Pomarine Skua (Stercorarius pomarinus) is normally 46–51 cm in length (including tail up to 19 cm) with an average weight of 125–135 cm. Rather larger and distinctly bulkier than Arctic Skua (or Parasitic Jaeger). It has a relatively heavier body, deeper belly, broader-based wings, heavier head and bill, and stouter neck. So, overall bulk can recall Herring Gull (Arctic’s bulk recalls Mew Gull).
Bill is obviously heavy and two-toned (with a pale base and dark tip) at all ages, recalling 1st-winter Glaucous Gull and visible even at long range. The major identification problems revolve around small Pomarines and large Arctics. In-flight, typically looks Broad-bodied and even pot-bellied, with a length of ‘rear end’ (i.e. extension of the body behind wings to the tip of the tail, excluding extensions).
Therefore, about equal to wing width (at a point roughly midway from carpal joint to base); ‘rear end’ is longer than wing width in both Arctic and Long-tailed. But markedly shorter in Great (and vagrant South Polar). Wing structure differs, with ‘arm’ being longer and broader than in Arctic while primaries are relatively shorter.
Flight progression generally steady, being slower, heavier and less inclined to glide or ‘shear’ than Arctic. Away from breeding grounds, where feeds mainly on lemmings, eggs and young birds, more of a direct feeder (on fish) or scavenger (feeding on carrion) and less kleptoparasitic than Arctic. More aggressive in pursuit of other seabirds than Arctic, frequently attacking (and sometimes killing) victims as opposed to harrying them to make them disgorge.
Adult summer has broad and blunt-ended tail projections, not narrow and pointed as in Arctic, which are twisted at the tips and appear to form a diagnostic spoon-shaped ‘blob’. Unless these broken off or re-growing, identification is simple.
Plumage otherwise rather like Arctic, varying from birds with almost all-whitish underparts to uniform blackish-brown overall. Compared with Arctic, pale morph has a breast band usually broader and more mottled.
Although sometimes absent (especially in males. Many of which have only a dark wedge on each side of breast) and often a stronger buff washes to sides of the neck. While flanks and lower belly are typically dusky or heavily mottled in females (less so, or even not at all in males).
Arctics often have distinct breast band and brownish lower belly, but flanks are usually clean (except in intermediates). Coloration of the cap, upperparts, breast band and ventral area is darker in tone in Pomarine. Dark morph all dark, with browner throat and sides of neck, looking more uniformly dark than dark morph Arctic. Which shows a more contrastingly darker cap. Intermediates exceptionally rare.
Adult winter has shorter tail projections (or lacks them) and in pale morph body plumage is barred to varying extent, with irregular pale feather tips on upperparts. Especially mantle, scapulars and upper tail coverts), recalling immature (but without barring on underwing coverts, which are uniform blackish). Best separated from Arctic by different ‘jizz’.
Juvenile variable, but great majority fairly dark, differing from juvenile Arctic chiefly in overall bulk. This is very pale juveniles are rare and very dark juveniles are uncommon, juvenile Pomarine typically being far less variable than either of the other two Stercorarius skuas. Dark tips to whitish greater underwing primary coverts divide the whitish bases of primaries from a whitish primary-covert crescent.
This double whitish flash is often a very helpful character in the short to medium range. (Note, however, that palest Arctics can show a suggestion of such a crescent and the very darkest Pomarines may lack it.) Body generally slightly darker than underwing coverts in Pomarine, more uniform in Arctic, and ground color to underwing coverts often paler, making barring stand out more.
In a few very dark Arctics, underwing coverts appear very dark and apparently unbarred. Feather fringes to upperparts and barring on upper tail coverts and underparts less rufous in fresh plumage than in typical juvenile Arctic, with barring on both upper tail and under-tail coverts more even, less wavy.
Head more uniformly colored (typically a deep grey-brown), with little or no sign of a paler nape, of darker streaking or of a paler area (‘noseband’) above bill base (all characteristic of Arctic), and upper tail coverts paler.
Arctic often shows a contrast between paler nape and darker crown. The darker birds showing a combination of pale upper tail and under-tail coverts and uniformly dark head (i.e. without paler nape) are Pomarine. While those showing obviously paler nape than upper tail coverts are Arctic. However, that 1st-summer Pomarine does show contrasting paler nape like Arctic.)
Additionally, although bill color and pattern much the same in both species, the blacktip to the larger, deeper bill of Pomarine is more conspicuous at a longer range. Usually lacks obvious whitish tips to primaries shown by all but darkest Arctic and often visible at rest (although a few pale Pomarines do have them) and has darker marginal coverts along leading edge of wing than in most Arctic.
Also has blunt, not pointed, tips to central tail feathers, but this feature usually of limited value in the field. Because of body bulk, larger individuals could be confused with Great Skua and especially with vagrant South Polar Skua at long range, but the latter has a more extensive white patch on the upper wing and shorter ‘rear end’ (see above). See also vagrant South Polar Skua.
Sex and Age
Sexes similar, but adult male summer usually cleaner on breast and flanks (often lacking dark breast band and flanks entirely). Most of moult to winter plumage takes place in winter quarters but projecting central tail feathers often lost (or damaged) earlier. Adult winter pale morph resembles immature, but underwing dark, not barred, and distinctive twisted tail projection often present.
Adult winter dark morphs little different from summer plumage but can have only short tail projection. Paler 1st-summer individuals like juvenile (see Identification), but have a more prominent tail projection, pale nape (like Arctic), and pale fringes to scapulars and upper wing coverts narrower or absent; dark individuals inseparable from dark juveniles.
2nd-winter pale morph much as adult winter pale morph but underwing barred like a juvenile. 2nd-summer pale morph recalls adult summer pale morph, but tail projection short and extensive barring still present on underbody and underwing. Virtually as an adult by third or fourth summer, but often retains some winter plumage on head, body, and underwing.
Fully adult plumage attained by the fifth summer. Immature dark morph much as adult from second winter, but even more uniformly dark, without much contrast between darker cap and paler sides of face and neck. It becomes an adult by the third summer.
Pomarine Skua rather silent away from colonies, where gives a series of high, nasal screams during territorial disputes: ‘g-waer, g-waer, g-waer’. Also gives a low harsh ‘kek’ when alarmed.
Status and Habitat
Pomarine Skua is not uncommon. Migrates SW and S, joined by breeders from Siberia, to winter in tropical Atlantic and the Indian Ocean. Most move off the western seaboard of Europe and Africa on migration. But minor passage route through Baltic and presumed to move high overland to reach the Indian Ocean.
It breeds on coastal tundra; the population in a given area varies according to lemming abundance. Away from breeding grounds pelagic. Only exceptionally observed inland.
Originally posted 2019-12-28 18:55:54.