American Crows, Corvus brachyrhynchos, frequently show problems with pigment deposition. Approximately 1% of nestlings that I band in New York show some white in their feathers, and four times that many have spots of white on their toes, bill, or other part of the body. I find young like this every year, and the occurrence is about what I would predict based on seeing abnormal crows in large foraging flocks.
This nestling from my study in 1993 showed rather extreme amounts of white relative to what I usually see. (It was found dead on the Cornell Campus soon after fledging.)
"Albinism" or some problem with pigment production can be arrived at in a number of ways, many of them quite distinct from each other. You can think of any complex, enzyme and protein mediated process rather like starting your car. Many things can go wrong, all of which result in you not being able to start your car. You may not have the key, the battery may be dead, the distributor cap may be missing, the car may be out of gas, etc. Similarly, many factors can interfere with the proper distribution of pigment in the feathers of birds, resulting in white instead. The problems can be localized in time and location (e.g., a damaged cell bed or a short term poisoning), and result in white in restricted areas, such as the crow illustrated above. If the problem in more basic (e.g., a genetic mutation) or occurs earlier in development, the entire bird can be affected.