Leucistic Bar-tailed Godwit

Leucism is the absence of all pigment in the subject, most commonly partially (we have a blackbird that visits our garden that has white tail feathers), and is differentiated from albinism (which if my understanding is correct is the total loss of pigment, but linked specifically to melanin (I may have that completely wrong...).

The extra shows the leucistic bird alongside its normally coloured fellows.