Spotting a Scarlet Tanager in the upper story of an Eastern Forest transports me to the tropics. And it is a tropical forest dweller in our winter months.
Our Northern Cardinal is almost as rosy and House Finches a bit less. Their red plumage is thought to come from carotenoids obtained in the diet (originally beta-carotene, the abundant precursor in plants—think carrots). Since tanagers are mostly insectivorous and insects have a lot of carotenoids, there is an easy explanation for why tanagers are bright red, except for an awkward fact.
While cardinals remain much the same color the year round, male Scarlet Tanagers are much less rouge when they go south, although they still have black wings and a black tail. They look like female tanagers, olive-yellow. Their diet is still based on insects in the tropics, so that is unlikely to account for the change. Since their testosterone levels plummet to female levels after the breeding season, does that suppress gene expression in the liver where carotenoids are converted to pigments (by p450 cytochrome enzymes)? Please enlighten me, readers.