Q&A YOUR QUESTIONS
QI took this pic (above) in the south of France and wondered if it could be a Pallid Swift. I would appreciate your expert view. Rod Pattison, via email
AAndy Stoddart, Birdwatch’s Identification Consultant, replies: “The separation of Common and Pallid Swifts is one of the toughest identification challenges in Europe. The features are subtle and highly dependent on the light and quality of the views, while photographs need to be interpreted with care. The key features relate to the head and underwing patterns and, thanks to the quality of this image, both can be assessed here.
“The head looks overall rather dark with dark ear coverts contrasting with a small, neat whitish throat. This suggests Common Swift (Pallid Swift shows a paler, more uniform head with a more obvious isolated eye and a larger and less contrasting throat patch). The best feature, however, is the underwing. Here the solidly darklooking lesser and median coverts contrast with the paler greater coverts and the underside of the flight feathers, again pointing towards Common Swift. Pallid Swift shows paler median coverts which blend in more with the rest of the underwing, leaving a narrow dark leading edge formed by the lesser coverts. Structural features are very subtle and hard to evaluate, although the rather smalllooking head and long-looking rear end also indicate Common Swift.”
QWe have what we think is a yellow-headed Greenfinch that has been visiting our garden for around a year. Then this one (below) turned up in July and seems smaller, so we wondered if it was the offspring. We wondered if this sort of unusual colouration happens sometimes in Greenfinches, and what it is caused by? Heather and Edward Shepherd, via email
AHein van Grouw, Senior Curator at the Dept of Life Sciences, The Natural History Museum, replies: “The yellow feathers on the head are the result of the absence of melanin pigment in these feathers. What remains is the yellow carotenoid pigment (melanin in combination with carotenoid pigment gives the green colour). The lack of melanin is most likely the result of ‘progressive greying’, an aberration causing the progressive loss of active melanin-producing cells (comparable with humans getting grey hair).
“There are many different forms of progressive greying and normally after every moult a bird will show more feathers without melanin. Most forms of progressive greying are not straightforward heritable, and the causes are still unknown. The Greenfinch in the photo is an adult male, so did not hatch this year. As it has adult wing feathers, and does not seem to be in moult yet, I reckon this bird is at least two years old, but can be older. It may be the same finch as you saw previously.”