In barn swallows, enhanced coloration triggers other changes
A new study conducted at the University of Colorado and involving Cornell University demonstrates that the appearance of female barn swallows, specifically the shade of their chestnut-colored breast feathers, can affect their health.
“There is some mechanism that allows the bird to somehow assess what they look like, and match their physiology to what they look like,” said Joanna Hubbard, a sixth-year doctoral student in ecology and evolutionary biology at CU.
It was already established that in North American barn swallows of both sexes, those with darker breast feathers had higher reproductive success than those with lighter colors, according to Maren Vitousek, a Cornell senior research associate. She led the new study while a postdoctoral researcher at CU.
There is evidence that breast feather color is significantly influenced by genetics, but melanin-based plumage color such as that in barn swallows also has been linked to social status as well as circulating testosterone, she said.
“Another bird can look at the individual (bird) and sort of know something about that individual,” Hubbard said. “Is it worth starting some sort of interaction as a potential mate or a potential competitor, or is it not worth it?'”
The new study, published Wednesday online in Biology Letters, showed that naturally darker barn swallow females — and those with artificially darkened breast feathers — also had lower levels of oxidative damage, which could ultimately make the birds healthier.
Oxidative stress results when the production of harmful metabolites known as free radicals exceeds antioxidant defenses in the birds, which can lead to DNA, protein and fat damage in the birds, Vitousek said.
For the barn swallow study, Vitousek, along with CU assistant professor Rebecca Safran and their team of undergraduate and graduate students, captured 60 female barn swallows in Boulder and Jefferson counties.
Thirty of the birds were used in the control group, while the other 30 had their ventral plumage darkened using a non-toxic Prisma marker. The testosterone, oxidative damage and antioxidant levels of each bird was measured at that time. Birds were then released.
After one to three weeks, 19 of the artificially darkened females and 17 birds from the control group were recaptured, tested again for testosterone, oxidative damage and antioxidant levels, then released once again.
“Intriguingly, females whose feathers were darkened to resemble ‘attractive’ birds rapidly adopted the physiological state of darker birds, decreasing their level of oxidative damage,” Vitousek said in a news release.