This is a condition where birds lose part of their pigmentation. This results in white or pale areas of the feathers. Similar to Albinism but an albino bird would be completely white and would have pink eyes. The cuticles of an albino condition are white and therefore the blood in the eyes shows through. Leucism as seen here is only partial and can occur in other animals and not just birds. A Leucistic bird may look quite attractive though! I have listed a few I have come across here.