Subject: Why are there partially leucistic birds ?

In France , we noticed birds partially leucistic and they seem to 
become more numerous (carrion crow, blackbird ). 
-I read that it's possible that leucism has a nutritionnal origin : 
how is it possible for melanin ? (it's well-known for carotenoid)
-Most of leucistic birds have only a part of their body concerned 
(head for Blackbird, wings for crows): how is that possible ?
-For Blackbirds, it's well-known that there are very much leucistic 
male than female : why?
Thancks for your help

Re: Why are there partially leucistic birds ?