Cool. It would have to be genetic I think. I've not encountered or heard of choughs like this - might interest people if you wanted to share this to the COG chatline?
Leucism (resulting in 'leucistic' animals) occurs infrequently throughout many vertebrates including humans (eg friends with a white spot in their dark hair). Its effect looks like a partial albinism. These individuals can be positively selected by breeders (eg think of piebald and skewbald horses) or nature (the white Bennett's wallabies on Maria Is) but more often white seems deleterious (eg the regularly reported white kangaroos around the ACT rarely seem to last). There are claims, e.g. by Jerry Olsen, ACT, that raptors target individuals in a group that look 'different', so if confirmed, such behaviour by predators would be expected to result in negative selection for leucism.