To answer the question of what makes a white bird white we first have to ask what makes a black bird black. The answer to that question is a pigment called melanin. Melanin is a black pigment that produces the black feather color in birds.
Melanin is an important pigment to birds, even those that are almost white in their normal plumage. Melanin adds strength to the feathers. Many gulls and the American White Pelican are mainly white, but tips of the otherwise white wings are black. The wing tips have the most stress and the melanin provides greater strength. Birds with less melanin in their feathers have weaker feathers that tend to wear faster.
Leucism is a complex condition that causes birds to have abnormally pale or white feathers. The entire bird may be effected or just the feathers on certain parts of the bird. In Leucistic birds other pigments, such as carotenoids, which creates yellow or orange colors, may still be present and active or may also be inhibited from some other cause.
The term leucism covers several different conditions that are difficult to distinguish.
In a leucistic bird or leucistic feathers, the melanin-producing cells are absent and melanin is not produced.
Another condition is sometimes referred to as Dilution. In this case the chromatophore (pigment cell) is present but produces less pigment than normal, producing feathers with a washed-out look.
So what about an albino bird?
Technically Albinism is caused a genetic mutation causing an absence of tyrosinase in pigment cells, which means the bird can not produce melanin. As with leucistic birds, carotenoid pigments may or may not still be present.
While the root cause of the two conditions is very similar, eye color is way to tell the two conditions apart. Albino birds have a pink eye, leucistic birds have a dark eye.
True albino birds are rare and seldom live very long in the wild. Poor eyesight is thought to contribute to their short lives.
With albinism, unlike leucism, there can not be a partially albino bird.
There is much conflicting information about the two conditions on the web. The British Trust for Ornithology, a highly-respected organization, says this:
“As with leucistic individuals, albinos can retain carotenoid pigments if normally present in the plumage.”
The widely-read About web site has this conflicting, and inaccurate, information:
“Albinism, on the other hand, affects all the pigments, and albino birds show no color whatsoever in their feathers. Furthermore, an albino mutation also affects the bird’s other pigments in the skin and eyes, and albino birds show pale pink or reddish eyes, legs, feet and a pale bill, while leucistic birds often have normally colored eyes, legs, feet and bills.”
The most complete explanation that I have found is provided on the Sibley Guides web site. It discusses both brown and black melanin and has illustrations of a normal cardinal, an albino cardinal, partially leucistic cardinal, leucistic cardinal, a cardinal lacking phawolwmanin, a cardinal lacking eumelanin and a cardinal in Dilute plumage.
Melanism is the opposite condition. In melanistic birds more than a normal amount of melanin is present in the feathers and the bird is darker than normal. This condition is generally rarer than leucism in small birds. Several raptor species, however, have both a light color morph and a very dark color morph.