I think albino gulls are not at all rare. I've seen albino (to varying
degrees) Mew, Ring-billed, California (specimen), and Glaucous-winged
(several) gulls and a white gull or two that I couldn't identify to species
(not Ivory, Iceland or Glaucous, although there are at times attempts to
turn albinos into one of these species). White gulls do not stand out
terribly much from their flocks, so they don't attract predators or
aggressive social responses as albinos do in some other birds.

Pure albinos, with red eyes, are quite rare, but albinism falling
short of that occurs frequently in birds-I've seen virtually all white
individuals of a half-dozen species and lots more in museums. The effects
can be odd, as when loss of melanism unmasks other pigments. Partial albino
male Red-winged Blackbirds often show orange on their breast! Albinism can
be symmetrical or asymmetrical. White feathers often show up on birds, and
this is probably because the feather-producing follicle has a nonfunctional
pigment gland, rather than from any genes for albinism. This may be the
case in most birds with scattered white feathers, and it is also correlated
with nutrition. I think young crows with largely white wings may be like
that as a consequence of inadequate nutrition as they were growing up. They
just didn't have enough metabolic energy to produce melanins for all their
feathers.

Dennis Paulson, University of Puget Sound