Stercorarius parasiticus (L.)
Family: Stercoraridae

The Arctic Skua is a dashing aerial pirate of our marine coasts. It pursues other pelagic bird species, in almost falcon-like flight, with fast twists and turns, until the quarry drops or regurgitates its meal, which is usually caught and swallowed in flight by the Skua. Breeding only in the very north of Scotland and some of its islands in the British Isles, this species has undergone rapid decline in recent years.

In brief

• A highly pelagic species which is generally only seen along the coast
• Best looked for in autumn passage movements from August – October
• Red listed in UK Birds of Conservation Concern as a declining breeder but only occurs in Northern Ireland as a passage migrant
• Reasons for the decline are not certain, but may be linked to effects of global warming on the food chain

Species description
A gull-like, polymorphic species, which has dark, pale and intermediate forms. At distance, even pale forms look darker than gulls; whilst close to, they are easily separated by all-brown upper-parts, except for a pale neck collar, running up from the all-white underparts, excepting brown under-tail coverts. A brown breast band is sometimes present. Dark forms are chocolate brown all over, though both forms show a pale flash in the outer wing, more obvious from below than above; and both, in adult plumage, have elongated central tail feathers projecting.

Life cycle
Not known to breed in Northern Ireland. Predominantly a breeding bird of the Arctic tundra, this species will use moorland and islands further south in its range. Colonies are usual, although isolated pairs do occur. Clutch size may vary, but two is commonplace, in a nest on the ground, which is little more than a depression in the vegetation. Adults can launch aerial stoops at both humans and animals which approach their offspring. Fish is the mainstay of diet; however, during breeding, eggs and young of other birds, small mammals and invertebrates are the main prey. Incubation takes four weeks, with a further four weeks to fledging.

Similar species
Separation of this species from Pomarine Skua and Long-tailed Skua can be very easy, or very difficult! In full breeding plumage, with a good view, the central tail projections of each are distinctive: Arctic has a short spike of a few inches; Long-tailed may have a projection exceeding half the body length; while Pomarine has a round-tipped, spoon-like projection. Distant and juvenile birds, which are extremely variable, are sometimes indeterminable; factors such as the chesty, broad wing - based Pomarine, as compared to the Kittiwake - like Arctic and the Tern - like Long-tailed come into play. Experience is an invaluable tool in Skua identification.

How to see this species
Arctic Skua may be seen anywhere along the sea coast of Northern Ireland, but particularly so from headlands, such as Ramore, Portrush and St. John’s Point, Co. Down, especially during onshore autumnal gales. This species does occur less commonly at other times of the year and is described as rare in winter and rare inland.

Current status
World populations of Arctic Skua appears stable, rendering the Birdlife classification as Least Concern. In the U.K. this species is Red Listed, with a breeding population decline >50% in 25 years. In Northern Ireland annual numbers on passage fluctuate, but this is more likely symptomatic of weather conditions and observers reporting the species, rather than population changes. Double figures of this species on a good passage day are not unusual.

Why is this species a priority in Northern Ireland?

• Red listed in UK Birds of Conservation Concern, due to breeding decline >50% in 25 years. Currently on the Priority List by default

Threats/Causes of decline
Many European seabirds have undergone breeding failures for a number of recent years. It appears that sea temperatures have risen, which would have an effect on the distribution of pelagic prey species. A species such as Arctic Skua, which is on the southern edge of its breeding distribution in the British Isles, may well be substantially impacted.

Conservation of this species

Current action
Arctic Skua is a U.K. Biodiversity Action Plan listed species.

Proposed objectives/actions

• Maintenance of the food chain – sustainable fishing
• Monitoring of population trends

What you can do
Join RSPB or volunteer as a temporary warden. Report any counts of this species to NIBA online or at 028 91 467 408.

Further information

Links
UK Priority Species data collection

Birdlife Factsheet

Video of nesting birds

UK Birds of Conservation Concern

Birds of Conservation Concern Ireland

Literature
Harrison, P. (1983). Seabirds Christopher Helm.

Mullarney, K., Svensson, L., Zetterstrom, D. & Grant, P.J. (2009). 2nd Edition. Collins Bird Guide. Harper Collins, London.

Text written by:
Allen & Mellon

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