Hello again Kate
We ran this past the experts in our Ornithology Department, and they have responded to your query as follows:
The black colour in magpies and other birds is due to melanin pigments deposited in the feathers. There are two types of melanins, called “eumelanins” and “phaeomelanins”. The former produces greys and black, the latter produces varying shades of brown and chestnut. What color you see depends on how much of each type of melanin is present. More eumelanin, you get grey or black. More phaeomelanin, you get brown or chestnut. If you have neither, you would get white feathers.
What may be happening is that there is some genetic mutation that is preventing eumelanin production. If the phaeomelanin is still produced, however, you would get brown feathers, rather than white (what’s known as leucism).
Also, the occurrence of this colour mutation is higher in NE NSW and SE Qld for some unknown reason. Up there this colour also commonly occurs in the Torresian Crow and could account for misidentification of this species as the Asian House Crow. This is a rare type of plumage aberration, but not as rare as albinism.
Hope this helps!