Here's the reply we received from Cornell:
Dear ,
That's a great shot! The red in the
Rose-breasted Grosbeak's breast is under the control of carotenoid
pigments. When there's too little of a specific pigment the red
becomes orange. If there's even less, it becomes yellow. Sometimes
we see a bird that is missing the pigment in a part of its feathers, a
white-headed robins or perhaps a crow with a large white patch on its
wing. Sometimes a bird just has a little too much or a little too
little pigmentation in its feathers and occasionally birds are missing
all pigmentation, albino birds. There are a number of causes of such
lack of pigmentation. You can read more about birds at our All About
Birds web site at
http://www.birds.cornell.edu/AllAboutBirds/attracting/challenges/strange_birds/
.
Anne
Hobbs
Public Information Specialist
Cornell Lab of Ornithology
http://www.birds.cornell.edu
