Diet is certainly a good hypothesis for the extent of the orange-red
coloration on the head of male western tanagers. The extent and intensity
of the red color on male house finches is related to the amount of
carotenes in their diet at the time they are growning those feathers.
I believe this has also been determined for cardinals as well. To the best
of my knowlege, this has not been tested in western tanagers. An interesting
finding with tanagers is that the red color on male scarlet tanagers has a
very different derivation physiologically than this same color on western
tanagers.

While it is easy to assume that the extent of color is related to the
diet, we should consider that there might be other factors that could
explain it.

Western tanagers do molt their head feathers in the spring, but the peak
of their molt is in April. So I wouldn't expect the individuals that
Maureen saw to be molting.

Mike Donahue
Seattle