Connie Masters of Balclutha asks :-
A blackbird in our garden has more white than black on its body. The white parts are stark white. It used to be chased by the black blackbirds but now chases them. Is it unusual for a blackbird to be born like this?
John Darby, a retired zoologist, responded.
We usually refer to these birds as partial albinos, and they are somewhere between uncommon and rare. It occurs as a result of the lack of normal colour, (pigment) in the feathers.
The absence of colour can happen for one of two reasons. The more unusual results from injury or damage to feather tissue resulting in a few, sometimes only a single white feather. The more normal cause results from a genetic error and interestingly, it is a genetic error that has to be carried by both parents for it to show up in their offspring, yet it may not show in the parents or if it does at all, it may be in only one of them.
Such genes are called recessive genes and are responsible for many factors including characters such as eye and hair colour as well as many diseases both in humans and other animals. Partial albinos and full albinos (completely white birds) get a pretty rough time in the wild. They are much more obvious to predators like cats, and normal plumaged blackbirds will tend to attack or avoid anything that appears to be different to themselves. Birds with white feathers are usually unaware that they are different and when they approach normal birds will tend to be chased away, giving the impression of chasing and being chased.