birds, where fancy can be as fleeting as flight, the color of the bird apparently has a profound effect on more than just its image. A new study of barn swallows reveals it also affects the bird's physiology.

In an experiment that involved artificially coloring the breast feathers of male barn swallows, a team of researchers found that the testosterone levels of the manipulated birds soared in a short period of time. The jump in testosterone, recorded after one week, was unexpected because it was observed at the time in the breeding cycle when levels of sex steroids like testosterone are typically declining.

"The traditional view is that internal processes of birds determine their external features—in other words, physiology forms the feathers," said Kevin McGraw, an assistant professor at ASU's School of Life Sciences and a member of the research team. "But our results indicate that a perceived change in the color of an animal can directly affect its internal physiological state. A barn swallow's hormonal profile is influenced by its outward appearance."

The researchers reported their findings in the June 3 issue of Current Biology in the article "Sexual signal exaggeration affects the physiological state in a social vertebrate." Authors, in addition to McGraw, are Rebecca Safran of the University of Colorado, Boulder and James Adelman and Michaela Hau of Princeton University. This group has been studying the coloration and behaviors of barn swallows for several years.

"The experimental manipulation didn't just improve the males' looks in the eyes of the females, it actually changed their body chemistry," said lead author Safran.

"The speed with which the internal qualities of the bird were affected by the plumage color manipulation was surprising to me," added McGraw. This suggests a dynamic system, he added, one that "speaks to the complexity of sexual signaling systems and the way people should think about how phenotype interacts with physiology."

The new study is the first to show significant feedback between physical appearance and physiology in birds, and has implications for better understanding the ecology and evolution of physical signals such as feather color, the researchers said.

In the animal world, sexual signals by males—from the antlers of elk to the gaudy tail feathers of peacocks—have evolved to convey honest, accurate information about the animal, McGraw said. Evolutionary biologists believe the top males in a population can afford the physiological costs of expressing the most exaggerated forms of sexual signals, like a conspicuous dark feather color that is either biochemically costly to produce or makes those individuals more susceptible to predators, he said.

The new study has evolutionary implications for North American barn swallows since their breast colors are used to convey status, health and the ability to successfully raise young. A 2005 study by this same group, published in Science, showed male barn swallows that were "made over" with darker breast colors bred earlier in the season and fathered more young, and the females that chose them cheated less often with other male suitors.

In the new study, the researchers captured 63 male barn swallows from six colonies in New Jersey at the start of the breeding season as the birds arrived and started forming pairs. The breasts of roughly half the birds were colored with a non-toxic marker to match the darkest, most attractive feathers of males within the population.

The marked birds were released back into the wild, re-captured a week later and administered blood tests to measure androgen levels, including testosterone. In addition to showing increased levels of androgens, the marked birds also lost weight, perhaps because they were more active than their "duller" neighbors, or simply couldn't measure up to the expectations of other barn swallows because of their "counterfeit" sexual signals, the researchers said.

"Increased testosterone levels are often associated with increased rates of activity and competition in animals, which might be why darkened birds lost body mass," McGraw said. "If there were social responses to the color changes we made to birds, they could come from either males or females. Whether colored males looked meaner to rivals or had increased self-esteem by looking fancier to females, we just don't know."

But from the studies the team has done, a clearer picture is emerging.

"These studies cumulatively show that colors are revealing of the bird's individual qualities," McGraw said. "We previously found that the darker guys were more fit, by fathering more offspring, but still didn't know what made a good, dark male. Here it suggests that testosterone and its associated behaviors are closely linked to a male's color and fitness."

"Since the ‘currency' of evolution is successfully raised offspring, the message from our work is that darker males, at least in North American populations of barn swallows, are favored over duller ones," Safran added. "The fact that darker males have naturally higher testosterone levels might be a clue as to why they are more successful."

The researchers now want to turn their attention to why the physiological changes in the male barn swallow happened so rapidly.