To add to my earlier posts on head color of male western tanagers...

The Birds of North America account of the western tanager by Jocelyn Hudon states:

...."rhodoxanthin produces red hue of head. The latter pigment is thought to be acquired directly from the diet without modification, unlike the red carotenoids in scarlet, summer and hepatic tanagers, and other emberizines [finches], which are believed to be derived metabolically from common yellow dietary precursors. Rhodoxanthin is an uncommon pigment in birds, having been reported only in pigeons of genus Ptilonopus, several cotingas and a manakin. Small quantities of rhodoxanthins have also been detected in northern cardinal and orange-tailed cedar waxwing."

He also states that the rarity of this pigment in nature strongly suggests a dietary origin, but that "preliminary analysis of available food items have failed to identify obvious sources."

Mike Donahue
Seattle