They're both Ortolan Buntings, both 3+cy males — but the bird on the right virtually lacks any trace of yellow. We checked BWP, found that “birds like these may occur in any part of range (examined from, e.g. western Europe and Turkey)” and thought little more of it. Then, last week, Aron sent me a photo of this bird, ringed by Fabian Meijer in Israel:
This time it's a Cretzschmar's Bunting but it shows the same lack of colour on the throat/moustache as the male Ortolan. Aron had been wondering about this; it seemed logical that an Ortolan could lose the yellow on the head but keep the orange/red on the belly, but why would a Cretzschmar's Bunting lose the orange on the head but keep the orange on the belly?
I tried to cobble together an explanation with my limit knowledge of birds' plumage. First of all, I knew that colour in birds had a few origins. Relevant in this case were melanins, which I knew caused colours from black to reddish-brown, and carotenoids, which I knew caused red and yellow colours. My guess was that the body colour of these birds was caused by melanin whilst the colour of the throat, moustache and, in the case of the Ortolan, yellow 'bloom' to the head was caused by carotenoids. It fitted nicely with what we could see, and it also meant that were were looking at one aberration shared between the two species rather than two different aberrations with similar results. The aberrant Ortolan at Landguard some years back (mutation “brown”, I think), whilst not directly proving anything, certainly did nothing to detract from that theory — although the body plumage has been drastically changed by the reduction in melanin (black parts have become brown), the throat/moustache still appears to be a normal yellow colour.
With no actually reliable evidence for our bunting theory, I decided to contact Hein van Grouw (author of the excellent “Colour aberrations in birds” paper in January's British Birds) to see if he could shed some light on the matter and confirm our thoughts. Here is his very helpful reply:
Indeed the orange brown colour of the belly is phaeomelanin, while the yellow (orange) on the throat is carotenoid.So, mystery solved! Desk birding isn't always boring...
Gene mutations causing a diluted colouration of carotenoids are known but rare. Especially as it is found in two different species I would assume the lack of colour is more likely to be diet related.
Thanks for Hein for the helpful comments and to Aron for prompting the discussion, and to Fabian for the photo.