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Astaxanthin is responsible for the pink plumage flush in Franklin's and Ring-billed gulls - McGraw, Hardy; May, 2005
McGraw & Hardy speculate that farm-raised salmon may be the cause for a "flushed" pink plumage not usually found in gulls (Franklin's and Ring-billed). They take care to eliminate the easy and obvious reasons, like dye, then went to the trouble to assure their results. Indeed, they found one (1) carotenoid when a few were expected.
This issue now is why?
Truely, they are speculating. They both know quite well that there are a variety of reasons, and as (Hill 99) concludes, there are "many inconsistencies between what is known (...) and what is proposed" on carotenoid's function (in relations to immunity).
Is There an Immunological Cost to Carotenoid-Based Ornamental Coloration? Geoffery E. Hill, The American Naturalist, Nov. 1999
pg 3. Levels of circulating cartenoids in various species of vertebrates
Conclusion if immune facility is satisfied, then excess carotenoid might go to plumage.
Carotenoids Produce Flush in Elegant Tern Plumage
Plainly I passed this up, but it was important to McGraw & Hardy. Worth noting is the initial bibliographic note (Stresemann 1927-1933, et. al). It should be clear that these investigation were done before the production of synthetic astaxnatin (via Hoffmann La Roche in 1985).
The Influence of Cartenoid Acquisition and Utilization on the Maintenance of Species-Typical Plumage Pigmentation in Male American Goldfinches (Carduelis tristis) and Northern Cardinals (Cardinalis cardinalis) McGraw, et al., 2003
Conclusion, feeding Cardinals, which should be red, seed that are mainly seed with natural yellow pigment does not effect the color. Hence, they oxidize the seed to make the plummage red.
Animal Nutrition Handbook; Section 17: Fish, Dog and Cat Nutrition and Feeding
Good bullets on the subject.
Major Types of Chemical Compounds in Plants and Animals
Excellent primer.