Crab Grebes
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Crab Grebes |
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| ( Linnaeus , 1758) |
The crab grebe ( Alle alle ) is a bird in the family of the alkenvogel (Alcidae) and the only species in the genus Alle . It is the smallest alken bird that lives in the Atlantic and is characterized, among other things, by its short beak. Crab Grebes breed in large colonies in the Arctic and can migrate to the North Sea in winter .
features
Auk are provided with a body length of 19 to 21 centimeters and a wingspan 34-38 centimeters approximately half as large as Puffin . They weigh between 130 and 160 grams. Apart from the beak length, there is little difference between the two sexes. However, males are between 2.4 and 16.6 percent heavier than breeding females.
In adult crab divers in splendid plumage, the head, neck, upper chest, top of the trunk and top of the wings are black, the underside of the trunk and the under tail-coverts are white and the underside of the wings are gray. The shoulder feathers have a white border. The tail, like the black beak, is very short. In winter, the throat and front side of the neck, chest and sides of the head down to the black cheeks are white. The iris is black and the rather short legs are gray.
The flight is straight, with fast, whirring wing beats. Between the dives it lies very deep in the water with hanging wings and neck drawn in.
voice
The crab grebe is very shouting during the breeding season in the colonies, otherwise mostly silent. The call is a chirping, chattering and ending in laughter "krii-ek ak ak ak ak", which creates a humming chorus in colonies. The warning call in flight is a howling "huhuhuhuhu ...".
Habitat and Distribution
Crab Grebes breed in large colonies in the Arctic on steep coastal cliffs and coastal mountain slopes. Its colonies lie between the 68th and 82nd parallel north. The largest breeding colonies are found in the Thule region in northwest Greenland and on Svalbard . The breeding birds located there represent around 90 percent of the world's breeding crab grebes. Smaller colonies can be found in Upernavik and Scoresbysund in eastern Greenland, as well as on Franz Josef Land , Novaya Zemlya and Severnaya Zemlya . Other colonies can be found in Baffin Bay , Iceland , Jan Mayen and Bear Island . A very small isolated population is found in the Bering Strait , which apparently also breeds. However, the population of this Pacific population is estimated to be less than 1,000 birds. Non-breeding birds reach the Canadian Arctic during the summer and fall and penetrate to the northwest of Hudson Bay and north of the Foxe Basin .
During the winter months, crab grebes can be found from the drift ice border south of Nova Scotia in the west of the Atlantic to Great Britain in the east. Exceptional guests even reach the Azores and Florida during this time . Crab divers are observed again and again in large numbers on inland waters. This is relatively common, especially in the northeastern United States. Such wanderers mostly occur in the months of November to December and then often for several years in a row. Inland crab divers were mostly seen in the UK in 1895 and 1912. It was noticeable that in 1895 a large number of females and only later males were observed. This indicates a gender-differentiated wintering behavior. The causes of the appearance of crab divers in inland waters are not entirely clear; unfavorable wind conditions can play a role, but also changes in the food supply.
There are two subspecies that breed on arctic islands. Here is A. a. all on Greenland , Svalbard and Novaya Zemlya . A. a. polaris breeds on Franz Josef Land .
Reproduction
Crab grebes are colony breeders, with colony sizes ranging from 1,000 to several million breeding pairs. The breeding colonies are usually found on rocky coasts with an inclination of 25 to 35 degrees. Crab divers also use mountain slopes that can be up to 30 kilometers inland. The nests are built up to a height of 400 meters above sea level. Within the colonies, sub-colonies can be distinguished, which can comprise groups from a few dozen to several thousand breeding pairs. The birds in these sub-colonies tend to stick together in flocks. Basically, they take off or land synchronously with one another, without any synchronized behavior with other sub-colonies being ascertained. The clutch consists of only one egg. In western Greenland, 65 percent of eggs hatch and 77 percent fly out of these.
The nest is built on scree fields and is often in crevices up to a meter below the ground. There are 0.3 to 1 nest per square meter. The single egg that makes up the clutch is placed in a hollow made of small pebbles that may have been brought in from the outside. A conspicuous rock near the nest is used both for courtship behavior and as a starting point for flights and is vigorously defended against other crab divers.
The egg is usually laid in June. It is incubated by both parent birds, which alternate on average four times a day in the breeding business. The breeding period averages 29 days. The young birds usually take two to four days and sometimes up to seven days to hatch from the egg. During their first two days of life they are constantly brooded , then intermittently for a period of up to ten days. They are fed an average of four to six times. The female provides most of the food in the first days of the nestling's life; from the fifth day of life, the male alone takes over the feeding of the nestling.
The young birds of the grebes grow up very quickly. The contour feathers grow from the 9th day of life and by the 15th day of life the down plumage on the head and largely from the back and wings has disappeared. The nestling time on Greenland is on average 28.3 days, on Svalbard it is 27. Usually the young bird flies away from the nesting site in the dark and is accompanied by one or more adult birds. The leaving of the breeding colony by the young birds is largely synchronized and most of the young birds have disappeared from the breeding colony within two or three days. The calls of the young birds seem to be the cause of the synchronized leaving of the breeding colony. As they leave the colony, the young birds are exposed to high predator pressures from arctic skuas and ice gulls .
At sea, the chick is also accompanied by an adult bird, and this is probably the male parent bird. So far, however, it has not been possible to determine with certainty whether the adult bird is still feeding the young bird at sea or only accompanying him to suitable feeding grounds.
food
Crab Grebes catch their food underwater. Mainly they eat zooplankton less than 3 to 30 millimeters in size, as well as fry. Crab grebes usually gather in large schools before they go looking for food. Far from the coast they are often widely scattered, on the other hand they are found in dense schools near coastal waters. Crab grebes are occasionally seen up to 100 kilometers from the breeding colonies. Crab grebes that raise nestlings stay within a radius of 20 kilometers from the breeding colony and generally do not move more than five kilometers away. The dives observed so far are very short and averaged 24.5 seconds. During the dives, they cover distances of up to 25 meters underwater.
Existence and endangerment
It is difficult to keep track of the crab grebes that breed in large colonies. Estimates of the colony in Thule vary between 14 and 30 million birds. The colony on Svalbard has 1 million birds and the one in East Greenland between 100,000 and 1 million breeding birds. The population in the Russian Arctic is not known, but apparently it is still as large as that in Greenland and Svalbard. Inventory trends are not available for this species.
The crab grebe is a frequent victim of marine pollution following oil tanker accidents. The overfishing of the seas has little impact on the crab grebe due to its specialization in crustaceans.
The most important predators of adult crab divers include the arctic fox and the ice gull. Gyrfalcons and snowy owls are also among the predators of the adult crab grebe, but do not have the same meaning as the first two species. Arctic foxes also eat the eggs and juveniles of the grebes. Polar bears have also been observed on Franz-Josef-Land, digging breeding birds out of their breeding niches.
swell
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 et al. 1998, ISBN 0-19-854032-9 .
- Lars Svensson , Peter J. Grant, Killian Mullarney, Dan Zetterström: The new cosmos bird guide. All kinds of Europe, North Africa and the Middle East. Franckh-Kosmos Verlags-GmbH & Co., Stuttgart 1999, ISBN 3-440-07720-9 .
Individual evidence
- ↑ Alderfer, p. 281
- ↑ NN Kartaschew: Seagulls and Alken . In: Prof. Dr. Bernhard Grzimek, Dr. Wilhelm Meise, Prof. Dr. Günther Niethammer, Dr Joachim Steinbacher (ed.): Grzimeks Tierleben Vögel . tape 2 . Bechtermünz, Augsburg 2000, ISBN 3-8289-1603-1 , p. 233 .
- ↑ Gaston et al., P. 161
- ↑ Gaston et al., P. 161 and p. 162
- ↑ Gaston et al., P. 162
- ↑ a b Gaston et al., P. 163
- ↑ a b Gaston et al., P. 164
- ↑ a b Gaston et al., P. 166
- ↑ a b c Gaston et al., P. 167
- ↑ Gaston et al., P. 165
- ↑ Gaston et al., P. 168
Web links
- All all in the endangered Red List species the IUCN 2008. Posted by: BirdLife International, 2008. Accessed January 30 of 2009.
- Videos, Photos, and Sound Recordings on All All in the Internet Bird Collection
- Crab Grebe's feathers