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THE JUVENILE SKUA IN DECEMBER 2001
Keith Vinicombe


During the late morning of Friday 7th December 2001 a juvenile skua was discovered off Woodford Lodge, where it remained until dusk. It was identified as a Pomarine but, despite being only the second record for the lake, news of its presence was withheld. Fortunately, it was still there the following morning but heavy fog rendered viewing conditions extremely difficult. Brief and rather distant views in very bad visibility revealed that it was a dark brown bird with heavy barring on the undertail-coverts and it also seemed to show blunt central tail feathers. This indicated either a Pom or a Long-tailed, but the very poor views led to considerable uncertainty about its identity. The bird did not appear to be particularly big, which led me to think that it might be a Long-tailed.

When the fog eventually lifted in late morning, newly arriving observers confidently identified it a juvenile Arctic, one of the main reasons being that it showed narrow pale tips to the closed primaries. Despite some remaining uncertainty, nobody seemed to have the confidence to seriously question this identification. It remained until 11 December and the record was subsequently accepted as a juvenile Arctic Skua and published as such in the 2001 Avon Bird Report. It should be mentioned at this point that one thing that confused the issue was that, during late morning, views of the bird overhead revealed that it had an obvious gap in the centre of its tail: the central tail feathers seemed to be missing. What we initially thought were blunt central feathers in fact appeared to be the adjacent t2 feathers.

Nearly six years later, in November 2007, I happened to be browsing through some local bird websites when I came across a good photograph of the bird taken by Paul Bowyer (reproduced below). Up to that point I didn't even realise that any photographs of it existed. On examining the photo, I began to have doubts about it being an Arctic. It was a dark brown bird, with no warm tones, but what was most obvious was the very thick, heavy and contrasting barring on the vent and undertail-coverts (with messy barring extending onto the flanks). In addition, it had a rather rounded head, but with a flat crown, and a thick neck. The colour of the head was a slightly paler greyish and it was very uniform with no streaking.

Arctic Skua Stercorarius parasiticus
Juvenile skua  Chew Valley Lake, December 2001 (Paul Bowyer)



Other features included:

(1) a distinctly darker area before the eye;
(2) the upperparts were fairly plain with only limited buff tips to the wing coverts, with no buff fringes to the greater coverts and
(3) the bend of the wing was darker than the breast sides.

In combination, all these characters indicated Pom and not Arctic. However, four other features suggested Arctic:

(1) the primaries showed buff fringes (but these were very fine and not that obvious);
(2) the bill looked rather slim for a Pom (and, although it was grey with a black tip, this was not that well-defined);
(3) it had a narrow pale band over the base of the bill and
(4) in the field it did not seem to be particularly big; in my field notes I described it as being about the size of a Common Gull.

The relevant length measurements in The Collins Bird Guide are as follows (in ascending order): Black-headed Gull (35-39 cm), Arctic Skua (37-44 cm), Common Gull (40-46 cm), Pomarine Skua (42-50 cm) and Herring Gull (54-60cm). Note that other references, such as BWP, give radically different length measurements for the skuas as they include the adults' central tail projections. The Common Gull size for the Chew bird would, therefore, tie in with it being either a large Arctic or a small to medium sized Pom. Another point that transpired (from BWP) is that skuas vary in size according to their sex but, unlike gulls, it is the females that are larger than the males. In the light of this, it is quite possible that a small male Pom would not look as big as expected, both in terms of its overall size and its bill dimensions.

Having spent some time comparing Paul's photograph of the Chew bird with the relevant literature (see 'References') and with photographs on the Internet, particularly some recent ones on the Dutch Birding website, I became more and more convinced that it was in fact a Pom. A particularly useful link was www.jeaniron.ca/2005/jaeger.htm, which outlines the identification of a juvenile Pom at Barrie, Ontario, in November 2005. This article has some excellent photographs and, as will be seen from the link, that bird was extremely similar to ours. Another photograph of the Ontario bird is reproduced below - compare it with Paul's photograph above.

Pomarine Skua Stercorarius pomarinus
Juvenile Pomarine Skua  Barrie, Ontario, Nov 2005   (Brandon Holden www.peregrineprints.com)


Confusion between juvenile Arctic and Pomarine Skuas is not new. For example Lowe (1982) described a juvenile Arctic Skua scavenging on a dead rabbit in Norfolk in January 1979, but a photograph published with the note subsequently enabled it to be re-identified as a Pom (Kemp 1984; Sellors and Smith 1984). Another example was a Pomarine Skua at Willen Lake in Buckinghamshire in November 1982. A photograph of this individual appeared in British Birds 76: plate 42. It was an intermediate phased bird and Jonsson (1984) re-identified it as an Arctic mainly, it seems, because it had a slim bill and pale tips to the primaries, which Jonsson considered to be diagnostic of the species. However, Blincow (1985) subsequently proved that this was not the case. He provided four further photographs that clearly showed the bird to be a Pom. Interestingly, the January 1979 Norfolk bird, a darker individual than the one at Chew, also showed faint pale primary tips.

Despite Jonsson's misunderstanding of the significance of the pattern of the primary tips, his article provided much useful information on the separation of Pomarine and Arctic Skuas and this was taken further by Olsen and Christensen (1984). These findings were then brought together and summarised in a seminal paper by Olsen and Jonsson (1989).

The characters of juvenile Pom are as follows:

UPPERPARTS
Pom is much more consistent in its appearance than Arctic and most have dark brown underparts and upperparts with narrow, pale sandy-brown to rusty-brown fringes to the wing coverts. The folded wing looks quite dark, in contrast to the paler breast. The greater coverts have pale tips but, unlike Arctic, they usually lack a pale edge to the outer web. Juvenile Arctic generally shows a rustier body and broader pale orange to rusty feather fringes above (a few may be whitish) the overall effect producing a warmer tone with a slightly paler, rusty tinged leading edge to the wing in flight. Because of this warmer tone to the forewing, at rest there is less contrast between the wing coverts and the body.

UPPERTAIL and UNDERTAIL-COVERTS
The pattern shown by the two species is different. Arctic often looks barred, but Pomarine always does. The bars of Pomarine are more even and parallel, whereas those of Arctic are less distinctly barred and more wavy or scaly. Pom shows barred uppertail-coverts in combination with a uniformly dark head, whereas in Arctic, pale barring on the rump is present only in combination with a pale head or a pale hind neck. [Note that, in flight, the Chew bird indeed had pale barring on the rump and uppertail-coverts].

HEAD
Pom's head is drab grey-brown, darkest before the eye, and it is far less likely to show a light buff hindneck than is the case with Arctic: usually only a lighter grey wash is present. Pom never shows a contrasting pale hindneck. Also, its head is uniform or only lightly barred or lightly spotted. Arctic tends to have a dark cap, contrasting with the paler hindneck, which is tinged rusty, or even orange. In addition, Arctic typically shows prominent dark streaking on the head and neck, the longest streaks on the chin and neck being about 20mm long and 4 mm broad. The very few Arctic Skuas that lack a contrasting light hindneck are normally solidly blackish-brown dark phased birds that lack barring on the belly and flanks. Also, such dark Arctics are not strongly barred on the undertail- and uppertail-coverts. Arctics show a pale forehead stripe but this feature is not mentioned for Pom [note that the Chew bird showed this].

HEAD SHAPE
Structurally, Pom is a full-bodied bird with a broad, rounded head. Arctic has a smaller and almost triangular-head shape.

BILL
Classic Pomarine has a heavy, pale bill with a black tip, producing an effect reminiscent of first-year Glaucous Gull. However, note that males are smaller-billed than females, and juveniles are smaller-billed than adults (and juveniles have a less well developed hook than adults; BWP). Note also that, while Pomarine's bill is always heavy at the base, it frequently tapers towards the tip [as it does in the photo of the Chew bird].


To progress the matter further, I felt that I needed to obtain some independent views on the Chew bird, so I sent Paul's photo to Lee Evans, who has considerable experience of the species, and to Klaus Malling Olsen, author of some of the aforementioned papers and also of the book Skuas and Jaegers (published by Croom Helm). From the photograph, both Lee and Klaus considered the bird to be a Pom. Klaus made the point that, although it showed some features that favoured Arctic (pale forehead, pale tips to the primaries and smallish bill), the other features (plain rounded head, relatively plain upperparts and very distinct barring on the vent and undertail-coverts, favoured Pom).

I discussed this with Richard Andrews but he still favoured it being an Arctic, and he was particularly concerned that basing the ID on a single photo could be misleading. To make sure that I have covered all the angles, I telephoned Paul Bowyer to see if he had any other photos of the bird. Fortunately, he came up with another six, which I then sent to Klaus. Having seen these additional photos, Klaus changed his mind about the identification, agreeing after all that it was an Arctic. His reply is as follows:

"Thanks a lot for sending another series. As we all are aware, judging a bird from just one photo could be dangerous. The first photo you sent is by far the most Pom-looking of them all! From photo 7 I can now see that the bird, which here offers comparison with other species, looks too small and slender, and especially too small-headed for Pom. Photo 6 offers opportunities to judge the head and bill better. The head-shape in that photo looks perfect for Arctic: flat-crowned and "merging into" the slender bill - all in all creating a triangular head-shape which is perfect for Arctic. By enlarging the photo I can see traces of some hindneck streaking, which is never seen in Pom, but is typical for Arctic. The bill too looks much longer and slenderer than in the photo first mailed, therefore fitting only Arctic. So I am sorry to change my mind and identify this bird to Arctic.

Atypically, the undertail-covert barring is very regular, the wing-coverts have too narrow pale edges to fit the general impression in Arctic, and the primaries lack the [prominent] pale tips which are typical for Arctic. As you will know, almost any plumage character overlaps, and I often rely more on general shape and flight. In many cases the head-and-bill shape has been the clincher.

It is puzzling that after so many years of skuas I still come across birds with characters not fitting into the general picture. Having said this, most birds are indeed now correctly identified".


Arctic Skua Stercorarius parasiticus
Juvenile Arctic Skua   'photo 6' referred to by Klaus (above) (Paul Bowyer).


Arctic Skua Stercorarius parasiticus
Juvenile Arctic Skua   'photo 7' referred to by Klaus (above) (Paul Bowyer).



In his first reply, Klaus stressed that, when identifying juvenile skuas, no single feature is 100% diagnostic so you have to ID them from a combination of characters. Although this bird was correctly identified in the field, the photographs nevertheless illustrate that, in many respects, it was not a particularly straightforward individual. It always amazes me that distant skuas seen flying past at sea always seem to be confidently identified, but whenever they turn up inland at point blank range, they always seem to be spark a controversy! Identifying skuas at long range may be fair enough if you're an experienced sea watcher, but living here in the Bristol area we see them so infrequently that we never build up this experience. Maybe there a lesson to be learned from this!

Keith Vinicombe
12th December 2007


Acknowledgements
I'm very grateful to Paul Bowyer and Brandon Holden for letting me use their photos and to both Lee and Klaus for taking the time to provide some very helpful comments.

References
Blincow, J. 1985. 'The Buckinghamshire skua' British Birds 78: 669-671.
Kemp, J. B. 1984 . 'Identification of first-winter Pomarine Skua'. British Birds 77: 27.
Jonsson, L. 1984. 'Identification of juvenile Pomarine and Arctic skuas'. British Birds 77: 443-446.
Lowe, A. R. 1982. 'Juvenile Arctic Skua scavenging inland during hard weather'. British Birds 75: 32-33.
Olsen, K. M. and Christensen, S. 1984 'Field identification of juvenile skuas'. British Birds 77: 448-450.
Olsen, K. M. and Jonsson, L. 1989. 'Field identification of the smaller skuas'. British Birds 82: 143-176.
Olsen, K. M. 2006. 'Juvenile skuas'. Birdwatch 170: 33-36.
Sellors, G. and Smith, T. G. 1984. 'Identification of first-winter Pomarine Skua'. British Birds 77: 27.


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