Some colors dependent on food source
You are what you eat. Or at least that's what we've heard all our lives. Of course it's not meant in a literal sense. I don't consider myself a pizza although I indulge in the vegetable, meat, and cheese pies with some regularity. A quick Internet search of the phrase's origin reveals a 19th century French origin. We English borrowed it sometime later and it's come to represent the notion that food controls our health according to phrases.org.uk. Good enough.
Every organism obviously depends upon a steady diet of whatever it is that keeps it going. A certain competitive edge goes to those which can vary their intake depending upon prevailing food sources. And so it was with some immature Baltimore orioles in eastern North America some years ago which led researchers down a mysterious trail of detective work, according to a 2007 article in "Birding" magazine by Flinn, Hudon, and Derbyshire.
The palette of coloration which birds display represent virtually the entire spectrum of imaginable hues from fluorescent greens and deep azure blues to bright reds and the darkest blacks. The nature of these various colors, however, stems from different sources, some structural, some of pigment origin, some a combination.
One can get deep into head-spinning biochemistry if a person lifts the lid on the subject of feather pigmentation. But the Cliff's Notes version is this: there are basically three groups of pigments which control the coloration of feathers.
The first is melanin. Yes, the same group of pigments which mandates skin and hair color in humans is responsible for the blacks, some reds and even pale yellows of some birds' feathers and skin. Melanin-rich feathers are structurally much stronger and longer lasting than other feathers. This could be the reason birds such as American white pelicans and snow geese maintain black wing tips.
Another class of pigment is porphyrins. These are producers of such colors as pinks, browns, reds, and greens such as those found in pigeons and pheasants. Curiously, under ultraviolet light porphyrins fluoresce a bright red.
While the two pigments above arise from physiological processes within the birds, the third type - carotenoids - is different. It originates in plants. The birds pick up carotenoids by eating plants or something which ate plants. They are then modified and expressed by oranges, reds, and yellows in feathers such as that found in the American goldfinches at your feeder.
The red of the northern cardinal is another example of carotenoid pigmentation, as is the pinkish blush found on quite a few Franklin's and ring-billed gulls. Even flamingos -- which lose their pink coloration if not exposed to a natural diet -- are painted with carotenoids.
(Most blue feathers, by the way, are not pigmented as such. That's right, the blue jays, indigo buntings, and bluebirds are in a way, pretenders. The blue coloration appears as a result of light refraction from the structure of the feathers. Pick up a blue feather some time and shine a flashlight through it from behind. It's brown.)
Now back to the mystery. A strange orange coloration was first noticed in the Northeast in the 1960s at the tip of some immature cedar waxwings' tails, a stripe normally yellow. By the 1980s a deep red carotenoid called rhodoxanthin was identified in the these feathers. But what plant, apparently fairly new to the landscape, was the source of the red pigment? In 1990, after testing several berry-producing shrubs it was found that two bushes - Morrow's honeysuckle and Tartarian honeysuckle - did indeed produce rhodoxanthin.
The exotic plants had been introduced in the Northeast some years earlier and have now become well established. Work done by Hudon in 2006 on the aberrantly red-hued Baltimore Orioles confirmed the presence of this same carotenoid making it the second species in which it was recorded, according to the authors. It is highly suspected that these wild birds are picking up the pigment from the bush honeysuckle berries at the time feather molt is taking place.
In the long-term, this could produce some negative biological complications for the birds. Say, for instance, female orioles prefer the deeper reds displayed by some young males over the usual orange mature ones. Might this lead to poor reproduction due to the relative lack of skills on the part of the immature males? The article states, "The potential impact of widespread rhodoxanthin ingestion on the evolutionary ecology of the Baltimore Oriole remains to be determined."
In a sense these birds really 'are what they eat,' at least in terms of coloration. And in this case, it might not be the best thing for them. Neither is the pizza I had last night for that matter.