The pinkish tone of this bird's feathers is a natural occurrence in a small percentage of Ring-billed Gulls. The pinkish tone is caused by the presence of carontenoids in the feathers. The presence of the carotenoids is believed to be caused by high levels of astaxanthin in the bird's diet during the period of feather growth. (see http://www.jstor.org/pss/27639295). Notice also that the bird's bill and legs are much more orange than those of a typical Ring-billed Gull in breeding plumage.