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COSEWIC Assessment and Update Status Report on the Ross’s Gull in Canada

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COSEWIC
Executive Summary

Ross’s Gull
Rhodostethia Rosea

Species Information

Ross’s Gull (Rhodostethia rosea) is a small, tern-like gull with a buoyant flight. It can be distinguished by a unique combination of a wedge-shaped tail, grey underwing and a narrow black collar that completely encircles the rather dove-like head. The sexes are alike. In breeding plumage, the head and body take on a rose colour strongest on the breast and belly. In flight, the dark grey of the underwing coverts contrasts with a broad white trailing edge to the wing. The immature plumage has black outer primaries and a broad black diagonal band across the inner wing, forming a broad white triangle on the rear wing, and a broad black tail band. 

Distribution

Ross’s Gull is an Arctic species with a circumpolar distribution. It breeds primarily in northeast Siberia, with small scattered colonies in Greenland, Svalbard, and Arctic and subarctic Canada. In Canada, only four nesting locations have been found, three in Nunavut and one in Manitoba. Winter distribution is poorly known but likely populations winter along the edge of the pack ice in the northern Bering Sea and the Sea of Okhotsk, and in the open waters of the Arctic.

Habitat

Ross’s Gulls breed in widely varying Arctic habitats, from marshy tundra to gravel reefs. All sites are located near water, and many are close to Arctic Tern (Sterna paradisaea) colonies. The site used in Churchill, Manitoba, consists of hummocks supporting grasses, lichens and dwarf willows, lower areas with grasses and sedges, small pools and some shallow lakes. Occupied breeding sites on the Cheyne Islands and in Penny Strait were on low-lying gravel reefs, close to nearby polynyas, which attract birds when open in late spring.

Biology

Ross’s Gulls are thought to reach sexual maturity in their second year. Weather conditions affect the timing of nestings; in Canada, poor weather conditions in spring in some years may dissuade the birds from nesting at all. Nests can be a depression in the ground (a scrape), a moss cup, or located in sedge tussocks. Clutch size is usually 3. Both parents incubate nests, for 21-22 days and chicks fledge at 20+ days after hatch. Nests are often far apart from each other, frequently among colonies of Arctic Terns, and the number of nests in each colony is usually up to 8 pairs, and has never exceeded 20 pairs.

Ross’s Gull eggs and chicks are preyed upon by both avian and mammalian predators, and nesting success is low. The gulls are probably opportunistic feeders, at sea, feeding on small fish and invertebrates and during the breeding season on insects.

Dispersal/Migration

After breeding, Ross’s Gulls move north into the Arctic Ocean, apparently exploiting drift ice and shelf breaks as far north as there is open water up to the North Pole. There is a pronounced fall migration eastwards past Point Barrow, Alaska, to feeding grounds in the Beaufort Sea in fall, and a return movement westward in early winter in response to ocean freezing and foraging opportunities at the edge of the pack ice.

Population Sizes and Trends

Censuses of Siberian breeding grounds indicate a population of about 50,000 Ross’s Gulls and it is considered vulnerable/apparently secure and not globally threatened. There is little recent information to indicate if the global population continues to be stable. There appears to have been a significant increase in the number of Ross’s Gulls reported south of traditional wintering areas in the last thirty years, for example, in the British Isles, Iceland, southern Canada and the United States outside Alaska.

In Canada, the Ross’s Gull occurs at low population numbers, scattered throughout the low and high Arctic region, with the total known breeding population in any one year ranging from 0-10 pairs. The Churchill population has ranged from 1-5 pairs since 1980, the colony at the Cheyne Islands peaked at 20 individuals when 6 pairs and 8 unpaired birds were present in 1978, and possibly 10 individuals (5 pairs) were present at the unnamed island in Penny Strait in 2005. In 2006, 3 pairs were found in locations on the Cheyne Islands where they had nested previously, and one pair was relocated on the unnamed island (G. Gilchrist, pers. comm. 2006).

Limiting Factors and Threats

Oil development in the Beaufort and Chukchi Seas poses a potential threat owing to the large concentrations of birds during fall in the Beaufort Sea. Breeding sites in Canada are relatively remote and are not at risk from industrial development at present, although there are known significant oil and gas reserves in the Canadian Arctic that might be exploited at some future date.

Given the rate at which climate change is affecting the Arctic, any obligate Arctic-adapted species should be considered under imminent threat. Annual snow and ice patterns are probably major limiting factors influencing the decision to breed in any given year, since one critical variable is the presence of open water close to the nesting site. Climate change represents an unknown effect on the reproductive ecology of Ross’s Gulls.

Disturbance by humans at nest sites has caused nest abandonment, and there likely still is a black market for the sale of Ross’s Gull eggs. Fledging rates are low, with hypothermia invoked as a frequent cause of chick mortality.

Special Significance of the Species

The Ross’s Gull has a remarkable mystique among the bird-watching community for its rarity, resulting in local economic boosts for the community (see Churchill). Subsistence harvest of Ross’s Gulls in Canada is probably negligible.

Existing Protection

Ross’s Gull was designated by COSEWIC in November 2001 as Threatened. The Species at Risk Act (SARA) prohibits damaging or destroying the residence (= nest) of Ross’s Gull. It and its nest are protected under the Migratory Birds Convention Act of 1994. No collection of adults, young or eggs is allowed. Hunting of Ross’s Gull is also prohibited in Russia. It also receives some protection from the Canada National Parks Act and the Churchill Special Conservation Area.