Steve Piotrowski and Chris McIntyre
Progressive greying on adult Whitethroat captured at Minsmere On 9th June 2013, Waveney Bird Club ringers involved with ringing demonstrations at RSPB Minsmere captured a family party of Whitethroats that included an aberrant adult showing a number of white feathers. The inner-most primary on the right wing, four primary coverts on both wings and two of the left outer-tail feathers were pure white. Birdwatchers often describe such birds as partially albino, but a recent paper published in British Birds (Hein van Grouw, 2013) states that there is no such thing as a partial albino bird and the white feathers are caused by progressive greyness.
It is suggested that progressive greying is caused by a form of food deficiency as birds are unable to extract enough tyrosine from its food resulting in pigment loss. The condition is said to be nonhereditary as demonstrated by the family group trapped at Minsmere, the offspring of the aberrant adult showing no white feathers.
Van Grouw states that “albino birds are rarely found in the wild as the absence of melanin in the eyes makes them highly sensitive to light and gives them a poor depth of vision. Most die soon after fledging.”
Whitethroat back
Whitethroat wing
Progressive greyness is an ageing process and more white feathers could appear each time the bird moults. I suppose we could compare birds with ourselves – as we grow older our hair turns grey, but when birds grow older some feathers become white following each moult?
Acknowledgement: Thank you to Adam Rowlands (RSPB Minsmere) for alerting the ringing team to the paper published in British Birds that discusses this condition. Reference:Van Grouw, H. 2013. The causes and recognition of common colour aberrations in birds. Brit Birds 106: 17-29.
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