Leucistic birds have affected plumage which lacks the melanin pigment due to the cells responsible for melanin production being absent. This results in white feathers, unless the normal plumage colour also comprises carotenoids, these remain unaffected by the condition.

Below is a Grey Wagtail I photographed and posted earlier in the year.

Today in the New Forest we spotted A Grey Wagtail with a Leucistic head cap.