Birds of the
English Channel & Bay of Biscay
Skuas - Family Stercorariidae
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Skuas are a typical component of any good autumn seawatch from the coast of Britain and Ireland. They are medium-sized and piratical in habits. They are powerful and agile fliers and survive largely in their role as raptors of the sea. They rarely catch and eat seabirds but usually harry them to the point where the victim will disgorge its food. They also take other prey on their breeding grounds such as lemmings, other rodents, carrion and small birds.
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The three smaller Stercorarius species which occur in the area are all northern breeders which pass through the area on autumn migration. Pomarine and Arctic Skuas have been seen in the spring but only in very small numbers, partly because the area is under-watched at this time of the year. Long-tailed has only been recorded in the autumn months and is rare. Great Skua, which is a larger species is also a northern breeding species and is the commonest and most typical Skua of the area. They occur in large numbers on autumn passage and during the winter.
Pomarine Skua - Stercorarius pomarinus
World Distribution:
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Pomarine Skua is circumpolar in distribution, breeding on tundra habitat north of the Arctic Circle. They are highly dependent on numbers of lemmings, which is their main food source. When numbers are low, adults often disperse early in July and August. Typically, however, successful adults move south in late August and juveniles linger until later in the year.
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In successful years large numbers of juveniles can move as late as mid-November. They winter in two main areas, in the Gulf of Mexico and off the West Indies, and off the coast of tropical West and South Africa. Some birds, however, remain in North Atlantic waters throughout the winter.
Survey Area Distribution:
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Pomarine Skuas are regularly encountered between the months of August and November and are seen on most trips, although it would be unusual to see more than ten on a single trip. In spring occasional birds have been recorded on late April research trips, when they are likely to be passage birds destined for a North Sea route north. They can be seen anywhere but the northern celtic shelf edge and the southern canyons are the best place to encounter them.
Identification: Flight and Structure:
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The identification of Skuas can be difficult and problematic and needs to be treated with more detail than these short accounts enable. However, one of the best ways to identify Skuas is through their flight action and jizz. This approach, when backed up by a sound knowledge of plumage, moult and ageing should clarify most difficulties. Of course jizz is not something that can be read in a book and it takes great practice and experience to accurately identify distant Skuas using this method.
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The flight of Pomarine Skua is powerful, direct and impressive. Their wing beats are slower and more measured than Arctic Skua and they appear to be rather stiffer and shallower, especially in direct flight. In banking flight they also appear more stiff and measured although their wings are often more angled than Arctic Skua.
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One point which emphasises how different these two species appear is that Pomarine can be confused with Great Skua in typical flight whereas Arctic never will be. Pursuit flight is not as agile as Arctic Skua which accelerates rapidly and can twist and turn quite acrobatically, with often quite fluttery wing beats when pursuing vertically or high above the sea surface.
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Structurally the main differences between the two species are as follows. Pomarine is usually much bulkier, with a deep chest which gives them a very front heavy appearance. The base of the wing is very broad whereas the primaries are comparatively narrower and shorter than in Arctic. The bill is rather longer and broader than Arctic with a pronounced hook tip and the tail is always broader than in Arctic whether the bird has tail spoons or not. Adults, of course have broad, twisted tail streamers, or spoons which tend to be very obvious and can project up to 20 cm.
Plumage:
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There are certainly reliable differences between adults of both species. Adult Pomarine in breeding plumage has a more extensive dark cap than Arctic Skua, reaching down below the eye in most cases. The bill has a large, markedly pale base with a dark tip - this contrast also holds true for most immature birds and is comparable to the difference in bill colour in first-winter Glaucous and Iceland Gulls. Pomarine usually has a strong dark breast band and rather thick and ragged dark markings on the flanks and vent. Adult birds, like all Stercorarius species, have two distinct colour forms, dark and pale. The former is rare and is all dark except for the pale wing panels.
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In immature plumages, differences are complex and rather variable. Often features can be used as clues rather than definitive identification marks. For further reference, the best guide to Skuas is undoubtedly ‘Skuas and Jaegers’(Larsson and Olsen). With juvenile and immature birds (adult plumage is usually reached after the third year), there are three basic colour forms, pale, intermediate and dark. Probably the most reliable plumage feature to look for on immature birds is the larger, more extensive pale ‘flashes’ on the upperwing in comparison to Arctic Skua. On the underwing, the pale area often extends to the primary-coverts which is rarely the case with Arctic.
Parasitic (Arctic) Skua - Stercorarius parasiticus
World Distribution:
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Arctic Skua has a circumpolar breeding distribution and is the commonest European Skua species. They breed in a large range of climactic zones from cool temperate (e.g. northern Scotland) to high Arctic (e.g. Spitzbergen) They occur mainly on coastal tundra and moorland although some birds nest inland, mainly in Iceland, where they breed virtually throughout the interior. In Britain, around 3-4000 pairs breed, the bulk of the population can be found on the Orkney and Shetland island groups. They return to their breeding grounds late April/May, and eggs follow May/June.
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Birds migrate south to winter in southern latitudes from late July onwards, peaking late August/mid-September. The main passage coincides with the peak autumn movements of Terns, chiefly down the North Sea coastline. Fledging occurs largely in August. Birds winter as far south as the tip of South America, although European breeders seem to spend the winter months in two main areas: The Patagonian shelf off Argentina and over the Benguela Current off Namibia and South Africa.
Survey Area Distribution:
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There is some evidence of a light spring passage through the survey area during spring. Although there is only data available from a few late April research trips. The spring passage along British coastlines is much more concentrated (mid-April to mid-May) than in autumn and more direct. Many birds migrate round the western coastlines of Britain and Ireland although good numbers are sometimes recorded in a usually steady passage along the English Channel, especially from sites such as Dungeness in Kent and Brighton Marina in Sussex.
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Autumn passage takes place mainly from mid-July through to October in the survey area, peaking in August and September. Relatively small numbers are recorded in the Bay of Biscay itself. 10-20 would be a good total for a trip during the peak months.
Identification:
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For an account of the differences between Arctic and Pomarine Skuas see the previous species account. The identification of Arctic and Long-tailed Skua is clarified under the latter species account.
Long-tailed Skua - Stercorarius longicaudus
World Distribution:
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The breeding distribution of this species is circumpolar in high arctic areas, generally north of the Arctic Circle. They nest on healthy fells above the tree limit and on drier tundra. In Europe, they breed inland in Norway and along the northern Scandinavian coastline round to Russia, as well as on Spitzbergen. Exact numbers are not known because of inaccessibility of many areas and species tendency to wander from year to year according to food supply, much like Pomarine Skua.
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The wintering areas of Long-tailed Skua are poorly known but are all in the southern hemisphere as records north of the equator are very rare after November. Evidence suggests that they winter in a wide area in the south Atlantic to about 50° S, mainly off South America, Namibia and South Africa.
Survey Area Distribution:
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In the survey area, they are rare with only a few records each year, usually in August - October. Caution is advised in separating this species from the very similar Arctic Skua.
Identification: Flight and Structure:
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Except when dealing with full plumage adult birds the best clues to identification are to be found again in the flight and structure of birds. With experience, identification is not usually problematical. Long-tailed Skua is smaller than Arctic Skua, about the length of a Black-headed Gull but with longer and thinner wings making it appear larger. In flight they are often quite light and buoyant with rather Tern-like wing beats. In light winds, they often fly much like a large Shearwater in a lazy, looping flight making slow but steady progress. In strong winds, however they are very agile and use the wind more than other Skuas often switch-backing and employing shearing flight high over the waves before flapping on loosely held wings. Birds chase less often than other Skuas but are very agile when doing so and capable of changing direction and angle of flight very quickly.
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Structurally, they are slimmer than Arctic Skua on the chest and fore-body with long, slender, more pointed wings and a longer and more bluntly pointed tail which appears V-ended in comparison to the sharply pointed yet more square-ended tail of Arctic. They have small, often more rounded heads with a short bill which shows more dark on the tip than Arctic.
Plumage:
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Adult birds in breeding plumage have distinctive tail streamers which can take up almost half the body length. They are less variable than Arctic or Pomarine Skua at this age and easy to identify given reasonable views. Immature birds on the other hand are just as variable as Arctic and can be difficult to identify.
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The best features to look for are those mentioned under structure and flight although there are some clues to look for. The wing flashes of Long-tailed are usually rather more restricted than on Arctic Skua. This is best seen on pale or intermediate phase birds the former of which generally have rather greyer, colder tones than Arctic. Juvenile Long-tailed shows barring on the rump and upper tail coverts which juvenile Arctic does not. However, second winter Arctic also shows this feature, as well as barred underwings, so be careful !
Great Skua - Stercorarius skua
World Distribution:
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‘Bonxies’ only occur in the North-East Atlantic. They breed on Iceland, The Faroes, the Shetland and Orkney islands, and on the mainland of northern Scotland. They winter, at their most northerly point in the western English Channel, Ushant and Bay of Biscay sea areas. They range as far south as NW Africa during these months. They return north from late March and lay eggs between April and June. They begin to disperse south during August and peak between mid September and mid October.
Survey Area Distribution:
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They are present year round, suggesting the presence of non-breeders in small numbers in the spring and summer. They peak during September-October, but their numbers hardly drop off after this. It is normal to see in the hundreds on a single winter research trip, November-February. They are found throughout the area during this time but seem to concentrate in the Ushant and western English Channel areas. It is of course possible that high numbers occur in shelf edge areas at this time of year but the lack of daylight hours hinders surveying during this period.
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Fish are the main food source. They can be taken from other birds: Gannet, Kittiwake, and Cory’s Shearwater seem to be the species most favoured: but also from the water surface and from behind fishing boats. Indeed, checking fishing boats is one of the best ways to record this species in Biscay.
Identification: Flight and Structure:
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Identification is not difficult. They are much the largest and heaviest of the four European species of Skua. Around the size of a Herring Gull, with similarly broad wings and bulky body, head and a stout, powerful bill. They have a much heavier flight than the other species with little of the agility. They do, however, have great powers of acceleration when in pursuit flight and are certainly difficult to elude !
Plumage:
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They are rather uniform in colouration and adult/immature and juvenile birds can be difficult to tell apart. Adult birds tend to be a warmer brown with coarse yellow-brown streaking over the upperparts, upperwing-coverts and head, especially in breeding plumage. The underparts are also rather patchy and mottled. Juvenile and immature birds are rather darker with dark heads and often a reddish tinge to the plain underparts The wing flashes are usually quite restricted on young birds and can be much narrower than on adults.
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As with the other Skuas, immature birds tend to congregate around the northern Celtic-Biscay shelf edge. During the winter months around half those aged are immature.
Copyright of BDRP, 2003