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

Biology

Life Cycle and Reproduction

Ross’s Gulls are thought to reach sexual maturity in their second year. Weather conditions affect the timing of nesting. Hatchings have occurred in Canada during the second week of June, although the usual hatch date is in mid-July (Macey 1981). Presumably poor weather conditions in spring in some years may dissuade the birds from nesting.

Ross’s Gull nests are approximately 10-15 cm in diameter and can be a depression in the ground (a scrape), a moss cup, or located in sedge tussocks (Macey 1981). Nests are lined with vegetation such as dry grass, sedge, moss, willow or birch leaves, or seaweed (Macey 1981) or unlined with vegetation (Mallory et al. 2006). Eggs are olive with faint reddish-brown markings and are approximately 30 x 43-46 mm. Clutch size is usually 3 (range 1–3) (Macey 1981, Mallory et al. 2006). Nests are incubated by both parents, for 21-22 days and chicks fledge at 20+ days after hatch (Ehrlich et al. 1988).

Studies conducted on nesting colonies of Ross’s Gulls in their Siberian range (e.g., Degtyarev et al. 1997) produced the following results. The average nearest-neighbour distance was 43 m, and some nests were separated by 100 m or more (n = 85). Colonies never exceeded 20 breeding pairs, and the usual number was between two and eight pairs. After five days, chicks were visited by the adults only for feeding, and by 15 days chicks were fed only four times a day. From this time to fledging, at about 20 days, the colony appeared to be deserted; the adults were absent, foraging elsewhere, and the chicks were camouflaged, hiding in the undergrowth. The colonies were usually quiet, since calls were only given in defense against predators.


Predation

Ross’s Gull eggs and chicks are preyed upon by Glaucous Gulls (L. hyperboreus), Herring Gulls (L. argentatus), jaegers (Stercorarius spp.), Arctic fox (Alopex lagopus), weasels (Mustela spp.), and polar bears (Ursus maritimus) (Densley 1991, Alvo et al. 1996). Adults have been taken by Peregrine Falcons (Falco peregrinus) (Burger and Gochfeld 1996). 


Diet

Ross’s Gulls are probably opportunistic feeders. At sea, the birds forage at the edge of the pack ice, where a large proportion of their food is comprised of Arctic cod (Boreogadus saida); 79% of Ross’s Gulls (n = 24) collected in the Chukchi Sea in September and October contained Arctic cod (Divoky 1976). Amphipods were also frequently taken, especially Apherusa glacialis (Divoky 1976), but a wide variety of other prey including Coleoptera, Decapods, Polychaetes, Copepods, Euphausids and Mysids were also found. On nesting grounds in Siberia, the main food of Ross’s Gulls is insects (Macey 1981). Bechet et al. (2000) observed adult Ross’s Gulls hovering low over the water or walking along the shoreline, probably foraging on insects and small invertebrates, and at Churchill, birds were observed taking small items from below the surface of ponds (Macey 1981). Other reports of diet and foraging include feeding on wave-washed “scum” along beaches, presumably picking up plankton; feeding on walrus (Odobenus rosmarus) dung; and gathering around dead animals (Burger and Gochfeld 1996).


Foraging

Ross’s Gulls may forage in small loose flocks or solitarily, occasionally joining Sabine’s Gulls and phalaropes. They will follow ships that are breaking through ice, capturing organisms on the undersurface of disturbed ice. Ross’s Gulls feed by aerial dipping, sometimes surface dipping, and walking (Burger and Gochfeld 1996).


Physiology

There are no studies to date on Ross’s Gull physiology.


Dispersal/Migration

Non-breeding Birds in Summer

During July, nonbreeding Ross’s Gulls move into the Arctic Ocean north of Norway and Russia, probably coming from Russian breeding grounds (Hjort et al. 1997). The birds apparently exploit drift ice and shelf breaks as far north as there is open water, up to the North Pole. The gulls associate with shelf-breaks owing to high food productivity resulting from upwelling of nutrients. In 1996, Hjort et al. (1997) reported Ross’s Gulls to be the most common bird in the central parts of the Arctic Ocean, north at least to 87º30’N, with large concentrations in the bathymetric corner between shelf-breaks and the St. Anna trough. Meltofte et al. (1981) also reported large numbers in the Arctic Ocean in July, from Franz Joseph Land, Svalbard and Greenland north to 82º30’.


Migration

The fall migration eastwards past Point Barrow on the north shore of Alaska has been known for some time (Fisher and Lockley 1954, Burger and Gochfeld 1996), and estimates of 20,000 birds have been reported heading east to previously unknown destinations. It is now known that Ross’s Gulls move east from the Chukchi Sea to the area around Point Barrow, starting in August and peaking in late September, and then to feeding grounds in the Beaufort Sea in late September or early October. There is a return movement westward in late October and early November, presumably moving in response to the freezing of the ocean and foraging opportunities at the edge of the pack ice. The gulls move south via the Bering Strait to winter in the Bering Sea and Sea of Okhotsk (Zubakin et al. 1990, Degtyarev et al. 1997), although the wintering grounds are still poorly known.


Interspecific Interactions

During the breeding season, Ross’s Gulls frequently nest in or close by colonies of Arctic Terns, presumably taking advantage of the predator-mobbing behaviour of the terns. Nests located in Penny Strait were close to Arctic Tern colonies and sometimes nests of Sabine’s Gulls; Sabine’s Gulls appear to be much more aggressive toward humans than are Ross’s Gulls (Mallory et al. 2006). However, Ross’s Gulls will defend their eggs and young, despite their “invisibility” (Burger and Gochfeld 1996, Degtyarev et al. 1997), and will show vigorous tern-like nest defence towards humans (Mallory et al. 2006).

Little is known about the behaviour of Ross’s Gulls away from their breeding grounds. Small loose flocks have been reported foraging along the edge of the pack-ice with Sabine’s Gulls, with little interaction among flock members. Individual Ross’s Gulls at Churchill have been harassed by Bonaparte’s Gulls when the two are feeding close together (pers. obs.) but the harassment tends to be momentary.


Adaptability

The level at which Ross’s Gulls will tolerate disturbance by humans is not known.Disturbance by bird-watchers, photographers, and tourists is a potential threat to the Ross’s Gull in Canada (Macey 1981). The trends emerging from the nesting sites at Churchill provide an indication of tolerance levels in this species. At least one nest at Churchill was abandoned because a photographer was too close (Alvo et al. 1996).  However, Ross’s Gulls are reported to nest successfully beside hunting camps in Russia (Alvo et al. 1996). Such contradictions indicate a need to investigate tolerance of disturbance levels at all stages of the breeding cycle.