Silberalk
| Silberalk | ||||||||||
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Silberalk ( Synthliboramphus antiquus ) |
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| Scientific name | ||||||||||
| Synthliboramphus antiquus | ||||||||||
| ( Gemlin , 1789) |
The silver alk ( Synthliboramphus antiquus ) is a very small species from the alken family . It breeds mostly on the Aleutian Islands and other islands off the coast of Alaska . It has black plumage with conspicuous white and gray areas, without a pronounced seasonal difference in the plumage. Two subspecies are described. Within the genus Synthliboramphus it is the only species that is not classified as endangered by the IUCN . In Canada, however, the species is listed as potentially threatened, as the breeding colonies on islands suffer severely from introduced mammals.
Appearance
The Silberalk is a relatively small alkenvogel with a black and white and dove-gray pen drawing. As is typical for alkenbirds, the legs sit very far back on the body. Unlike guillemots or the puffin , he does not assume an upright posture on land, but usually lies on his stomach. Movement on land is usually accompanied by violent wing flapping. It can either fly up from the water directly or after only a very short run-up. Landing silver alkes often go under immediately. The sexual dimorphism is not very pronounced. Females only have slightly longer wings. Silver bars weigh an average of about 213 grams.
In adult birds are in breeding plumage of the back, the upper wing-coverts and upper tail-coverts bluish gray. The head, the back of the neck and a throat patch are black. The black headstock is framed by white, somewhat elongated feathers. Such feathers can also be found occasionally on the back. The underside of the body is white, the wings of the hand and the tail feathers are dark slate gray. The beak is pale pink to yellowish horn-colored, the base of the beak is black. The legs and feet are pale blue-gray. The webbed feet occasionally have an almost flesh-colored color. The toes are black and the iris is dark brown.
The plain dress is largely similar to the magnificent dress, but the black throat patch is missing. The chin and the sides of the neck have individually different amounts of soot-gray feathers. The number of elongated white feathers is somewhat reduced. Fledglings resemble the adults in their simple dress, but have a shorter and slimmer beak. They largely lack the white border of the black headstock.
During the winter months, silver hawks can be confused with the marmel hawks . However, this one has more noticeable white body sides and lighter wing-coverts. The Silberalk differs from the similarly similar short-beaked lalk mainly in its different beak shape. The also very similar Aleutian alkali has a plump build and the underside of the body is not white, but brownish. Off the Asian coasts there is also a possibility of confusion with the Japanalk .
Distribution area
The distribution area of this species is predominantly in the cold temperate zone in the northern Pacific, but in Asia it also extends into subtropical waters off the coast of China. During the winter, silver hawks show a noticeable southward migration. They can then be observed off the California coast, among other places. Their breeding colonies are located on islands between the 52nd and 60th degrees of north latitude in the eastern Pacific and between the 35th and 62nd degrees of north latitude on the Asian coast. They are common birds off the Aleutian Islands and in the Gulf of Alaska , among others . Silver alkes can also be seen in large numbers in southeast Alaska and off Haida Gwaii . There are 13 breeding colonies on the Queen Charlotte Islands, each with well over 10,000 breeding birds.
During the winter months some birds stay in their breeding area. Basically, however, they migrate south. They can be observed off the coast of the US state of Oregon, for example, between October and November and then again in March. Numerous silver owls overwinter off the California coast, where the birds arrive at the end of October. On the Asian side, silver hawks migrate to Taiwan. Most of them are off the Japanese and Korean coasts.
Food and subsistence
Silver hawks search for their food in schools of up to fifty individuals who dive more or less simultaneously. They are often associated with swarms of other bird species, typically are in swarm near Silberalken also kittiwakes , Nashornalken and other krill-eating seabird species. Silver hawks are usually found on the edge of such swarms. They attract seagulls because their dives drive sand eels and other schooling fish to the surface. The diving depth of silver hawks has not yet been explored. However, they are usually less than 45 seconds under water, which suggests a diving depth of ten to twenty meters. Their main diet is young sand eels and krill as well as young fish of other species. Young herrings are also among the fish that adult silver alks feed to their nestlings.
Reproduction
Breeding colony
Silver hawks breed in colonies. The nests can be found up to 400 meters from the coastline. In moderate climates, the colonies are often in forests, where the alks dig their burrows, up to two meters long, under trees. In places where there are no trees, for example in the Aleutian Islands, they prefer to breed in dense vegetation and build their burrows under tall tufts of grass. They also take advantage of crevices and cavities under boulders. In British Columbia, they are often associated with rhinoceros, Aleutian and storm petrels in their breeding colonies . Silver hawks keep a relatively large nest distance for colony-breeding alken birds. On the Queen Charlotte Islands there is one building every 10 square kilometers.
Breeding birds gather about one to three kilometers from the coastline of their breeding colony. Such collections of brooding silver owls can be observed from around six hours before dark. They usually return to their breeding colonies in the dark. The first-time arrivals arrive around 90 minutes after sunset. They only return earlier on moonless nights. Incoming birds initially fly at a great height above the colony, where the colonies are located in wooded areas, silver hawks collide more often with branches and twigs when they fly to their burrows. Breeding birds usually land ten meters from their burrows and then run quickly and directly towards them. In the morning, the last birds usually leave the breeding colony one hour before sunrise. Such collections of breeding birds can also be observed in the vicinity of the breeding colony in the morning hours.
Social interactions between the birds of a breeding colony can be observed both in the morning and in the late afternoon. These include loud calls, the spreading of the crown feathers and noticeable jumps out of the water. The birds reach a height of about 30 centimeters above the water before they drop back again. In English usage, this behavior is called flop display , because they let one of the wings hang down and tilt back into the water over this side of the body. Occasionally there is also aggressive behavior. They grab each other by the beaks and chop one after the other.
From the second half of the breeding season onwards, large numbers of non-breeding silver owls visit the colonies and stay near the colony until the last chicks have left it. These non-breeders are usually silver hawks in their second or third year of life.
Burrow and nest
The actual nest is a hollow that is only sparsely covered with small twigs, leaves and dry grass. The corridors of the burrows are usually somewhat curved, so that brooding silver beams are not visible from the entrance. In buildings where the corridor is too short, the breeding birds occasionally pull vegetation over the entrance from the inside, so that the building appears unused from the outside. As a rule, silver hawks prefer freshly constructed burrows, so that there are often a large number of unused burrows in a breeding colony.
Egg laying and young birds
The beginning of the oviposition varies depending on the area of distribution. In China, egg-laying peaks around March 15th; on the other hand, eggs are laid on the Kamchatka Peninsula from the end of June to the beginning of July. Typically, the water near the breeding colony has a surface temperature between six and eleven degrees Celsius at the time of oviposition. The females usually lay two eggs. The incubation period is 28 to 37 days. The eggs are not sensitive to interrupted brood. Young birds can also hatch successfully from clutches that have been left for 24 hours. Young birds of a clutch usually hatch less than 24 hours apart and the chicks are led out to sea by their parents for the next two days. They are not fed under construction. In contrast to many other alken birds such as Lummenk and Craverialk , which also show this behavior, research has been carried out on how long the parent birds care for their offspring on the high seas. They are fed by their parent birds for up to a month.
Breeding success
The breeding success in largely undisturbed breeding colonies is unusually high. 96 percent of young birds hatch from eggs that have been incubated for at least 30 days. Breeding pairs usually raise between 1.44 and 1.69 fledglings per breeding cycle. This is also confirmed by observations on the high seas. Most family groups sighted consist of two adult birds and two juveniles. However, it happens more often that silver hawks do not start the breeding business.
Predators and life expectancy
The predators of the silver hawks include deer mice , which are native to many islands off British Columbia. They prefer to eat eggs just before hatching or kill newly hatched chicks when the parent birds are not present. The predators of adult silver falcons include ravens, eagles, seagulls and eagle owls as well as foxes, raccoons and rats, if they occur on the islands with breeding colonies. All of these mammals are introduced to the islands. One of the main predators of the silver hawks is the peregrine falcon , whose food in some places consists of up to fifty percent silver hawks during the breeding season.
About 23 percent of adult silver falcons do not experience the next year of life. Breeding colonies typically have the following age structure: 30 percent are annual birds, 29 percent are two-year-old birds that are not yet breeding, and 41 percent of the birds are breeding birds that are three or more years old.
Subspecies
Two subspecies are described, of which S. a. microrhynchos differs from the nominate form by a smaller beak and a less intense spot on the sides of the neck . It only occurs on one of the commanders' islands on the southern edge of the North Pacific Bering Sea.
- Synthliboramphus antiquus antiquus ( Gmelin , 1789)
- Synthliboramphus antiquus microrhynchos Stepanyan , 1972
Duration
The population of silver hawks is estimated at one to two million individuals. As with numerous other alken birds, the silver alk is particularly threatened by the introduction of mammals to the islands on which their breeding colonies are located. Mammals that negatively impact population numbers include rats, foxes, and raccoons. It is believed that this reduced the populations on the Aleutian Islands by more than 80 and on the Queen Charlotte Islands by more than fifty percent. The breeding colonies off Korea, Japan and China, where the population is generally smaller and silver hawks are not subject to protective measures, are also endangered.
supporting documents
literature
- Jonathan Alderfer (Ed.): National Geographic complete Birds of Northamerica. National Geographic, Washington DC 2006, ISBN 0-7922-4175-4 .
- Anthony J. Gaston, Ian L. Jones: The Auks (= Bird Families of the World. Vol. 4 (recte 5)). Oxford University Press, Oxford u. a. 1998, ISBN 0-19-854032-9 .
- Richard Sale: A Complete Guide to Arctic Wildlife. Christopher Helm, London 2006, ISBN 0-7136-7039-8 .
Web links
- Synthliboramphus antiquus in the endangered Red List species the IUCN 2008. Posted by: BirdLife International, 2008. Accessed January 31 of 2009.
Single receipts
- ↑ BirdLife Factsheet on the Silberalk , accessed on October 19, 2010
- ↑ Alderfer, p. 289
- ↑ Gaston et al., P. 214
- ↑ Gaston et al., P. 216
- ↑ Alderfer, p. 289
- ↑ Gaston et al., P. 216
- ↑ Gaston et al., P. 216
- ↑ Gaston et al., P. 218
- ↑ Gaston et al., S: 219
- ↑ Gaston et al., P. 219
- ↑ Gaston et al., P. 219
- ↑ Gaston et al., P. 219
- ↑ Gaston et al., P. 220
- ↑ Gaston et al., P. 220
- ↑ Gaston et al., P. 221
- ↑ Gaston et al., P. 222
- ↑ Gaston et al., P. 222
- ↑ Gaston et al., P. 222
- ↑ Sale, p. 272
- ↑ BirdLife Factsheet on the Silberalk , accessed on October 19, 2010
- ↑ Gaston et al., P. 216