I posted some comments on Facebook the other day in relation to ageing and sexing Dusky Thrushes. The comments were prompted by a photo posted by Pete Morris and the discussion resolves around that bird, a photo of a bird posted by Steve Rooke, and the Derbyshire bird. Although the comments were very much intended to add some interest to the Facebook thread and not particularly go much further, I've seen a few people mentioning them since and asking where they can see them. With that in mind, I'm copying and pasting them here, where they should be more accessible for those who don't have Facebook or the relevant "friend connections" to view the original post. Give the copy-and-paste nature, you may have to infer context in a few places. I'll thank Pete, Steve and Mike Watson in advance for having nabbed their photos and posted them here.
Since discussion on the Derbyshire bird seems to have centred itself [on this Facebook threat], I thought I'd add my comments on this bird (and the two males above). I'm in the office at the moment and had planned to write a few short words but, well... Sorry Pete Morris for taking up half your Facebook wall!
First, the two males discussed here. For me, Pete's bird is a 2CY (first-winter) and Steve's bird is an 3CY+ (adult). The photos are taken at more-or-less the same time of year, so the wear on the two birds can be compared. Dusky Thrush undergoes a complete post-breeding moult and a partial post-juvenile moult, both undertaken on the breeding grounds. There's no further moult until after the following breeding season, so any retained juvenile feathers following the partial PJM will be retained throughout the bird's first winter.
In addition to the overall higher level of wear on Pete's bird (though see my comment on the greater coverts below), I've tried to focus on structural and plumage features that aren't too affected by time of year. I've focused on four features, which I detail below.
– Primaries. As it happens, I'm currently writing up a short note on ageing passerines using primary shape, with thrushes being a good example of a species where it can be particularly useful. Juvenile primaries are generally more narrower and/or rounder at the tip; adult primaries are broader and ofter more squared-off at the tips. Pete's bird shows rounded-tipped primaries, perfect for an immature bird. In contrast, the primaries on Steve's bird show a bit of an 'angle' to the outer web. You kind of have to take my word for it on this one, but the shape matches what I'd consider fine for an adult. The innermost visible primary may be an odd accidental loss. Sometimes primaries covered by tertials are considerably more protected than exposed primaries and there can thus be quite a contrast between the two, although the contrast here seems too strong for that. The level of wear and extent of fringing between the primaries on these two birds is also in line with expectations for immature vs juvenile. The difference in pattern, too, seems to be age related with immature birds showing a concentrated orange area at the base of the primaries and adult males showing orange extending much further down the primaries.
– Tertials. As with the primaries, there is a similar difference in shape between juvenile and adult tertials. Pete's bird shows rather narrow, quite rounded tertials; Steve's bird show much broader tertials in comparison. Again, the angle of the photo isn't great for assessing this, and this also comes with the caveat that the tertials could theoretically (but probably in reality very rarely) be moulted. Nonetheless, evidence points again to immature vs. adult.
[Edit: As is typical, it wasn't until I hit send that I noticed the inner tertial on Pete's bird, which appears to be adult-type. It's subtly broader than expected for a juvenile feather, has the same thin white edge as in the moulted GCs, vs. the more extensive and more diffuse pale tips to the juvenile middle and longest tertial, shows a more defined contrast between the black and the orange, and has an orange colour that matches that of the moulted adult-type GCs but contrasts with the paler orange or the unmoulted middle/longest tertial, unmoulted GCs, and secondaries/primaries. This doesn't change any conclusions but it does add another piece of evidence to the suite of characteristics that age this bird as a 2CY. It also proves the point that moult limits in thrushes can be subtle!]
– Primary coverts. My own assessment of this matches what's already detailed in Svensson (1992; Identification Guide to European Passerines), which is actually a great starting point for anyone wanting to age a bird, even in the field. In immature males, the dark tip of the primary coverts runs up the outer web of the feathers such that the dark tip on the closed wing is rather diffuse. On adult birds, the black tip is restricted in how much it extends up the feathers, and thus the dark appears on the closed wing as a neat black tip (as in Steve's bird). This is perhaps the most obvious ageing feature for a (male) bird in the field, since moult limits (esp. in the GCs, see below) can sometimes be subtle in thrushes.
– Greater coverts. I think there might be too much weight being put on the white at the tips of the greater coverts. Certainly, it appears to be more extensive on immature GCs but it doesn't seem to be as clear-cut as white/no white. Pete's bird shows a moult limit in the greater coverts, with the outer four being retained juvenile coverts. The most obvious difference between the two ages of feathers is the overall ground colour – paler more wishy-washy orange on the juvenile coverts. Notice that the white tip is more extensive, extending some way up the feather, and also more diffuse. Comparing the GCs to the rest of the wing in general, there is a contrast between the moulted inner GCs and the orange of the tertials/secondaries/primaries/primary coverts. There is no contrast between the retained juvenile greater coverts and these feather tracts, with the same orange tones across all of the retained juvenile feathers. In contrast, there is no moult limit visible on Steve's bird, neither in the GCs themselves nor between the GCs and the rest of the wing. Indeed, all of the flight feathers are a much richer orange-brown, matching in colour the moulted adult-type inner GCs on Pete's.
I said above that there was a "see below" on the wear of the greater coverts. The inner greater coverts on Pete's bird appear to be quite worn around the edges and may suggest worn juvenile feathers. All of the other evidence available points to them being moulted adult-type feathers and I suspect this bird has has a scrap with a thorn bush at some point. Indeed, one of the primaries is pretty mangled and while that is a juvenile feather, that level of damage wouldn't be expected from general wear and tear.
One final point on these birds: as is often the case, Killian's illustration in the Collins Bird Guide go way beyond what is stated in the text and annotations. The differences in for example primary covert pattern, primary colour, and greater covert pattern, have all been illustrated perfectly!
So, for anyone still with me, moving to the Derbyshire bird:
– Primaries. The primaries on the Derbyshire bird are extremely narrow, almost pointed. This is entirely typical for an immature bird.
– Tertials. The tertials, too, are narrow and entirely typical for an immature bird that has retained juvenile tertials.
– Greater coverts. The greater coverts on the bird's left side are all uniform with not apparent moult limit. This could mean one of two things: they're all juvenile or all adult. Overall, base- and fring-colour of the wing in general looks rather uniform, suggesting the greater coverts are the same generation as the primaries, secondaries and tertials (i.e. juvenile). Turning to the bird's right side, GC9 (second-from-innermost) has been moulted (slight asymmetry isn't too unusual) and shows a clear difference in base colour, fring colour and length from the neighbouring GCs. It also contrasts with the primaries/secondaries/tertials, confirming our assessment from the bird's left wing.
In conclusion, the Derbyshire bird can be aged as a 1CY (first-winter).
Assuming this bird is a pure eunomus, which I have no reason to doubt – although I acknowledge the atrogularis/ruficollis/eunomus/naumanni complex seems to be a bit of a mess – the very dull overall plumage would suggest a female. Svensson (1992) states that there are "very slight differences between the sexes, and many are intermediated, difficult to sex". He points out two useful differences between the sexes: mantle/scapular pattern, and primary coverts pattern. His illustrations of a scapular feather show a well-defined black centre in males (as in the two birds discussed above) and a dark, rather than black, feather centre on females, sometimes reduced to just a dark smudge. The Derbyshire bird shows next-to-no contrast in the centre of the scapular/mantle feathers and sits right down and the female-end of the scale. The primary coverts in males are illustrated and are as discussed for the two birds above. In females, the primary-covert tips are dusky and diffuse, lacking the sharply contrasting black tips of males. The primary coverts of the Derbyshire bird (ignoring the feather fringed) appear almost uniform in their base colour, and again puts this bird right down at the female end of the scale.
In conclusion, I would call this bird a female.