Yes, carotenoid pigments are apparently responsible for many of the pink,
orange, and red colors on birds. Zookeepers have found that you can feed
flamingos and Scarlet Ibises *carrot juice* to keep their colors up in
captivity. It's a fascinating system. When a male Red-winged Blackbird is
partially albinistic, there are often red patches on other parts of its
body where the black pigment is missing! The subcutaneous fat in redwings,
flamingos, and even White Ibises is bright red or orange, while that of
most birds is quite pale. Where do redwings get their carotenoids? Probably
from seeds. There have been recent papers about this in House Finches--the
effect of diet on redness.

But I wonder about the pink that is found on gulls and terns. Do Franklin's
Gulls eat shrimp in the breeding season? Or do freshwater crustaceans have
a good supply of carotenoids (but do Franklin's even *eat* freshwater
crustaceans, or are there carotenoids in grasshoppers?). How about Elegant
Terns--do they eat shrimp along with their primarily fish diets? Do the
*fish* eat shrimp and therefore contain carotenoids that color the birds?
Is the pink color an accident of carotenoids building up in the preen
gland, or is the pink color adaptive in signalling breeding condition, or
special vigor, or whatever? There are plenty of shrimp-eating birds that
aren't red, orange, or pink.

How many of the world's gulls and terns show this? Right now I can't think
of any others, although I have vague feelings that maybe Sandwich and/or
Roseate terns show it too. Why don't any of the larger species do so?

This is another example that shows us that (a) yes, Virginia, there are
simple answers, and (b) however, things are always more complex than they
seem.

Dennis Paulson phone: (206) 756-3798
Slater Museum of Natural History fax: (206) 756-3352
University of Puget Sound e-mail: dpaulson at ups.edu
Tacoma, WA 98416