Wednesday 25 February 2015

Dusky Thrush variation

What a difference a camera makes. One great thing for me is being able to turn it on common species and see the kind of detail that I'd probably never have taken the time to discover without being able to scrutinise images. In an earlier post, a couple of weeks ago I talked about a Dusky Thrush that had caught my eye and I'd suspected may have had a touch of Naumann's in its family history, this post is to follow up on that.

There were two or three farmers beginning to plough their fields in Mie when I was last there on 8 February and this provided a good opportunity to look at a few Duskys at close range. From a flock of about 50 birds in one field I was able to get reasonably good images of a handful that came close to my van.

None of the features I though were suggestive of past introgression on the previous bird now look convincing in the light of the variation I saw in this random selection of photographed birds. All the points that caught my eye then could be seen on one or more of the birds on the more recent visit. Indeed a couple of these birds might have made better candidates for showing traits of past introgression but I now feel more inclined to think these features must be within normal Dusky variation. If so then there must be a few Duskys out there that combine most or all of these features and could be confusingly suggestive of an intergrade.

Firstly, no apologies for re-posting old digiscoped shots of an April intergrade in Kyoto a few years ago. This bird is a stand-out example that really doesn't leave any doubt what it is.

Dusky x Naumann's intergrade in April, Kyoto.

Following are images of seven birds that came within camera range, none of them were photographed because I thought they were of particular interest at the time. Two are obviously adult males showing all the distinctive features birders would love to see in a vagrant, one appears to be an adult female and the rest first winters.

Bird one
These shots show an already very smart looking male, not many look as good as this in early February and it was definitely the sharpest bird I saw well that day.

Bird two
This bird has much fresher looking, broad pale fringes still partially masking the bolder plumage of the previous adult male.

Bird three
A first winter with oddly asymetrical tertials. There are reddish feathers on the rear flanks which seems to be normal at this age, though they aren't usually as prominent as on this bird. The extensively brown tail with a rather uniform bright rump and bright tail base look quite like a Naumann's but the blackish longest upper-tail coverts don't.

This bird was photographed in Kyoto city in January 2011 and shows a more typical contrast between the reddish rump and darker tail.

Bird four
Even in dull conditions the proximal three quarters of the tail are brown contrasting with a blacker distal area. In sunlight this brown is surprisingly warm and conspicuous creating the impression of a blackish terminal tail band.

Bird five
This seems to be an adult female and as with the earlier adult males the tail is blacker from most angles. Most birds appear to have blackish-brown markings on the under-tail coverts, if any at all, however this bird is distinctly reddish. Though it doesn't show too well in these jpegs it also has partial reddish fringes along the length of the flanks and more prominently at the sides of the breast. The sides of the neck are also flushed orange to a greater extent than I've noticed on other birds.

Rusty markings on the vent and under-tail coverts plus a few marked fringes along the flanks are unusual.

There are two or three clearly rusty-fringed feathers on the side of the breast and warmer markings across the flanks.

The sides of the neck have an obvious orange flush. Those at the bend of the wing are probably from the under-wing coverts which are in any case orange on Dusky.

Bird six
An odd feature here is the pale dashes on the outer webs of the scapulars, I didn't come across this on any other bird.

Bird seven
This bird looks a nice standard no questions raised first winter.

So there's plenty of variation to be going on with in this small sample, and enough of interest to keep me checking all the birds I see.

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