Pigments in the feathers of birds with dark plumage may help the creatures rid their bloodstream of some toxic metals, thereby helping them better survive in urban or other polluted areas, a new study suggests. While the finding may help explain the higher proportion of darker colored pigeons in cities, some researchers suggest that other factors may play a larger role.
The pigment melanin, in its most common form, provides black and brown color to skin, feathers, and hair. It also binds to metal ions such as zinc and lead, says Marion Chatelain, a theoretical ecologist at Pierre and Marie Curie University in Paris. Previous studies suggest that bird populations exposed to higher levels of these heavy metals lay fewer eggs and have reduced male fertility.
If melanin indeed provides birds with an opportunity to remove heavy metals from their bloodstream, the pigment could give darker birds an evolutionary advantage over light-colored individuals that can’t detox as effectively, Chatelain suggests. According to that notion, dark-colored birds could better rid themselves of the noxious substances, which bind to the pigments during feather growth and are thus removed from the bloodstream. Generation by generation, they would have more hatchlings than lighter birds, which would over time lead to an increase in their proportion within the population. Indeed, Chatelain says, previous studies have noted a higher proportion of darker colored birds in urban areas.
So Chatelain and her colleagues checked to see if melanin might truly be giving urban pigeons a detox advantage. They captured 97 free-ranging pigeons (Columba livia) in the highly urbanized suburbs of Paris and kept them in cages outdoors for 1 year, feeding them a diet of corn, wheat, and peas. The researchers measured the zinc and lead concentrations in two large feathers removed from each bird’s wings when they were first caught, and then did the same 1 year later for replacement feathers that had grown from the same follicle.
During the birds’ year in captivity, zinc levels in their feathers dropped from an initial average of 328 parts per million to 89 ppm . Moreover, after 1 year, the darker pigeons in the group (as measured by the percentage of wing surface covered by dark feathers) had higher concentrations of zinc in their feathers than lighter pigeons did, the researchers report online today in Biology Letters. That suggests that when fed the same diet and housed under the same conditions, darker birds remove more zinc—and possibly other noxious heavy metals—from their bloodstream than light-colored birds do. Locked away in feathers and out of the bloodstream, the heavy metals have much less of a chance to affect the birds’ health.
While the finding provides some support for the team’s hypothesis, future analyses, including direct measurements of heavy metal levels in the birds’ bloodstream, would likely provide a better assessment, Chatelain says.
“Although the idea is nice, I am not totally convinced yet,” says Marcel Eens, a behavioral ecologist at the University of Antwerp in Belgium who has also studied health effects of heavy metals in birds. Without conclusive data about blood levels of the toxic metals, evidence for the notion is lacking, he suggests. Plus, he contends, because birds only molt, or shed and replace feathers, once a year, it’s highly unlikely that they can detoxify their blood to a significant extent in a highly polluted area.
Instead, he suggests, the prevalence of dark-colored birds in urban areas may stem from other factors. For instance, research hints that darker birds are bolder and more aggressive—traits that could equally well explain their relative success in a competitive urban environment.