Among the bedrock assumptions ornithologists make about the role of color in bird plumage is that redness is an honest indicator of male quality and vigor. The more vivid the red, the thinking goes, the fitter the individual.
Scientists assume this because birds don’t synthesize the pigments that give them their yellows, oranges, and reds. Rather, the birds acquire the pigments, known as carotenoids, from their diet.
The relationship holds up from species to species — so long as the birds eat native foods. That’s what researchers from Ohio State University learned recently when they compared Northern Cardinals in rural and urban settings in central Ohio. Thanks to the carotenoid-rich berries of an abundant introduced shrub, the Amur honeysuckle, even urban cardinals whose body condition was poor looked bright.
Canadian investigators report in The Wilson Journal of Ornithology that the berries of Tatarian honeysuckle, another widely propagated exotic bush, have a similar effect. At the McGill Bird Observatory in Montreal in August 2006, they captured 15 Baltimore Orioles that were unusually red where the birds normally are yellow to olive.
Jocelyn Hudon, curator of ornithology at Edmonton’s Royal Alberta Museum, and colleagues collected feathers from the red orioles, extracted the carotenoids, and compared them with the berries. Rhodoxanthin, a pigment of deep red hue only rarely encountered in birds and normally absent in oriole feathers, was in both.
Any avian species that have carotenoids in feathers and ingest honeysuckle berries when molting, Hudon suggests, could potentially have their plumage imbued with unusual reddish tones. “The current availability of rhodoxanthin in much of eastern North America and the midwestern United States, through the bush honeysuckles, offers a natural laboratory to investigate the role of plumage coloration and redness in mate selection in many birds that display carotenoids.”
Read it yourself:
Todd M. Jones, Amanda D. Rodewald, and Daniel P. Shustack (2010) Variation in Plumage Coloration of Northern Cardinals in Urbanizing Landscapes. The Wilson Journal of Ornithology: June 2010, Vol. 122, No. 2, pp. 326-333. Abstract.
Jocelyn Hudon, Dan Derbyshire, Seabrooke Leckie, and Tom Flinn (2013) Diet-Induced Plumage Erythrism in Baltimore Orioles as a Result of the Spread of Introduced Shrubs. The Wilson Journal of Ornithology: March 2013, Vol. 125, No. 1, pp. 88-96. Abstract.
A version of this article appeared in the June 2013 issue of BirdWatching magazine.