Baltimore Orioles (Icterus galbula) of unusual redness over large sections of their plumages were recently discovered in southeastern Canada. Reddish feathers from six of nine specimens sampled at the McGill Bird Observatory in Ste-Anne-de-Bellevue, Montreal, Quebec in fall 2006 contained rhodoxanthin, a keto-carotenoid of deep red hue usually found in plants. Rhodoxanthin comprised ∼5% of carotenoids in many oriole feathers, and up to 18% in the reddest one. Redness in oriole feathers with rhodoxanthin correlated with amounts of that pigment, rather than with amounts of red 4-keto-carotenoids like canthaxanthin normally present in orange oriole feathers. Redness in feathers with rhodoxanthin also tended to be greatest in feathers with the least amounts of carotenoids. The anomalous rhodoxanthin altered the normal relationship between redness and 4-keto-carotenoid concentration, and total feather carotenoid concentration in Baltimore Orioles. We believe rhodoxanthin and the associated aberrant reddish tones result from consumption of berries of exotic bush honeysuckles (Lonicera spp.), now widely propagated in eastern North America and the Midwestern United States. We confirm the presence of rhodoxanthin in the berries of Tatarian honeysuckle (L. tatarica). Rhodoxanthin produces a shoulder at ∼520 nm of the reflectance spectrum of feathers in which it occurs.

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