Pomarine Skua (Stercorarius pomarinus)

[order] CHARADRIIFORMES | [family] Stercorariidae | [latin] Stercorarius pomarinus | [UK] Pomarine Skua | [FR] Labbe pomarin | [DE] Spatel-Raubmowe | [ES] Pagalo pomarino | [NL] Middelste Jager

Subspecies

Physical charateristics

The Pomarine Jaeger is the largest of the jaegers. Adults have long central tail feathers that are twisted 90 degrees, but these are usually lost after the breeding season, in mid- to late summer. Pomarine, Greek for ‘lid-nosed’, refers to the sheath that covers the base of the bill. The large, bi-colored bill contrasts with the dark face. Adults and juveniles have light and dark morphs, but 90% of the adults are light. The light morph has a blackish cap and dark brown upperparts, white underparts and collar, a yellow wash on the sides of neck, and a bold brown band across the breast. The dark morph is similar except the underparts, sides of the neck, and collar are entirely dark brown. Juveniles are brown with uniform head and neck and strongly barred coverts on the tail and underwing. In other parts of the world, jaegers are known as skuas.
Pomarine Jaegers forage by taking fish from the surface or diving, scavenging, preying on smaller birds, and stealing prey from other seabirds (kleptoparasitism). During the breeding season, they forage over land by hovering and swooping down on lemmings. They may be nomadic during the breeding season, in search of areas with high lemming densities. They sometimes dig to find lemming nests. Breeding pairs defend large territories for nesting and foraging. They migrate singly or in small groups. Pomarine Jaegers are powerful and fast in flight.

Listen to the sound of Pomarine Skua

[audio:http://www.aviflevoland.nl/sounddb/P/Pomarine Skua.mp3]

Copyright remark: Most sounds derived from xeno-canto

Range

North America, Eurasia : North

Habitat

When they are not breeding, Pomarine Jaegers are highly pelagic. They winter in productive regions of tropical and subtropical oceans, and concentrate over upwellings and boundaries of currents. They may be seen around large fishing vessels where they steal food from other seabirds. Their Arctic breeding grounds are on low-lying, wet, coastal tundra.

Reproduction

Arrival in breeding areas depends on lemming densities. Adults often form pairs, occasionally with their old mates, before establishing territories. Both sexes build the nest, a depression they form by trampling the ground with their breasts and feet. The female usually lays 2 eggs, and both parents incubate. The young are able to leave the nest within two days after hatching, but stay near the nest where their parents feed them pieces of lemmings. The young are dependent on parents for food for two more weeks after fledging, which occurs at 21-27 days. The parents leave the breeding area as soon as young reach independence.

Feeding habits

Their diet includes fish, smaller birds, carrion, and refuse. Breeders rely on lemmings for successful reproduction.

Conservation

This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be stable, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Migration

Migratory, but details inadequately known due to pelagic nature, absence of ringing recoveries, and difficulties of identifying juvenile and non-breeding birds at sea. Chief wintering areas in Atlantic Ocean are north of Equator, from Caribbean to Sargasso Sea and north to Cape Hatteras (North Carolina), and in cool upwellings off West Africa, especially 8-17 degrees N. Some cross Equator and move further down West African coast to c. 30 degrees S.
Movement noted northern seas from early August. Principal movements through North Sea and temperate North Atlantic occur September-October; adults precede juveniles. Return migration begins April, with main North Atlantic movement in May. Small spring passage detected in British and Irish inshore waters spans early April to early June. Breeding grounds reoccupied about mid-June, though variable according to ice conditions.

Distribution map