We become so used to seeing birds such as robins and chickadees and sparrows that we notice right away when one of them is oddly colored. The abnormal color variations that are seen in birds often are the result of mutations that involve the reduction or lack of melanin pigments. When melanin is reduced, dark feathers become paler, even whitish. When melanin is absent, feathers are pure white.
Albinism involves a complete absence of melanin and usually results in a pure white bird with red eyes. The melanin pigment that gives birds brown eyes is absent, so the red blood is visible through the transparent cornea. This mutation does not affect other pigments, so if a bird has carotenoid pigments coloring it red or yellow, that color may remain.
Leucism is different from albinism in that it may affect all pigment types, reducing their concentration to produce a paler bird or eliminating them entirely to produce a white bird. This can occur in some or all feathers, making some birds a patchwork of normal and white or whitish feathers. A bird that is white with brown eyes is leucistic.
Odd variants are typically seen in the most common birds, so even when such a mutation is rare, we see enough individuals to eventually encounter a funny-looking one. Robins that were pale overall have been reported with some regularity. Dark-eyed Juncos are often reported with scattered white feathers, even a head pattern somewhat like a chickadee; pure white ones are much rarer, perhaps in part because they are very conspicuous to predators and don't last long! You can see why these mutations would remain rare.
Some populations of Black-capped Chickadees in the Seattle area have persistent leucistic genes, and individuals are seen year after year in a neighborhood with different combinations of whitish caps and backs and white outer tail feathers (looking a bit like a junco as they fly away). Apparently chickadees don't have enough visual predators to eliminate these genes entirely.
I know of at least four male Red-winged Blackbirds in Washington that had plumage almost identical to the one shown here. Apparently that leucistic mutation is widespread in the genome of this species, at least in the Pacific Northwest. Is this some basic blackbird color pattern that is suppressed by the males' black pigment?
Leucism can be hard to detect in birds such as gulls that are already gray and white, but a pure white one like this Ring-billed Gull usually indicates this mutation. Remember that egrets and swans aren't leucistic; they just evolved white plumage, obviously advantageous to them.