Su Anderson of Omakau asks :-
We have on our property a pure white sparrow and a pitch-black fantail. Neither have any markings on them. What causes such colouring?
Ilka Soehle, the Natural Sciences Curator at Otago Museum, responded.
There are two different things happening here. First, the white colouring of the sparrow is a condition named albinism - the sparrow would be called an albino. This word originated from the Portuguese expression albo, which means white.
Albinism results from a genetic mutation that interferes with the body's making of the pigment melanin. Melanin is the colouring pigment for the feathers but since there is no melanin produced to put it into those feathers, they are white.
Birds that lack not just melanin but all pigments are called true albinos. In addition to their white feathers their eyes, legs and bill have a pinkish tinge because the colour of the blood shows through when there is no pigment in the tissue.
This genetic mutation only shows up when a bird inherits the albino gene from both parents. Such birds are rare in the wild because a pure white bird stands out against its background, especially at night when it is more vulnerable to predators like cats and stoats. However, albino mutations are found in practically every animal species on this planet. Surprisingly Bristol Zoo in the UK has an albino penguin named Snowdrop.
The pitch-black colouring of the fantail is a condition called melanism. Melanistic birds have an abnormal high amount of dark pigments and the bird usually appears black or dark brown.
Some species have a naturally occurring melanic or normal dark colour variant (or "morph"), for instance the fantail. In fact, New Zealand has quite a high number of melanic bird species compared to other countries, e.g. variable oystercatcher, black robin and black stilt.