Tuesday, 12 March 2013

Spot the Difference

Catching new birds for the project has been put on hold recently due to the cold and windy weather of recent weeks. Instead we've been out re-sighting previously ringed birds. All our birds carry colour rings providing us with an essential way of identifying seemingly identical looking individuals.
Occasionally however, a bird will give you a helping hand by having a trait that tells them apart from the masses. For example A4, a male, is very tame and allows close approach which provides a clue to his identity. Some traits, such as plumage variations, are far more useful. Although male blackbirds live up to their name, every now and again one will have plumage that deviates from the norm of plain black. One such male is B6 who has a small white spot at the top of his left breast.
B6's white spot makes him easy to recognise in the field.
Plumage abnormalities can be caused by a number of reasons; genetics diet, injury, disease or even age. Where the cause is genetic the abnormal colouring will be consistent from one moult to the next. Abnormal colouring caused by environmental factors, such as diet, can often be reversible. Sometimes you will hear partially white birds referred to as a "partial albino" however this is incorrect - albinism is absolute, there being half way house. A bird is an albino or not.
Blackbirds are among the species most reported with abnormal feather growth. Whether this is because they are abundant and closely associated with humans or because they are just more prone to colour abnormalities is unknown. But research is under way on to the prevalence of abnormal plumaged birds in urban environments. The British Trust for Ornithology run an Abnormal Plumage Surveyand are asking people to report abnormally coloured birds they see in their garden.
Compared to some blackbirds, B6's plumage aberration is very slight but it is still very useful identifying mark, especially when he is feeding in long grass!

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