From Wikipedia: Xanthochromism (also called xanthochroism or xanthism) is a term that may be applied to birds, fish and other animals whose colouration is unusually yellow through an excess of yellow pigment, or possibly a loss of darker pigments that allows yellow pigment to be unusually dominant. It is often associated with the lack of usual red pigmentation and its replacement with yellow. The cause is usually genetic but may also be caused by diet. A Cornell University survey of unusual-looking birds visiting feeders reported that 4% of such birds were described as xanthochromistic (compared with 76% albinistic). The opposite of xanthochromism, a deficiency in or complete absence of yellow pigment, is known as axanthism.