The brightness of plumage for the American Goldfinch is an indicator of theSo the yellow guy may be a superfinch.
amount of carotenoids the bird ingests during the molting period. A friend
of mine, who works with AMGO and their carotenoid pigments, suggested that
the bird may have ingested a larger amount of carotenoids during the molting
period (vs. his flock mates). Maybe the reason he is so yellow is because
he is an "aggressive" bird and able to outcompete his buddies. Another
possible argument is that he may have lived in a different area during his
molt where the foraging was better. Hope this helps.
I was perusing through my new copy of The Sibley Field Guide to Birds of Eastern North America last night, and while I like the format overall and the amount of information given for a small, pocket-size guide, I came across a couple of statements that I questioned. About the Great Blue Heron: "Nests in colonies in dead trees..." I have seen several great blue heron nesting colonies, and the trees they are in were all very much alive. Sometimes the nesting activity of the herons, and the accumulation of waste, ends up killing the trees, but I have seen one particular colony at least once a year since 1971, and very few trees have been lost.
The other statement, on the immediately preceding page, regarding the American Bittern: "Uncommon in marshes, where it hides among grasses and reeds, its cryptic plumage blending in with the vegetation." I once believed that bitterns were so secretive that I had little chance of ever seeing one. That is, until I encountered this brazen bittern last summer.
Despite these discrepancies, I think Sibley's guide clarifies some things that my 20 year old Petersen's and 35 year old Birds of North America failed to. For example, I was mystified by the woo-woo-woo sound produced by the wingbeats of one bird here in the spring, and neither of my books offered much help. Sibley clearly identified it as a Wilson's snipe, which must be a relatively new name for that species.
I'm hoping to get out in the marshes and woods more this spring and identify more of the species that are no doubt here, but which I have not taken the time to see or hear.