Ragu went
birding near to Buraydah recently and found an odd looking Dove. The bird
appears to be an abnormally plumaged Laughing Dove, although the trouble is
knowing what abnormality has caused the colour change is dificult to prove. In
leucistic birds, affected plumage lacks melanin pigment due to the cells
responsible for melanin production being absent. This results in a white
feathers, unless the normal plumage colour also comprises carotenoids (e.g.
yellows), which remain unaffected by the condition. Although leucism is
inherited, the extent and positioning of the white colouration can vary between
adults and their young, and can also skip generations if leucistic genes are
recessive. The reduction of pigment in leucistic birds causes feathers to
weaken and be more prone to wear. In some situations this can hinder flight,
which, in addition to leucistic birds usually being more conspicuous, can
heighten risk of predation. There is also evidence that leucistic birds might,
on occasion, not be recognised or accepted by a potential mate. Leucism is
being used as an umbrella term to encompass a number of plumage irregularities
that can be difficult to distinguish from each other. One of these is called
‘progressive greying’, which also results in white feathers. While leucism is
heritable, progressive greying is not – but without knowing the history of a
bird, these two conditions are difficult to tell apart. ‘Dilution’ is
another condition where plumage colour often appears ‘washed out’ (i.e.
‘diluted’). In dilution, melanin cells are present (unlike in leucistic birds)
but produce less pigment than normal. White feathers can also be caused by
chromatophore (pigment cell) defects, rather than an absence of
melanin-producing cells. The information on plumage aberrations is taken from
the BTO website.
