South Polar Skuas at a glance

I’ve been through the entire list of Polar problems. I knew it would be hard, but it’s harder than I ever thought it would be. I’ve suffered from blisters, a high-altitude cough, frost nip, and I even managed to break a ski they told me was unbreakable.

-Lewis Clarke

Over these past weeks I’ve been doing seabird surveys off Galicia, I’ve been able to study loads of big Skuas, what made me aware of how hard, and sometimes how easy, South Polar Skua identification can get. The constant SW-W winds I got during the almost 3 weeks I spent offshore helped big numbers of seabirds gathering together around Fisterra area, feeding on the rich upwelling produced by those oceanic conditions. Great Skuas were particularly abundant, with up to 120 individuals counted together feeding on our discards. Among the thousands seen, 3 South Polar Skuas (all aged as 2nd cycle+) stole the show, given the species is still considered a mega in this side of the Atlantic.

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Figure 1. Big numbers of Great Skuas Stercorarius skua congregated to feed on our discards. Among them, 3 South Polar Skuas were seen. Galicia, September 2019. ©Marcel Gil Velasco.

All the birds behaved the same way, keeping larger distances towards the boat than Great Skuas and offering just a couple of close-by glides. The identification process, therefore, needs to be quick, especially if photos are still required. With such a big number of ‘big skuas’ following the boat, it’s worth to dismiss as many obvious birds as possible and to do it rapidly, so we don’t miss a chance with the actual candidates. I want to make clear first that I’m not very experienced with SPS: I’ve just seen 4 individuals in two consecutive years off Galicia (plus 2 probable SPS back in 2011 and 2012), but I thought it could be useful for some to share here a captioned collection of ‘big-skuas’ photos, starting with the most obvious Great and ending with the real SPS, trying to depict pretty much everything in between. Let’s start by the

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Figure 2. Old Great Skua Stercorarius skua. Galicia, September 2019. ©Marcel Gil Velasco. There was a time when South Polar Skuas were meant to be strikingly pale, almost white from the distance. At this time, this old Great Skuas attracted some attention. However, we now know Great Skuas tend to become paler with age, leading to some funny blondie colorations. These birds are easy to rule out: they are all blond, heavily patterned all over, and they are all moulting inner-mid primaries in early October.

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Figure 3 & 4. Juvenile Great Skua Stercorarius skua. Galicia, September 2019. ©Marcel Gil Velasco. The lack of notorious wing-moult and the more or less uniform plumage could attract our attention at first, but in reality, there’s nothing further from an SPS than this. In my opinion, similarly to what happens with raptors, Skuas also change their silhouette with age, adults becoming heavier overall. Young birds look very long-winged, long-necked and long-tailed and their bill is thinner. Although the difference in structure to what we are used to with adult Bonxie can be quite striking, it doesn’t make it any closer to SPS, much rather the opposite. That said, this characteristic orange plumage is truly beautiful and probably underrated by most seabirders. Note the incipient hood too.

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Figure 5. 2cy Great Skua Stercorarius skua. Galicia, September 2019. ©Marcel Gil Velasco. These eye-catching birds can get tricky, but most times are quickly ruled out after seeing the obviously spotted upperparts (including scapulars) and the well-defined dark hood. The lack of active moult in inner primaries could fit SPS, but not the coloration.

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Figure 6. 2cy Great Skua Stercorarius skua. Galicia, September 2019. ©Marcel Gil Velasco. The darkest individuals of this age class can make a major challenge. Despite the pale spots on breast-sides, nape and scapulars are evident when this close, from the distance this bird would look much closer to SPS. The spots in the nape would make it up for the classical SPS nape-patch and the hood wouldn’t be so apparent, leading to a uniform head apparently concolorous with the body. I find this kind of birds especially difficult when seawatching from a cape.

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Figure 7 & 8. Juvenile Great Skua Stercorarius skua. Galicia, September 2019. ©Marcel Gil Velasco. I’d put [ordinary] dark juvenile GS more or less at the same level of difficulty as dark 2cy, although they are easier or harder depending on distance and light conditions. Again, from the distance, they look very uniform and not actively moulting, but close views reveal the spots on the upperparts. The primaries are very pointed too, strongly indicating a juvenile bird. From a cape, the identification should be based mainly on structure, keeping the classical elongated GS slender appearance (read the captions of the Figures 3 & 4). The lack of a nape-patch rapidly makes me suspicious too, given I’ve seen this feature in every SPS here.

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Figure 9, 10 & 11. Juvenile Great Skua Stercorarius skua. Galicia, September 2019.  ©Marcel Gil Velasco. Now a real challenge. This bird got me hesitating during several days, changing my mind from one photo to another. In the end, I think it’s just the trickiest Great Skua I’ve seen so far. I saw it under very bad weather conditions: 4-5m swell, rain and gusts of up to 50kt, so I didn’t look at it through my bins much. Instead, I focused on getting bare-eye comparisons with the nearby Great Skuas and the best photos possible. Fortunately, it behaved quite well and came close to the boat on more than 4 occasions (differently to what SPS usually do) and I managed some decent photos for later study. The rain drops are still visible in front of the bird; don’t take them for spots! It looks very cold-toned below and entirely uniform above, with just a pale edge to some scapulars and uppertail coverts. The design of the head, though, doesn’t look right for SPS. It lacks the nape-patch and the coloration is uniformly dark, without the darker area around the eye typical of the real thing. It looks like the classical hood described above is more extensive in this case, but it’s still there, together with a much more extensive dark area in the rear belly and the undertail. Moreover, there’s no active moult, but the primaries are very pointed, indicating a juvenile. The small white flashes in the primary basis also support this age. Interestingly, what stroke me the most of this bird (or, indeed, what didn’t strike me) was the structure. It’s not as bulky as an adult Great, with a small head and especially a rather long tail. The bill looks thin, but also quite long. In some photos, it seems to be quite bull-necked, but in some others, it shows the classical GS profile. The slightly longer central tail feathers are intriguing, and remind me of either the ‘milky teeth’ shown by juvenile raptors (as I remember Andrea Corso calling them). The bird is ringed and sadly the ring doesn’t allow assessing the white markings on the tarsus. Despite I’m now quite sure it’s a dark and very uniform juvenile Great Skua, comments on this bird are more than welcome. Too bad it’s not colour-ringed; we’d know far more about it!

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Figure 12. South Polar Skua Stercorarius maccormickii, Asturias, October 2012. ©Martí Franch. This bird caused much trouble back then, but in my opinion it’s safely assigned to SPS now. Structurally, fits well with a South Polar: wing shape, tail length and  overall compact appearance. It does look slim though, but I don’t think this is a problem. The colour is also ok: nape-patch present, cold-toned overall and lack of hood. Although in the bottom right photo looks like it’s got some warm spots on the upperparts, I think the tone of the spots is light-related, given the sea in the background is also quite golden. The shape of these pale areas is right for SPS, being just restricted to the edge of the feathers and producing a scaly appearance. Finally, the moult is the ultimate evidence here. In my opinion, P9 and P10 are still retained, together with at least the outermost primary covert. P8 seems to be fully grown, potentially indicating some sort of suspension. I’ve previously seen this kind of stuff in another long-distance migrant such as Pomarine Skua, but never before in SPS (with a very limitet n size though).

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Figures 13, 14 & 15. 2cy+ South Polar Skua Stercorarius maccormickii, Galicia, September 2018. Last’s years only South Polar Skua. Once again, this bird showed up under very unfavorable weather conditions. From the distance, it looked uniform and dark but before rising my bins I still didn’t know if it was going to be a 2cy or a dark juvenile GS. However, I soon realised it didn’t show any hint of hood, but a nice nape-patch instead. When it came closer, the moult became obvious: it was growing P9, with P10 still old. There were also 2 generations of secondaries. The upperparts were not uniform but scaly, but it was only because all the coverts were pale-edged. The underparts were uniformly pale all the way up to the throat. Structure wise, it looked compact even when it was stretching its neck (first photo above) but it was the tail what looked strikingly short. Probably due to that fact, the centre of gravity was closer to the prow than in a GS.

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Figure 16. 2cy+ South Polar Skua Stercorarius maccormickii (left) and Great Skua Stercorarius skua (right), Galicia, September 2018. Although it kept the distance most of the time, the bird stayed around for quite a while, allowing prolonged views and even side-by-side comparisons with an adult Great Skua. Again, the most striking feature was the short tail, together with the small head. I’d also say the wing length/width ratio is higher in GS. Finally, the white patch on the primaries was also much smaller above.

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Figures 17 & 18. 2cy+ South Polar Skua Stercorarius maccormickii, Galicia September 2019. The second bird this year. I only saw it under a very strong light, but it’s still a safe SPS in my opinion. Same structure as the bird above, paler nape, short and pointed tail and still growing P10. The upperparts show a handful of spots, but always on the tip of the feather and never along the shafts. In my opinion, this bird is an example of how, even with poorly documented observations, it’s still possible to clinch the id.

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Figures 19 & 20. 2cy+ South Polar Skua Stercorarius maccormickii, Galicia, September 2019. The most obvious bird I’ve seen, straightforward even from a big distance. Classical contrast between dark upper and frosty underparts, including the head. Note the back is also dark, concolorous with the wings, something that doesn’t happen in old GS. Sadly, when it came very close, it was with full backlit, but I still managed to catch some colours on the photos. The head is amazing: it’s got the expression of a small skua and the bill is just slightly deeper than in a Pomarine. The underparts are completely uniform and very cold-toned. I’d describe them as metallic.

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Figure 21. South Polar Skua Stercorarius maccormicki (right) and Great Skua Stercorarius skua (left), Galicia, September 2019. The photo is a bit distant, but it’s still a nice side-by-side comparison. Even sat on the water, when the structure is not so assessable, this bird is bloody striking. The entire body is pale and frosty and shows a very strong contrast with the wings. Nothing to do with the old blonde Great Skua.

South Polar Skuas are probably much regular in Europe than we think and feels like identification is still in a work in progress stage. Although there’s been some big steps during the last years (e.g. Newell, et al. 2012), some individuals are still causing trouble, especially from land but also when seen at decent distances. Although there’s already a lot of material relative to SPS in the North Atlantic (of course especially from the American side), I feel there isn’t much on dark juvenile/2cy Great Skua yet. Despite these tricky plumages prevent SPS from being safely identified from very big distances, there is a good number of features that are most-likely diagnostic when they can be actually assessed.

I want to thank Daniel López-Velasco for the very interesting conversations on this subject we’ve shared over the last years and Martí Franch for letting me include his 2011 bird and also for encouraging me to write this post.

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