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.
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| 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!
