Rockin' around

There's a Blue Rock Thrush in Gloucestershire at the moment. It's origins are up for debate and, as with every debate on origin, the age of the bird has come under scrutiny. I have to admit I'm not sure age really makes that much difference with some of these vagrants – and I'm staying out of the origin (and indeed subspecies) debate – but, since there seems to be some confusion over the age of this individual, I thought it was worth cobbling a few words on the subject (and cobble is the right word, since I've been keeping track of most of this debate on my phone from a delayed train heading back to London and haven't long been back at my desk). Initially, observers were calling the bird a 1CY ("first-winter"). I posted on Twitter yesterday to say the bird looked like an adult, and subsequent reports on the news information services went out as such. Today, though, I see opinion has swung back to it being a first-winter. A few people have been wanting to know why I aged the bird as I did, so here are my reasons for why the Gloucestershire Blue Rock Thrush is an adult:

First of all, as always, let's start with the moult of the species. The species has a partial post-juvenile moult in its first year and a complete post-breeding moult in subsequent years. In that respect, moult strategy is as in Dusky Thrush. Per BWP, the partial post-juvenile moult includes "head, body, lesser and median upper wing-coverts, occasionally tertial coverts (4 of 20 examined), and rarely some inner greater upper wing-coverts or central tail-feathers". Don't worry about what the tertial coverts are; you'd need to go rummaging around for them even if you had the bird in your hand. From a practical point of view, that moult means on a first-year bird we're likely to be able to see a moult limit either between the median coverts and the greater coverts, or within the greater coverts themselves. It also means, since birds have just one moult a year, those moult limits won't change until the bird undergoes its first complete moult after breeding in its second calendar-year. Blue Rock Thrushes don't moult primaries, secondaries or primary coverts during the post-juvenile moult, so they will always offer helpful clues to the ageing regardless of the extent of the rest of moult. BWP matches with my observations from photos that the moult in this species is usually quite restricted (e.g. just one or two GCs moulted, and thus 8 or 9 old greater coverts retained) but Svensson (1992) hints that some birds may be able to moult all GCs. If that were the case, a moult limit would be between the greater coverts and the always-retained primary coverts, primaries and secondaries.

So, what can we see on the Gloucester bird? This post on BirdForum seems (from what I can gather) to be the reason for the age switching from adult back to first-winter. I agree, the linked photo shows an apparent slight difference in colour across the greater coverts but this is not a moult limit. It's simply light reflecting off the central greater coverts in a way that makes them look paler than the inner – and outer – coverts. There are plenty of other photos of the bird that show the greater coverts to be uniform all the way across; see below photos by Lee Fuller, for example, and multiple examples elsewhere online.

Adult male Blue Rock Thrush, Gloucestershire, December 2016 © Lee Fuller

Adult male Blue Rock Thrush, Gloucestershire, December 2016 © Lee Fuller

Adult male Blue Rock Thrush, Gloucestershire, December 2016 © Lee Fuller

Adult male Blue Rock Thrush, Gloucestershire, December 2016 © Lee Fuller

The greater coverts are all of the same generation, but are they all adult or all juvenile? First of all let's look at the base colour, which is deep black across the entire feather tract. In 1CY birds the base colour of the GCs is brownish. Secondly, the feather fringes are deep solid blue, and rather broad. Young males can show some blue to the feather fringes but it's duller, paler and thinner to the point that it often wears away by spring. See e.g. here and here, and also here for examples of young males. I'm not sure when the first two photos were taken, but the third bird is a 2CY ("first-summer") bird in early summer. That individual has helpfully dropped and renewed one of the central greater coverts. Despite the high level of wear across the entire feather tract, contrast between the retained juvenile GCs and the renewed central GC is still visible and the black base-colour and broad blue fringe to the renewed GC is apparent. The median coverts of the Gloucestershire bird show the same adult-type pattern as the greater coverts. Per references and observations, first-year birds always moult their median coverts, so a bird with retained juvenile GCs would show some contrast with the adult-type MCs, which we don't see in the Glos bird. All the median coverts and greater coverts are adult-type, but perhaps this bird is one of the rare (theoretical?) individuals that has moulted all greater coverts during its post-juvenile moult. The same black base-colour and blue fringing as shown by the GCs is present across the the primary coverts, the tertials, secondaries and primaries (see e.g. here); the entire wing is adult-type, a sure sign that the bird must be a 2CY+ (adult) bird that has undergone a complete post-breeding moult, and not a first-year ("first-winter") bird which would have retained at least its juvenile primaries, secondaries and primary coverts, which would contrast with any moulted flight coverts. There is also no contrast between the body feathers (which are always moulted during the post-juvenile moult) and the flight feather; the paler blue fringes to the wing feathers in first-years contrast with the deeper blue of the moulted adult-type body feathers; in adult birds, the shade of blue is uniform across the whole bird, as in the Glos bird.

Even if the bird can be safely aged as an adult on these features alone, it's worth checking that all the other features fit. The tail feathers on the Glos bird are broad and rounded, and also the same generation as the wing feathers (and body feathers). Primary shape is good for an adult, as is tertial shape and primary covert shape. Overall, this bird looks in fresh condition with no obvious wear or bleaching; you'd expect a first-year to be more worn around the edges by now regardless of origin. As an additional "don't put too much weight on it but it's worth mentioning anyway" feature, the bird's iris looks to be particularly warm-brown; in most first-year passerines the iris is a darker greyish-brown, although this is heavily dependent on light conditions and can vary both between and within species.

There's one thing I've deliberately left till last, and that's the white tips to the greater coverts and primary coverts. I've repeatedly seen reference to the pale-tipped GCs as evidence that the bird is a first-year. Yes, pale tips to the greater coverts are indicative of immaturity in some species but in other species, including Dusky Thrushes as discussed in the previous blog post, a pale tip in itself is of no use as an ageing feature. And just as with the Dusky Thrushes, Svensson's Identification Guide to European Passerines is a great starting point for attempting to age this species. Svensson says of adult Blue Rock Thrush, "All GC uniformly fresh, tipped white and edged blueish... ...Primaries and PC dark, fresh and glossy, very narrowly tipped white." He says of first-years, "Primaries and PC slightly worn, sometimes rather broadly tipped buffish-white or off-white. In some, outermost GC unmoulted, edged dull brown and contrasting with fresh inner [GC], edged as in adult". I've added the bold text myself. There's even a figure to go with the description of the primary coverts:

Blue Rock Thrush primary coverts, from Svensson (1992).

Blue Rock Thrush primary coverts, from Svensson (1992).


For me, the Gloucestershire bird – with its neat, thin, white tips to its blue-edged primary coverts – matches Svensson's description and figure for an adult male near-perfectly. Combined with the other characteristics mentioned above, I have no doubt that the Gloucestershire bird is an adult.

And for anyone who wishes to purchase a copy of Svensson's Identification Guide to European Passerines, you can do so from the BTO. £20 of any birder's money well spent!

Edit: 13/01/2017 Since I posted this blog post, debate over the Blue Rock Thrush’s age has continued. Rather than engaging in these debates on Twitter (where the 140-character limit simply isn’t enough to give an unambiguous explanation of subtle features), I thought I would address some of the issues I’ve seen raised with an edit to this blog post.

As a general overarching point with this bird – and others – I’m perfectly happy for people to have questioned my conclusion. It forces me to go back and critically assess the assessment I’ve made. I have to admit though, the more photographs I see of this bird, the more satisfied I become with the conclusion of my initial assessment.

Moult limit (1): I’ve seen several further mentions of a moult limit in this bird, based on the photograph that’s linked from the BirdForum post that I linked to in my original post above. My opinion on this hasn’t changed but it’s worth stating again that this apparent moult limit is, in my opinion, a trick of the light. Assessing a moult limit from a single image can be misleading and this apparent moult limit is not visible in any other photo of this bird.

Undertail coverts: I’ve seen some argument that the buffish fringes to the undertail coverts on the Glos bird make it a first-winter. At least some of this seems to have stemmed from a misunderstanding of text in Svensson (1992) that says, regarding sexing characteristics, “Applicable after post-juv. moult, which takes place in Jul–Oct.” and goes on to say of males, “Chin and throat bluish, tinged grey in fresh plumage. Under TC bluish-grey, tipped buffish-white and with dark grey subterminal marks.” The confusion seems to have arisen from the phrase “applicable after post-juv. moult”, which appears to have erroneously been taken to mean “birds in first-winter plumage”. In this case, “after post-juv. moult” is not referring to the plumage acquired immediately after the post-juvenile moult but rather any plumage after the post-juvenile moult; that is to say, these characteristics are not applicable in juvenile plumage but they are in all subsequent plumages. 

With regards to the undertail coverts themselves, it had been argued that adults show white fringes to the undertail coverts and first-winters show buff fringes. Unless I’ve misunderstood something, the birds on which this feature has been assessed appear to have been aged using the criteria laid out in my original post detailing why I believe the Glos bird to be an adult. An assessment of the undertail coverts on these birds seems to have been made, and that assesment applied back to the Glos bird; the same Glos bird that was originally the basis for the ageing criteria used to age the birds where undertail coverts were assessed. The result, as far as I can see, is a cyclic argument where the two joining ends of the circle are at odds with each other.

I haven’t studied the undertail coverts on this species with regards to ageing in detail myself, so this is one area where I will say that I don’t know for sure, although with a quick Google search it wasn't difficult to find e.g. this bird, a clear 1CY in October with, to my eyes, whitish tips to the undertail coverts; or this bird, a 2CY in May with buffish-white tips to the undertail coverts. My general impression of European/Arabian Blue Rock Thrushes (and the European/Arabian bit is important in this sentence) is that the underpart fringing is generally the same from the breast to the undertail coverts. Regarding the Glos bird, my impression of the breast and belly feathering is that it is fringed buffish-white with a dark subterminal band (see e.g. here). The undertail coverts of the Glos bird are, in contrast to the breast and belly feathers, fringed a much richer buff-pink or pale buff-orange, depending on which photograph you look at (see e.g. here). This strikes me as a feature which is more than likely related to the orangy/pinkish/rufousy outer webs to the outermost row of undertail coverts; and this in turn I’m sure must surly be a clue towards the subspecific identity of the bird, not the age of the individual. A colleague of mine analysed a large number of photos of Blue Rock Thrush from Spain and none showed any rufous-toned in the undertail coverts. It’s been reported that European birds can show some rufous on the undertail coverts but, if that’s correct, it must be extremely rare. From a very quick Google search when this bird first turned up, birds labelled as pandoo appear to be able to undertail coverts matching the Glos bird, although it seems this characteristic is variable – or at least not well studied and catagorised. On a similar note, this bird constantly gives the impression of being darker and more ultramarine in colour than European birds. Even in the field, European birds can look anything from powder-blue to almost black, so I'm being cautious in my assessment of the colour of this bird. Still, it's something to consider.

Alula: It’s been stated that this bird can be “conclusively aged as a first-winter” by the brown alula. This one is actually hard for me to argue against for the simple reason that I cannot – no matter how hard I try – make the alula on this bird brown.

Tail feathers: It’s been stated that the “tail feathers look pointed in flight images”. Again, I struggle to argue against this other than to point people towards photos such as this one, which show a good square-on image of the tail feathers and which, to my mind, are most definitely not pointed.

Moult limit (2): I should thank Gareth Jones for contacting me and passing on a link to his blog (scroll down to the bottom of the page), which includes comments from Javier Blasco on the age of the Glos bird: “these are not easy birds for ageing using photos since moult limits are not very clear. I enclose one of your photos [reproduced below] with one arrow pointing to that I think it is a moult limit in inner Greater Coverts .. if I am right then it is a 1st year male. But people who are arguing for an adult bird have good reasons too with a bird so fresh and having some bluish tinge in coverts and flight feathers. So .. I vote for 1st year but not 100% sure ... so send my opinion for other colleagues to consider if my argument is right or wrong.”

My response to Gareth was as follows: I respect Javier’s views on these matters but I do not believe the arrowed greater covert represents a moult limit. The inner greater coverts can often be slightly more protected from wear/bleaching, and on some species can show a subtly different pattern such as this. I’ve spoken to colleagues in Spain, who handle and see numerous Blue Rock Thrushes, and they have confirmed that a “false moult limit” such as this can sometimes be detected on adult male Blue Rock Thrushes. Additionally, there is no difference in colour between the arrowed GC and the tertials and secondaries. If we say, for argument's sake, that the GC had been moulted, we would expect to see a contrast between the adult-type GC and the retained juvenile tertials and secondaries. In this bird there is no such contrast within these feather tracts, which points to the tertials and secondaries being adult-type too. Only an adult bird would possess adult-type tertials and secondaries, since these feathers are never moulted during the post-juvenile moult.

Not particularly related to the age of this bird but something I was asked about: The apparent ‘thread’ around the bird’s leg in relation to the bird’s origin. I’ve seen adornments like this occasionally on wild birds caught for ringing – from Goldcrests to Blackbirds. It was actually relatively common on Goldcrests where leg bracelets like this often seemed to be composed of spider webs and pine resin. On other species the composition varied; mostly various fibres, sometimes bound together with dried mud etc. So, in terms of indication of origin, I'm not sure this apparent thread around the leg holds any weight on either side of the argument.

Finally, I’m always of the opinion that my arguments should stand on their own merits. However, it’s worth adding that I’ve spoken to several birder-cum-ringers with a particular interest in ageing birds in the field, all of whose opinion I respect greatly. All have agreed that the Glos bird is an adult for the same reasons as detailed in my original blog post.