Researchers find antioxidants key to barn swallow’s ability to thrive
Summit County correspondent
Glenwood Springs, CO Colorado
BOULDER, Colorado – North American barn swallows, which are widespread throughout Colorado, outperform their peers in reproduction by maintaining a positive balance of antioxidants, according to a new study conducted by researchers at the University of Colorado at Boulder.
CU researchers tracked concentrations of carotenoids – naturally occurring plant pigments commonly sold in health-food stores – in wild birds over the course of the grueling breeding season. The antioxidants can help protect cells from free radical damage.
American barn swallows migrate thousands of miles to their breeding grounds each year and immediately commence courtship, nesting and reproductive activities. Many lose significant amounts of weight and become physiologically compromised during the intense spring activities. But the new study indicates some individuals can bear those costs better than others, according to CU assistant professor Rebecca Safran, lead author of the study.
“Our results indicate the concentrations of these molecules are highly variable within individuals over time,” Safran said. “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.”
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.
“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. 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 said.
Safran and her team, which included dozens of CU-Boulder students and volunteers from the community, 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 two to four times over the breeding season.
The three carotenoids measured in the study – leutin, zeaxanthin and beta cryptozanthin – all are antioxidants sold widely as nutritional supplements for humans. The swallows obtain carotenoids from insects that feed on plants rich in the nutrients.
According to Safran, it makes sense that individuals should be able to signal their abilities as parents and mates over time, rather than just at the beginning of the season when pair formation takes place, since the barn swallow reproductive season lasts about four months.
“The swallows that maintained high levels of carotenoids throughout the summer got more reproductive attempts and produced more offspring,” Safran said.
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 added.
“Nutritional status is a 24-hour game, because many nutrients don’t carry over beyond the next day,” Safran said.
The top barn swallows appeared 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.
“Our findings in this study contradict the prevailing scientific views regarding the immense physiological costs of reproduction in birds. 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,” Safran said.
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 said.
A 2008 study by Safran and her colleagues showed the testosterone of male North American barn swallows skyrocketed early in the breeding season when their breast colors were artificially enhanced to the deep red most attractive to females. The birds likely had more testosterone racing through their bodies because of amorous interactions with the opposite sex and more run-ins with competing males.
According to Safran, the new study’s implications for humans is that antioxidant levels vary significantly day-to-day, depending on intake.
“All this is a long-term game with these water-soluble nutrients. They are only in our systems for 24 hours, so you can get yourself pretty far behind,” she said.
Barn swallows are one of the most widespread songbirds in the world, and are easily observed in Colorado, including Summit County, during the summer months. They migrate to Central and South America in the fall.
The birds can be recognized by their blue bodies, red-orange bellies and a long, slender, deeply forked tail. Females are slightly more muted in color and shorter-tailed than males.
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