For nearly 30 years Leonard and I have observed the red-tailed hawks (Buteo jamaicensis) in Big Valley (Modoc County CA). Not long ago as we were counting red tails along the road, I commented that the number of melanistic red-tailed hawks seems to have increased over time. Melanistic red-tailed hawks have dark breasts while most red tails have breasts that are more buff. I had no data to support this statement, it was only a feeling from hours of informal hawk-watching.
Menno Schilthuizen in his book Darwin Comes to Town reports on the work of N Yu Obhukova and Marion Chatelain. Working with urban pigeons, Obhukova demonstrated that darker “melanistic” pigeons were more common in polluted urban areas than in rural areas which, theoretically, are less polluted. Melanistic birds have more dark melanins (see “Melanins” 02-14-19) in their feathers. Chatelain, again using urban pigeons, showed that birds exposed to heavy metal pollutants (zinc, lead, copper, for example) are able to purge their bodies of heavy metals by transferring them to the melanin-laden dark feathers. This suggests that perhaps urban pigeons are evolving toward darker plumage because of the detoxifying properties of darker feathers.
Big Valley is an agricultural area where chemicals (herbicides, pesticides), fertilizers and organic compounds (manure) are routinely added to fields in large quantities year after year. These amendments are often high in heavy metals. I wonder if there is any correlation between my casual observation that there appear to be more melanistic red-tailed hawks and the constant addition of compounds containing heavy metals (or other pollutants) to the local environment?
These red-tailed hawks were photographed along Modoc County Road 87 near our ranch.