I don't see many Dusky x Naumann's intergrades in Kyoto; no more than two or three ever in fact. Those I have come across were fairly obvious, or perhaps I ought to say the ones I've noticed were fairly obvious. A first winter male I saw at the weekend got me wondering whether I might be overlooking less obvious birds. After all I don't look that closely at the thousands of Duskys I see. But is it even an intergrade at all? I certainly don't think it's an F1 hybrid as any bright individual such as this would have far more red and less black by comparison, however I suspect there must be some Naumann's influence in its ancestry.
Below are a couple shots of the last intergrade I found here, at Ogura five years ago. The red in the rump and scapulars is clearly different to any Dusky and the sides of the breast and rear flanks are strongly flushed reddish. There's no doubt about this one.
Dusky Thrush is a very common species throughout Kansai, especially in late winter, and there were quite a few on the beach were I found this bird. Until it flew a little it looked no different to all the others along the tideline but as soon as it did there appeared to be too much red towards the back end and this caught my attention. Looking more closely I felt there was too much red in the uppertail coverts, even the rump (which can be quite rusty on some) was redder than most birds, as was the base of the tail. However to an extent this depended on the angle and at times it could look darker. Occasionally the side of the neck or upper breast seemed to hint at orange-red very slightly, though this too depended on the angle and light. But the rear flanks undoubtedly had a small but obvious amount of red, something I would never expect to see on a Dusky.
Running along the base of the seawall in a brief spell of brighter light I could confirm my suspicion that the uppertail coverts and tail base were redder than I'm used to seeing.
Not only are the rump and uppertail coverts redder than I'd expect from Dusky but the rear flanks also have a few distinctly reddish feathers.
The median coverts are far brighter and more uniform than the blackish-centred feathers of adult males. Here (and the following images) there's a faint rusty wash across the proximal half of all but the central tail feather, this presumably accounts for it giving the impression of being redder-tailed in the field even though I was never able to explain why at the time, I could never see more than the rusty fringe to the bases that any Dusky will show.
In this view, I wouldn't have given it a second glance; this looks a straight forward Dusky. On close inspection some of the small feathers in the breast and at the base of the whitish gorget seem slightly reddish. I'm unsure whether this is an artifact but it may explain why the neck sides and upper breast sometimes seemed to subtly flush orange-reddish.