Peter Hill, of Mosgiel, asks :-

We are puzzled by a bird which has been in our garden frequently this summer and autumn, though not before that. It is the same size as a thrush, but has a plain brown chest. It's most distinguishing feature are several white spots, about the size of a fifty cent piece, on it's back and it has some white under its wing. It is mostly seen at ground level and is digging up the moss on the lawn to look for grubs. It appears to spend much of its time at ground level, though it is happy to sample the grapes we are growing on a pergola. What is this bird?

Yolanda van Heezik, a zoologist at the University of Otago, responded.

The bird you describe is almost certainly a female blackbird (which is brownish rather than black) with a plumage aberration called leucism.

Much more is known about plumage aberrations in Europe and North America. In 2012 The British Trust for Ornithology held an Abnormal Plumage Survey across Britain, and in less than a month received nearly 700 sightings of more than 35 different species. Three quarters of records were of leucistic birds and nearly half of these were blackbirds, mostly male.

In Dunedin another leucistic blackbird currently lives in the Town Belt. Plumage colour comes from pigments in the feathers: melanins (blacks, browns) and carotenoids (yellows, oranges, reds). Blue results from scattering of light depending on the structural properties of feathers, and greens come from a combination of pigments and structural properties of feathers.

Aberrations in plumage colour include: albinism (complete lack of melanin in the plumage and the skin), leucism (complete lack of melanin from all or part of the plumage), schizochroism (lack of a single pigment from part of or all of the plumage – well known among bird breeders but rare in the wild), melanism (abnormally high levels of melanin result in a darker bird overall), carotenism (a shift towards redder or yellower plumage) and dilution (a decrease in pigments overall resulting in a faded appearance).

Why does this happen? Genetic mutations of unknown origin cause leucism and other aberrations. Carotenoid pigments originate from the diet and might be affected if the diet is deficient, or the bird is diseased. In blackbirds, protein deficiency has been linked to whitened plumage, particularly in urban areas.

What are the consequences for the bird? It is possible that the white patches may make the bird more easily detected by predators, or affect its mating success, as such birds can be shunned or picked on by their own kind.