Rock gull
| Rock gull | ||||||||||||
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Rock gulls on Saint George in Alaska |
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Rissa brevirostris | ||||||||||||
| ( Bruch , 1853) |
The red-legged kittiwake ( Rissa brevirostris ) is a medium-sized sub-Arctic species of the subfamily of seagulls (Larinae) and, together with the Dreizehenmöwe ( Rissa tridactyla ) in the genus Rissa arranged.
The IUCN now classifies the rock gull as endangered ( vulnerable ) because there has been a significant decline in the population over the past four decades. The population is a maximum of 500,000 sexually mature individuals and appears to have stabilized in recent years. It is therefore possible that the IUCN will change the status to near threatened .
features
The 36 to 39 centimeter tall rock gull is easy to confuse with the slightly larger kittiwake. In the splendid plumage the head, neck, tail, underside of the trunk and most of the lower wings are white. The back and the upper side of the wings are dark gray, the tips of the outer hand wings are black. Differentiating features to the kittiwake are the coral-red legs and feet, which are predominantly black in the kittiwake, the shorter yellow beak and the darker gray upper side. Their dark brown eyes are slightly larger than those of the kittiwake. Outside the breeding season, rock gulls have a diffuse black mark on either side of the head behind the eye. Young rock gulls in the first year resemble adult birds in appearance. However, they have black bills and can be distinguished from other juveniles of related species by their white tail feathers.
nutrition
Rock gulls feed mainly on planktonic small fish , squids , crustaceans and mollusks . During their forays they often hunt together with the kittiwake. They pursue their prey, which is swimming just below the water surface, above the water, submerge them or collect them from the water surface. Their larger eyes also give them the advantage of being able to spot and hunt fish at dusk or at night.
Reproduction
It is incubated in large colonies together with other bird species such as the kittiwake ( Rissa tridactyla ), the thick-billed mum ( Uria lomvia ), the hornlund ( Fratercula corniculata ), the red-billed lalk ( Aethia psittacula ) or the yellow- capped lump ( Fratercula cirrhata ). They build their mud and grass nests on the cliffs along the coast on small ledges and ledges, preferably under overhangs and in the higher cliff regions from a height of 200 to 300 meters. They refer to very narrow ledges and are therefore not in competition with kittiwakes, who prefer wider ledges for nesting. The rock gulls are relatively safe from predatory mammals at their breeding grounds. A clutch consists of one egg; rarely from two eggs. Both adult birds are involved in nest building as well as in the hatchery and rearing of the young. The chicks leave the nest after about five weeks. If the young birds take in too little lipid-rich food during the development period , they grow more slowly and produce more stress hormones.
distribution
In contrast to the kittiwake, the rock gull has only a very small distribution area. The species occurs exclusively on the Bering Sea before the breeding colonies are limited to four places: the to Alaska belonging Bogoslof Islands , the Pribilof Islands and the Buldir Island and the Russian Commander Islands . More than 75% of the population breeds on the island of St. George, which belongs to the Pribilofs . The number of breeding pairs has declined sharply since the 1970s. According to the latest studies, however, the stocks are slowly recovering. After the young birds have left the nest, the adults move to the open Bering Sea until the next breeding season. Little is known about their way of life in the winter areas.
Causes of the population decline
Growing seagulls need a certain amount of fat in their diet. Climate changes decimated the number of high-fat prey fish and there was an oversupply of low-fat fish. Since the populations of other sea bird species in the Bering Sea in the North Pacific have declined sharply since the 1970s and a lack of offspring to maintain the populations could be ruled out, scientists have investigated how the warming of the Bering Sea and the resulting lack of high-fat prey fish influence the populations could.
swell
literature
- Dominic Couzens : Rare Birds - Survivors, Evolution Losers and the Lost. Haupt Verlag, Bern 2011, ISBN 978-3-258-07629-4 .
- National Geographic Society: Field guide to the birds in North America . Washington 1983.
Individual evidence
- ↑ BirdLife Factsheet on the Rock Gull , accessed on August 6, 2011
- ↑ Alexander Kitaysky, John C. Wingfield & John F. Piatt: Effects of food stress on reproductive performance of seabirds at Pribilof and Bogoslof Islands, Bering Sea. Preliminary Summary of 1999 Field Season. pdf, online
Web links
- The rock gull on www.audubon2.org (English)
- Rissa brevirostris in the endangered Red List species the IUCN 2012. Posted by: BirdLife International, 2012. Retrieved on June 10, 2013.
- BirdLife International: Species Factsheet - Rissa brevirostris . Retrieved June 10, 2013.
