Abnormal Plumage Survey: an Update

Abnormal Plumage Survey: an update

Dunnock wing-flicking, by John Harding

The BTO Abnormal Plumage Survey, which is exploring plumage irregularities of garden birds, has now received well over 1,000 records, involving 48 species. Here, the BTO’s Tim Harrison is joined by Hein van Grouw, an expert on plumage colouration, from the Natural History Museum.

We have received Perhaps the most obvious plumage While these broad categorisations have reports of birds abnormalities are those that involve some proved useful when recording birds at showing abnormal form of white feathering, either just a few distance in gardens, there are subtleties to plumage from white feathers on an otherwise black bird discover. across Britain & or an individual that is completely white Ireland. (such as the Dunnock pictured above). EXPLORING WHITE BIRDS Over three-quarters of the sightings A common misnomer for a bird with a few reported through the BTO Abnormal white feathers is ‘partial Albino’. This is a Plumage Survey have been categorised contradiction in terms; an individual is either as ‘Leucistic’, with Blackbirds and House Albino or it is not. Albinism is characterised Sparrows most frequently spotted. We have by a total lack of melanin pigments (i.e. used the term ‘Leucism’ to describe plumage black, grey and brown) due to an inherited abnormalities where feathers become paler absence of an enzyme that catalyses melanin or turn white unexpectedly, but where production. Albino individuals are totally eyes retain their normal dark colour. The white unless carotenoid pigments (i.e. pale latter feature has been used to distinguish yellow to scarlet red) are also present. An Leucistic birds from Albinistic ones, which Albino adult Goldfinch, for example, would have red or pink eyes, owing to blood being be white all over, except for its red face and gold wing bars. visible through colourless tissue. (back) Grouw and Hein van Tim Harrison (front)

14 Bird Table Spring 2012 Seeing an adult Albino in the wild is Leucism itself is an inherited absence Additional detail on very unlikely, since bad eyesight (one of of pigment cells, and can be total (causing the results to date the consequences of Albinism) means that whiteness of the whole plumage) or partial can be found via survival prospects are poor. It is also worth (causing whiteness in just parts). The white www.bto.org/gbw. noting that an apparently Albino bird might pattern is present throughout a bird’s life Alternatively, send be confused with an ‘Ino’ individual. In an and does not change with age. Far more a stamped, self- Ino individual melanin is still produced but common than Leucism is Progressive addressed envelope it loses its dark qualities. An Ino can appear Greying, which is acquired rather than to APS results, white all over and has reddish eyes, but its inherited. Progressive Greying sees the GBW, The Nunnery, eyesight is much better than an Albino’s and progressive loss or failure of pigment cells Thetford, Norfolk, so an adult bird matching this description is with age, although this can also be caused IP24 2PU. almost certainly an Ino not an Albino. by physical disorders. From the onset of the condition, the bird will gain increasing LEUCISM numbers of white feathers after every moult. ‘Leucism’, as we have used the term in our Should the bird live long enough, it will survey, should really be broken down into eventually be all-white. Progressive Greying a number of distinct types of abnormality. is particularly common in Blackbirds, House While Leucism, from the Greek Leukos, Sparrows and Jackdaws but is difficult to means ‘white’, amongst birdwatchers the distinguish from Leucism without knowing definition has drifted from white feathers to the history of the individual. include pale, washed-out colouration. The latter, however, is caused by other heritable WHAT NEXT? aberrations, of which ‘Brown’ and ’Diluted’ Please keep sending in your records and are the most common. In Brown, changes in photographs to the Abnormal Plumage the appearance of melanin pigments cause Survey, as these help us to clarify black feathers to turn brown. These, in turn, abnormality types in greater detail, identify are easily bleached by sunlight, causing which species are affected most often and them to turn whitish. Dilution, whereby where these birds are being found. Visit reduced melanin production causes www.bto.org/gbw to take part online, or individuals to appear washed out, might ‘phone 01842-750050 for a paper form. also cause confusion. Clockwise from top left: Brown House Sparrow (Martin Goodey); Progressive Greying Blackbird (Finlay Dowell); Progressive Greying House Sparrow (John Harding); Diluted Robin (Donna McGhee)

Spring 2012 Bird Table 15