We caught this bird during our normal MAPS operations on our previous visit to Maplewood Bog. No, it’s not a new species – it’s a Rose-breasted Grosbeak with some plumage abnormalities. We weren’t sure exactly what was going on here when we caught it. When we got home, I did some searching online (ah, the miracle of the internet) and turned up this discussion on the WhatBird Forums. In it they quote an article written by Julie Craves (Coffee & Conservation) where she discusses a similar bird.
The yellow breast (and underwing coverts), it turns out, are the result of a condition called xanthochroism. Just like albinism is a lack of pigment, and melanism is an excess of pigment, xanthochroism is an abundance of yellow pigment. It may be caused by certain abnormal items in the diet or dietary deficiencies, or may be genetic. The yellow may either be present in excess, or it may replace another colour (usually red, which is often itself controlled by diet).
This individual was an older bird, at least three years old (“After-Second Year” in banding lingo) based on certain plumage characteristics, but given the extent of white at the base of his primaries, he may be much older than that. Oftentimes, as a bird ages its markings grow in size or extent and can be a rough gauge of age. It’s not a perfect science and there’s always exceptions (who knows if it’s linked to his yellow breast), but this guy may have seen quite a few summers – I can’t recall ever seeing a grosbeak with such an extent of white on his wings.