Family Gulls, Terns, Skimmers (Laridae)
Ross's Gull (Rhodostethia rosea)
Taxonomy
Larus roseus
W. MacGillivray
, 1824,Melville Peninsula, Canada
.NE Siberia from Taymyr Peninsula E to R Kolyma; also locally in Greenland, and irregularly in N Canada. Winters mainly in Arctic. Limits of distribution very poorly known for all seasons.
Descriptive notes
29–32 cm; 120–250 g; wingspan 82–92 cm. Two-year gull. Dove-like or tern-like, with a buoyant tern-like flight, and unique combination of wedge-shaped tail (not always apparent), grey underwing and black nuchal collar; in fresh breeding plumage, head, neck, underparts and tail rosy-white; rose colour strongest on breast and belly, more intense than on any other larid, but fades with wear, although still visible on some birds in early Oct; narrow black collar completely encircling head; outer web of outer primary black; white trailing margin to secondaries; bill black; legs bright red to reddish orange; iris dark, with red orbital ring. In flight, characteristic dark grey underwing-coverts with broad white trailing edge to wing. Non-breeding adult lacks black collar and rosy hue, has pale greyish crown, indistinct dusky black flecks around eye, and small black auricular spot. Hydrocoloeus minutus has more rounded wings, and blacker underwing with narrower white margin. Immature has blackish outer primaries and broad blackish diagonal band across inner wing, enclosing broad white triangle on rear upperwing. Wedge-shaped tail is white with black tips to the centre tail feathers only .
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Voice
Calls are higher-pitched and more melodious than those of most gulls. A short, dry, yapping call "a-dac a-dac a-dac" or "de-urr de-urr" is heard on the breeding grounds. Alarm call is a short, tern-like series of notes "kik kik kik kik". Generally silent away from nesting grounds but a short, high-pitched "kew", recalling Black Tern Chlidonias niger' is reported in winter.#R#R
Habitat
Very few nesting colonies found. Breeds in boggy terrain of the upper taiga and tundra, preferring marshy tundra in river deltas or small ponds with stunted alders (Alnus) and willows (Salix). Often associates with Arctic Terns (Sterna paradisaea) and waders. In non-breeding season, found in open seas and at edge of pack ice.
Food and feeding
Little information. On breeding grounds chiefly insectivorous; on migration and in winter, mainly marine invertebrates (plankton, crustaceans, molluscs, and even priapulids) and small fish. Reported feeding on wave-washed “scum” on the beach, presumably plankton. Occasionally feeds on walrus (Odobenus rosmarus) dung. Forages in small loose flocks or solitarily; may join Sabine's Gulls Xema sabini or phalaropes. Often follows ships through ice, capturing organisms on undersurface of disturbed ice. Often feeds by aerial dipping; sometimes by walking, also by surface-dipping. Flocks sometimes gather around dead mammals.
Breeding
Lays in early to mid-June. Forms loose colonies of 2–10 pairs, rarely 18, in tussocks on islands in tundra or taiga pools, often with Sterna paradisaea; a nearly unique feature of colonies is their “invisibility”. Nest built of dry grass, sedge and moss, often damp, lined with dry grass, sedges and reindeer moss. Nearest-nest distances averaged 43 m, sometimes as much as 100 m, rarely less than 5 m. Social interactions among neighbours very rare; change-overs quiet and unobtrusive. Normally 3 eggs (1–3); incubation 19–22 days; incubating bird flies up at call of incoming mate, which lands and goes to exposed nest; non-incubating mate does not remain on territory; intruders do not elicit alarm calls; chick yellowish to rusty-brown, with dark spots and lines on crown, back and wings, and breast and central belly unmarked whitish; hatching weight 15 g; parental defence aggressive, and also with well developed distraction displays including “rodent-run”, “false feeding” and “impeded flight”; adults may lead brood away from colony; at 7 days, chicks left unattended and form groups at water's edge; extremely short fledging period reported to be 3 weeks or less. Fledging rate low, usually less than 33%; hypothermia a frequent cause of chick mortality.
Movements
Post-breeding movement started by late July, N to Arctic Ocean, and then migrating E or W along the coast or edge of pack ice; arrives back at breeding grounds in late May, depending on snow and ice conditions. Winter distribution poorly known, but presumably along edge of pack ice. Very rare even in the Bering Sea after winter. September–October records around Wrangel I and in Chukchi and Beaufort Seas, with birds often flying NE ahead of westerly winds; presumably these birds continue past Barrow, Alaska, where birds first seen in early August, with notable movements from late September to early October, when thousands pass NE to feed in Beaufort Sea; many or most return W in Oct–Nov to their unknown wintering grounds. No corresponding spring movement. From Siberia, birds disperse W in August–September, eventually reaching Franz Josef Land and Spitsbergen. An ocean transect from E Greenland to Franz Josef Land recorded 1000 birds, mainly in groups of 2–16, 90% in adult plumage. Two satellite-tracked adults, a male and a female, breeding in the Kolyma Delta, Russia, departed from the breeding area and reached the nearby Laptev Sea at the beginning of Jul, both birds moved NW, and the male staged until the end of Sept in an area of scattered sea-ice NE of Severnaya Zemlya archipelago; by mid-Oct, most likely escaping the polar night, this bird reached the coast of NW Alaska, and a few days later it arrived in the coastal wetlands of North Chukotka, where it remained until the transmitter stopped in early Nov#R. Three geolocator-tagged individuals from a breeding site in the Canadian High Arctic wintered in a restricted area of the Labrador Sea#R.
Vagrant to the USA , found mainly in the midwest and north-east, and south to California#R. Vagrants also occur irregularly in the Pribilof Islands, Manchuria and N Japan: where there was an influx of 100+ in Hokkaido in January 2001#R. Rare visitor to Europe, most frequently found in the British Isles and Iceland but recorded as far south as Spain and Italy.
Status and conservation
Not globally threatened (Least Concern). Information inadequate to assess true status. Very few breeding localities known, and none regularly monitored. Total population may be as low as 10,000 pairs, but probably closer to 25,000 pairs; even suggested as probably no fewer than 50,000. At least 20,000 (up to 38,000) individuals move past Point Barrow in autumn; c. 27,000 recorded in 2011 between 20 Sept and 28 Oct, peaking on 16 Oct when over 7,000 birds passed during a three hours period#R. The Canadian and Greenland populations are very small, perhaps no more than five pairs: breeding recorded at four sites in N & W Greenland, although permanence of these sites uncertain. All known Canadian colonies have contained fewer than six pairs, and indeed the entire North American population may not be permanent. Adverse factors on species' numbers include predation on chicks by gulls, skuas and Peregrine Falcons (Falco peregrinus). In Canada, nests at Churchill fail frequently owing to human disturbance. Some harvesting of subadults and adults continues at Barrow, and migrants are still shot for food by Alaskan Inuit. Oil development in Beaufort Sea a potential threat.
Resembles Hydrocoloeus minutus in size and plumage sequence, but with very different adult plumage; morphometric analysis suggests that it is an early derivative of a primitive larid or of a hooded larid. Monotypic.