01/05/2022
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Focus on: Pomarine Skua

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One of the most anticipated spring migrants in Britain isn't a passerine battling its way from Africa, but a seabird. Birders will be waiting on a blustery headland during late April to mid-May, with scopes ready for when small groups of Pomarine Skuas glide into view, trailing their spoon-like central tail feathers behind them, as if in readiness for the first knickerbocker glory of the summer season.

Typical adult birds have a rugged beauty: a black crown and face mask from which protrudes a bill flushed with dull pink, tipped with black; pale sherbet-yellow cheeks, throat and hindneck; bright white belly topped by an HB pencil-shaded breastband (always present in females, sometimes confusingly absent in males); and charcoal brown covering the mantle, wings and undertail, all the way down to the tips of its 'spoons'. In flight, each wing flashes white 'quotation marks' on the underwing and a broad white patch on the upperwing, formed by the white shafts of its primaries.


Nesting on the Arctic tundra and wintering at sea in the tropical oceans, Pomarine Skua is seen on migration in British waters (Ian Davies).

Not all the adults are so ostentatious; 5-20% are dark-morph birds – a deeper near-black plumage covering their bodies, with a grey bill base, but still with those white wing patches. This colouration can make them appear more intimidating than pale-morph birds.

And, to other seabirds, intimidating they are. Skuas are kleptoparasites: bullies that steal items of prey caught by other birds. They chase terns, gulls and even Northern Gannets to relieve them of their hard-earned gains, deftly manoeuvring like the Red Baron to force their unwitting providers to drop or regurgitate food.

In winter, skuas are the scourge of migrating terns as they head south. Autumn Pomarines are outriders to North Sea Kittiwake flocks as they, in turn, accompany inshore sprat shoals, the skuas switching to robbing similarly sized Common Gulls once the fish move on.

Skuas steal food whenever they can, but will also scavenge carrion and kill and eat smaller animals. Pomarines have been observed attacking Grey Herons, with one photographed killing and eating a Snowy Egret in Virginia, USA, in 2018.

 

Playing the spoons

The skua family, Stercorariidae, is divided into one genus, Stercorarius, and eight species. The larger brown skuas resemble juvenile gulls and are sometimes placed in the genus Catharacta. However, because Pomarine Skua S pomarinus is genetically closer to Great Skua S skua than the similar-looking Long-tailed and Arctic Skuas (S longicaudus and S parasiticus, respectively), the whole family is now treated as one genus. Great Skua is also more closely related to Pomarine than its three southern relatives.

Americans call the three smaller species 'jaegers' and their typical two-tone plumage is probably a more 'ancestral' trait, the streaky brown of 'Catharacta' skuas being a retained juvenile state. Striking dark and pale plumages probably make defending a territory easier in spring, although behavioural similarities between Pomarine and Great could also be a default factor of their size. Pomarine is about 50 cm in length – including tail streamers of about 10 cm – and, as with many predatory birds, females are up to 15% larger than males. Adults feature a small percentage of intermediate morphs among their number to make field identification trickier.


Dark-morph birds, which constitute up to 20% of the population, are particularly striking (Daniel Lopez Velasco / www.agami.nl).

The diagnostic 'spoons' are two broad, blunt-ended central tail feathers with a longitudinal half twist, only resembling cutlery on a side view. These moult out biannually in November-December and March-April, when birds become harder to separate from the pointy-tailed Arctic Skua. However, 'Poms' are always more heavy chested, with broader wing bases, and a heavier, more relaxed flight style (a bit like a European Herring Gull), plus those distinctive 'quotation marks' on the underwing – actually white outer primary and greater covert bases forming two narrow wing-bars.

Juveniles share the pale, intermediate and dark morphs of the adults, but their heavy barring makes them more resemble Great Skua. They have the Glaucous-Gull bill pattern, 'speech marks' and sturdy build of adults, but also feature thickly barred undertail feathers, sometimes visible from a distance.

Pomarine Skua breeds from European Russia through to Nova Scotia, Canada, nesting on areas of flat, sea-level tundra. It has very recently been proved to nest in far north-eastern Norway, its only site within the national boundaries of Europe, where there are substantially fewer than an upper estimate of 20,000 pairs. Breeding successes are closely linked to the availability of lemmings, but because of the rodents' boom-and-bust population dynamics, skua breeding times and productivity are also variable. Wandering summer non-breeders are also present in the Arctic, increasing during lemming 'bust' years.


Young birds resemble immature gulls, with a largely brown plumage characterised by intricate barring (Steve Young / www.birdsonfilm.com).

 

Poms on the move

Southward migration occurs in a broad movement from August to December, peaking in October – though stragglers often remain in the North Atlantic and North Sea; Pomarine is the most common skua on passage off north-eastern USA. Most wintering birds are pelagic and found in a broad belt from south of the Canary Islands, China and Hawaii to the tips of South America, South Africa, New Zealand and Australia, where rafts of up to 450 birds can congregate in Apollo Bay, Victoria.

The largest Northern Hemisphere wintering numbers are often found in food-rich marine upwelling areas, such as off the Mauritanian coast.

Pomarines migrate offshore in loose flocks of five to six individuals – sometimes up to 50 or more in Scotland. There are also overland migration routes, even across continental landmasses (mostly so high as to go unnoticed), using seas such as the Caspian and Arabian Gulf as stopover points; skuas regularly use a route from The Wash to the Bristol Channel in this way. This behaviour – only noted in the Old World – may have evolved when there were too few sea routes during the last glaciation.

British birders generally see small migrating groups of Pomarine Skua in May and October. The species moves a little later than Arctic Skua and for unknown reasons most spring birds noted are pale morphs. Skuas generally follow migrating shearwaters in the Pacific and terns in the Atlantic. They can cover between 200 and 500 km per day at a speed of 35-55 kph, battling into headwinds of up to 35 kph, and settling on the sea in anything stronger. Even so, these sturdy birds have been observed sustaining flight in storm winds of 167 kph in the central Pacific!


Adults are generally seen between mid-April and late May in spring, returning south again in early autumn (Alan Murphy / BIA).

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It is such powerful winds that sometimes drive this usually scarce species to our shores in larger numbers. While groups passing our coast normally number no more than 15 or 20, westerly winds in spring can drive the greater numbers of birds that pass along the edge of the continental shelf (which runs up to 300 km offshore from Biscay north past the coast of Ireland and halfway to the Faroes) closer to shore. This can result in day counts of more than 100, particularly in recent years.

On 25 May 2002, 152 'Poms' were counted off Bowness-on-Solway, Cumbria – where skuas traditionally cut across land towards Scandinavia – while on 19 May 2007, the same site produced 177; on both occasions strong winds veered from south-south-westerly to westerly. Similar conditions produced big numbers in the Channel on 5 May 2014, when 135 were counted off Splash Point, East Sussex.

Aird an Runair on North Uist, Outer Hebrides, is the best location for the species in Britain, the site of an early count of 156 on 22 April 2004 and several hundred in May 2013. The best southern site is Dungeness, Kent, which produced 121 on 6 May 2016.

However, the most unprecedented numbers occurred in May 2015 at Aird an Runair. Large numbers passed for much of the month, after 71 on 7th grew to peaks of 911 on 16th and 429 on 24th. Even these stupendous counts were far outnumbered by accompanying Long-tailed Skuas – usually the rarer species. Kudos to the counters – the weather was rough, squally and dotted with heavy showers, but the best numbers came in with south-south-west to north-north-west winds.

Overall, 3,004 Pomarine Skuas were logged that month, some amazingly close and in flocks of more than 100. This momentous movement perhaps indicates regular heavy skua migration out in the Atlantic, though without favourable conditions and co-ordinated counts we can only guess at its true scale.

Despite the well-documented 'Pom' migration routes overland, it is very rare to see individuals actually on the deck or even overhead. Strong westerlies can change this, and stand-out records don't come more central than one at Holme Pierrepont, Notts, in June 1973 or another at Edderthorpe Flash RSPB, South Yorks, on 16 November 2000. However, more dramatic was the flock of 13 seen at Queen Mother Reservoir, Berks, on 25 April 2003, when a heavy band of afternoon rain moved across the West Country after strong south-easterlies.


The species' diagnostic 'spoons' are formed of two broad, blunt-ended central tail feathers (Pete Morris / www.agami.nl).

 

Call of the skua

Few birders will have heard Pomarine Skua vocalisations because it has never bred in Europe away from Spitzbergen and most calls are territorial. A Pomarine long call has a 'laughing' tone, like a faster, harsher Mediterranean Gull. Alarm calls are similar, but more wavering and uncertain, while it has a 'squeaky metal' note serving as a kind of 'song'. An infrequent flight call like a gull on helium is occasionally heard and, in feeding groups, a high-pitched 'chattering'.

Arrival on breeding sites is controlled by tundra thawing and lemming availability, with territories usually established at the end of May into June. Intruders (including gulls and Short-eared and Snowy Owls) and rival skuas are greeted with raised wings and a bent-forward or raised-head posture, accompanied by a forceful long call. This aggressiveness is exploited by Dark-bellied Brent Geese on the Taimyr Peninsula of Russia, which place their nests closer to the skuas during lemming booms to defend themselves against Arctic Foxes. Lemmings are caught by hovering or pouncing and will be pursued by the skuas after they tear the roofs off their burrows.

The breeding display adds raised breast and central tail feathers to the imposing long-call stance, as the paramours face each other. Two eggs, the size of a chicken's, are laid five days later in a trampled depression on a dry raised tussock among wetter tundra. Laying is staggered over a fortnight in solitary nests, although dense colonies can develop during lemming booms.


Pomarine Skua nests in the far north of Eurasia and North America, laying two olive-brown eggs in grass-lined depressions (Matthew Studebaker / BIA).

Both sexes develop brood patches in their bellies to allow transfer of body heat to the eggs; incubation is shared equally. Chicks are mobile and downy on hatching, and can leave the nest after two days; they run and swim after a week and fly within six weeks; in a boom year, chicks form small bands which maraud around the area unchallenged. 'Baby food' consists of pulled lemming: strips off flesh torn off the rodents' corpses. Once airborne, the juveniles pursue local Glaucous Gulls, though parental feeding continues into September. Adults depart first and dominate migrating flocks into early October, when immatures take over. Non-breeding immatures can return early or linger in their winter quarters.

The world population of Pomarine Skua is around 400,000 individuals, 10% of which nest in European Russia; the species is considered 'Least Concern' by BirdLife. It was occasionally shot as fishermen's bait, but this rarely happens now. Fluctuations in lemming numbers appear to have little long-term effect on breeding success; Pomarine Skua lives for about 15 years and lemmings undergo boom and bust in a four-year cycle.

This means that you stand a good chance of finding your own from a headland vantage point as the species passes during migration (see below). Seawatching is always wise in spring and autumn as almost anything might be seen – but Pomarine Skua will certainly be a headliner on the bill if you do.

 

Where to see Pomarine Skua

There are several key locations from which to see migrating ‘Poms’ in Britain, and most seawatching hot-spots will turn up a few in April-May or August-November. Influxes occur, particularly in autumn, so check BirdGuides.com, too.

Spring

There is a more general spread during spring, with most seen in the English Channel or from Scottish Islands.

  • Dungeness, Kent: birds fly quite close to the seawatching hides; Beachy Head, East Sussex, Portland Bill, Dorset and other Channel headlands; news is often phoned ahead as ‘Poms’ head east up the Channel; 
  • Orkney and Shetland: low numbers pass frequently; Papa Westray, Orkney, and Esha Ness and Wats Ness, Mainland, Shetland, can be productive; 
  • Outer Hebrides: Aird an Runair headland at Balranald, North Uist, is usually the best, while Ardivachar Point and Rubha Ardvule are also worth trying.

Autumn

The North Sea coasts and South-West are most productive after breeding.

  • Yorkshire: Spurn, Flamborough and Filey get regular birds July-December; 
  • The Wash: one of the best places to see skuas heading inland is Lynn Point, Kings Lynn, Norfolk, in late September/early October;
  • Sheringham, Norfolk: all four skua species are seen regularly from the beach shelter; 
  • West Cornwall: try Pendeen in north-westerlies or Porthgwarra in southerlies or south-westerlies.
Written by: David Callahan

 David Callahan is a taxonomist and freelance nature writer. Follow him on Twitter: @Callahanbirder.