Unless they are Snow Geese or Pelicans, white birds are kind of unusual in our area. It's not hard to figure out why...they can't hide from predators very well, seeing as how we never have snow. What made this bird even more unusual was that it was just the right size to be a flycatcher, and it was behaving just like a flycatcher, perching and then flying out for bugs and then returning to the perch. But flycatchers aren't white, especially the flycatcher known as the Black Phoebe (Sayornis nigricans). The picture below shows the normal appearance of a Black Phoebe.
I could barely understand what I was seeing, given that I am still a bit new at the bird-watching business. But when I got the angle in the picture below, I was strongly suspecting that I had found a leucistic Black Phoebe, even though I had never heard of such a thing in the past. I tried googling right then in the field the term for a white color in birds but mistook leucocratic instead of leucistic, and kept getting references to granitic rocks instead of anything having to do with birds (I figured it out later at home).
Leucism is different than albinosim. Albino animals lack melanin, a natural pigment that provides dark color. They are generally pure white, and their eyes will be pink (click here for an example of an albino California Scrub Jay). Leucistic birds have melanin, but they are inefficient at using it in their feathers, so they may show some color, as seen in the Black Phoebe I saw today.