U. COLORADO (US)—North American barn swallows outperform their peers in reproduction by maintaining a positive balance of antioxidants, found naturally in plant pigments—and in health food stores around the world.
The study by researchers at the University of Colorado at Boulder is the first to track concentrations of carotenoids in a wild bird or animal species over the course of the grueling breeding season. The swallows obtain carotenoids from insects that feed on plants rich in the nutrients.
Carotenoids can offer the benefits associated with nutritional supplements that protect cells from free radical damage, says Rebecca Safran, lead author of the study reported in PLoS One.
Since American barn swallows migrate thousands of miles to their breeding grounds annually and immediately commence courtship, nesting, and reproductive activities, many lose significant amounts of weight and become physiologically compromised during the intense spring activities, says Safran.
The recent findings suggest that some individuals can bear such costs better than others, she notes.
While other studies have looked at carotenoid levels in captive birds at a single point in time, the new study is the first to monitor carotenoids within wild individuals as they feed, mate, nest, and rear young, explains Safran, assistant professor in the ecology and evolutionary biology department. “Our results indicate the concentrations of these molecules are highly variable within individuals over time,” she adds. “The season-long balance, rather than a sample at a single point in time, indicates which birds are the top performers as parents and mates.”
“By monitoring wild populations of barn swallows during the breeding season, we determined how individual birds managed their own health while enduring the costs of parental care,” says Safran. “Individuals who maintain a positive balance in their nutritional status through the breeding and nesting season are those with the greatest reproductive performance and tend to be darker in color and larger in body mass.”
Safran and her team trapped scores of barn swallows with mist nets in rural sites around Boulder County, measuring and weighing them and taking blood and feather samples before releasing them back into the wild. Each bird was sampled between two and four times over the breeding season.
Since the barn swallow reproductive season lasts about four months, it makes sense that individuals should be able to signal their abilities as parents and mates over time, rather than at the beginning of the season when pair formation takes place, she notes. “The swallows that maintained high levels of carotenoids throughout the summer got more reproductive attempts and produced more offspring.”
Many of the high-quality barn swallow pairs, which weighed more than their peers during the breeding season, produced two clutches of eggs rather than one, producing a greater number of young that fledged, she notes.
“Nutritional status is a 24-hour game, because many nutrients don’t carry over beyond the next day,” she adds. The “top” barn swallows appear to be very efficient at foraging and dealing with the costs of reproductive success on a day-by-day basis, which includes guarding the nest and feeding the young, both of which are physiologically taxing activities, Safran explains.
“Our findings in this study contradict the prevailing scientific views regarding the immense physiological costs of reproduction in birds,” Safran says. “While evolutionary theory says individuals that pay the greatest cost in parental care do so at the expense of self-preservation, we found some individuals are good at doing it all—maintaining their own nutritional status while bearing the costs of reproduction.”
The researchers also found that barn swallows carrying more carotenoids had deeper red breasts—a sign of healthy, robust individuals—and that those individuals darker in color had greater circulating levels of carotenoids at the start of the breeding season. Previous studies by Safran and her colleagues suggest females are more attracted to males with deep red breasts and that they “cheat” less on their male partners than other females. The breast coloring appears to be an indication of status, performance, testosterone, and nutrition, she says.
Researchers from CU-Boulder and Arizona State University contributed to the work, which was funded in part by the National Science Foundation, the Howard Hughes Medical Institute, and CU-Boulder’s Undergraduate Research Opportunities Program and the Biosciences Undergraduate Research Skills and Training.
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