You Are What Eats You
The wide-ranging swamp sparrow occurs across North America, from Alaska to Labrador and south to the Gulf Coast.
The swamp sparrow's plumage does not vary much across its vast range, except for a tiny pocket of birds that breed in salt marshes in Delaware and winter in coastal North Carolina.
These birds have a predominantly gray and black plumage, while their inland cousins tend to be more buff and rust. The reason for the difference in plumage has so far been unknown but recent studies suggest that bacteria may play a role.
Scientists from the Smithsonian Migratory Bird Center and Ohio Wesleyan University collaborated on a study to compare the feather-eating bacteria, Bacillus licheniformis, on the 2 types of swamp sparrows. One set of samples came from freshwater cranberry bogs in the mountains of Maryland while the others came from the coastal marshes of Delaware.
Only 40% of inland birds had feather-degrading bacteria on their feathers while 82% of salt marsh birds did. In addition, the bacteria were far more numerous on the salt marsh birds.
Feather-degrading bacteria do quite well in the hot and salty environment of a coastal marsh. It turns out that the pigment melanin, which causes black or gray colors, provides some protection against the ravages of these bacteria.
It may be that coastal plain swamp sparrows have evolved a darker plumage as a protection from quill quelling microbes.
This article summarizes the information in this scientific paper:
Dark Color of the Coastal Plain Swamp Sparrow (Melospiza Georgiana nigrescens) may be an Evolutionary Response to Occurrence and Abundance of Salt-Tolerant Feather-Degrading Bacilli in its Plumage. 2009. Peele, Ashley M., Burtt, Edward H., Schroeder, Max R., and Greenberg, Russell S. 2009. The Auk, 126(3): 531-535.
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