Hi Mark,

I think it is a genetic mutation known as melanism, whereby feather colour which is usually restricted to one part of the plumage actually occurs on other parts of the plumage as well. In extreme cases, such as your Sulphur-crested Cockatoo example, the coloration of the sulphur crest occurs across the enter plumage. Melanism in birds is not uncommon, particularly in parrots and cockatoos, but it is very rare for the condition to impact the entire plumage.

Interestingly, birds can see the UV part of the light spectrum. The sulphur crest reflects UV light, so that part of the cockatoo probably shines like a beacon to a bird. If the whole plumage is sulphur-coloured, it may be that the whole bird shines as a result of UV reflection.