I have a desire to pit myself against the Hudsonian Godwit. This rare vagrant arrives almost without warning, and I have long suspected that finding one amongst a flock of Black Tails must be one of the sternest tests of knowledge, skill, concentration, and luck.
So here I am, now having seen the long staying bird at Cattawade, and I have some answers...and some questions.
The Cattawade bird turned up in July, first dropping in to Minsmere (via Portugal, it seems, which is interesting in itself) in full breeding plumage. The dark and rich red chestnut underparts contrasting with the pearl grey head and neck, with gold spotted black upperparts and wings like burnt paper, obscured shadows of summer patterns on a few coverts. I don't think there is anything too difficult about a bird in such finery, even if it is worn and lost in a post breeding collection of black scattered ferrous Icelanders trembling in a heat haze.
No. It's the basic plumage of winter that must inevitably pose the biggest problems, and at Cattawade I had not only a perfect opportunity to follow a single bird processing through its moult, but more importantly, a perfect opportunity to familiarise myself with a 'Gray American'. I said 'Gray'.
Across four visits in August, I watched the bird gradually shed the last of its rufous flanks and cloak its back with grey, until all that was left of the summer was the sepia tertials and coverts, and an orange smattering between the legs.
I watched it in bright light and dull, morning haze at distance, close range with the sun overhead, and the soft neutral 'prime' of dawn.
Ok. So this is a limited case. A study group of one. That's a big caveat. Everything from here on in must be tempered by the question - just how typical is this. for the time being, I'm going to assume that nothing about the bird is out of the ordinary. Until another one comes along, I think it's pointless assuming otherwise. But always, at the back of my mind is the possibility born from years of looking at Black Tails, that this bird is doing something different, atypical or otherwise annoyingly aberrant.
There are two things to think about when trying to separate this bird from the Black-tailed Godwit. Form and colour. Both categories contain features that vary from the totally subjective and context dependent, to the (tentatively) objective and diagnostic.
The biggest trouble in identifying a Hudsonian Godwit is that the most diagnostic feature is rarely visible.
But having put in the hours, I think I can say that it's possible to be fairly confident without seeing the black underwing, even at distance.
Dealing with form first, the most striking aspect of the Cattawade Godwit was the proportions of its body. It looked small headed and large bodied in comparison to the surrounded islandica Black Tails. Shorter necked as well. Looking at the data gives a clue as to what's going on. Wing length across the two species is comparable - especially between haemestica and l. islandica. It's the tarsus measurements where the two part ways. The average male islandica has a tarsus measuring 68.7mm (as per BWP, Cramp et al) while even the largest female Hudsonian tarsus is no longer than 60mm (HANZAB) and the male average is a mere 52.3mm. Given their bills are equally long, this difference in leg length translates to a difference in neck length, with the Hudsonian looking more 'dumpy' and less 'svelte' than islandica. From the beginning, I was struck by how close to the structure of Bar-tailed Godwit this bird was. The similarity extended to the shape of the head - with the Cattawade bird showing a subtle peak to the forecrown (compared to the square head with at most a peak well behind the eye on the rear crown in Black-tailed) and bill slightly more upturned than that of Black-tailed, lacking the subtle 'bulb' at the tip shown by that species.
At the other end, the Cattawade bird showed a noticeably longer primary projection, another feature more akin to lapponica.
So that's the key. A bird that looks like a Bar-tailed Godwit, but with Black-tailed Godwit winter plumage.
And this is where it gets more difficult. It's not always easy to see structural details. If a bird is sleeping among a flock in belly deep water, things are going to get very hard.
So there has to be clues in the plumage, doesn't there?
I was lucky to see the Cattawade bird in the company of a few Icelandic Black Tails, and the comparison suggests that in winter plumage, Hudsonian Godwits are slightly (yet noticeably) darker necked. There's a smokiness to their upperparts that extends onto the flanks. In neutral light the foreparts look strikingly brown, made all the more distinct by the pale cheeks and bold supercilium.
In brighter lights the bird does look paler, though retaining the overall brown appearance, but the new scapulars and coverts acquire a blue hue totally unlike the grey seen in limosa. Well, maybe not totally unlike it, but I swear there is a difference. Describing colour is notoriously difficult. It requires a common understanding that is rarely communicated in the brief and poetic lexicon of personal experience. I'm not sure what it means to be smoky brown versus ashy grey, pale slate versus blue umber. Nevertheless, these are the terms I'm sticking to for now. Put simply, if the Black-tailed Godwit is grey, then the Hudsonian is both browner and bluer. That's a description dependent on time and place and I'm either talking nonsense, or I've accidentally unlocked the possibility of a four dimensional definition. It's a tenseless definition that must encompass, as far as possible, the ephemeral effects of natural light.
Even at long range, the pale cheeks stood out on this Cattawade individual. Throughout its stay, this feature was easily the most obvious difference between it and the Black Tails. I'm not sure how diagnostic it is. I've since seen quite a few photographs of Hudsonian Godwits that do not show it.
But....
Some of my photographs and footage don't show it either.
It seems that, at least on the Cattawade bird, the pale cheeks are sometimes lost in bright light at certain angles. Is this what's going on in these other photographs? Turning to other sources has been of limited help. It seems that there's a dearth of illustrations of Hudsonian Godwits - at least in my own library. Sibley and Hayman do not seem to have noticed the cheeks in any confident way. Both have illustrated them with not much more than a gesture. Lewington seems to see something there in his illustration of a juvenile, but nothing like as obvious as the Cattawade bird. The plumage description in HANZAB does mention it, though suggests it is a feature not observed in all birds. So while it might not be diagnostic, when it is there, I think it pretty much rules out other species. Even looking at the endless variations observed in Black Tails, this pale cheek is something I have not seen - at least not in the same way.
Finally, the colouration of the bill. There's a strong black colouration to the upper mandible (culmen) extending from the base to the tip. Black Tails do often show a dark ridge to the culmen, but it's never as extensive down the sides. In Hudsonian, the effect is of a black upper and pink lower half to the bill, whereas in Black-tailed, the contrast is between black outer half and pink inner half.
So there it is. A foolproof way of identifying a Hudsonian Godwit just before it opens its wings and flies off. Maybe.
It's not smaller than Black Tails, but it is shorter legged and necked. It looks small headed and large bodied.
Structurally like a Bar Tail, coloured like a dark Black Tail with paler cheeks.
One day I hope to test this ID outline on another bird. See if it stands up to scrutiny. Like with many identification problem species, accurate diagnosis depends on a suite of features that add up to a whole. When the most diagnostic of those features are not seen, the question becomes - how few are needed to make a correct interpretation? Structure seems to be the most reliable character, but this of course demands complete familiarity with the structure of Black Tails. Colour seems like it has potential to make things easier, but also the potential to cloud the issue. I can imagine seeing a bird that doesn't look noticeably different in colour to the other birds around it, and at greater distances, any slight difference in tone may be lost anyway.
All this is to say that it's very possible to miss a Hudsonian in a sea of limosa.
But then again, with a bit of luck and application, one might stick out even in winter plumage.
And yes, I realise I put 'shorter bill' in the comparison video - it seems that was a mistake, there is not much difference in bill length between all three species but I'm not going to try to edit the video now.
