Crested alf
| Crested alf | ||||||||||
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Crested lalk ( Aethia cristatella ) |
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| Scientific name | ||||||||||
| Aethia cristatella | ||||||||||
| ( Pallas , 1769) |
The crested lalk ( Aethia cristatella ) is a monotypical species from the family of alken birds. It is a relatively small alken bird, which during the breeding season has a very conspicuous beak shape and a feather hood that falls forward. Its distribution area is the North Pacific. It breeds in colonies that can contain more than a hundred thousand birds. The IUCN currently rates the species as harmless ( least concern ).
Appearance
general characteristics
The crested lalk reaches a body size of 25 centimeters. This makes it a relatively small alken bird with a body size that corresponds to that of the crab diver and the red-billed falcon. The weight varies a lot. During the breeding season, adult birds weigh between 211 and 322 grams, with the females being slightly lighter. Crested hawks have a very upright posture on land and, like the red-billed divers, are very agile on land. Excited birds stretch their necks very far forward, which is otherwise not observed in any of the small alkenbirds. The flight of the crested hawk is fast, the wings are narrow and pointed. Adult crested falcons have white irises, legs and feet are pale blue-gray with black webbed feet. However, there are also individuals who have brownish to brownish-pink colored legs and feet. Males are usually slightly larger than the females.
Splendid and simple dress
In the magnificent dress , the top of the body is dark gray, there is only a white line from the eyes to the neck. The wings and the tail feathers are black-gray to black-brown. The conspicuous, forward-falling spring bonnet consists of twelve to twenty, two to three millimeters wide and 15 to 50 millimeters long blackish feathers. Various studies have shown that crested falcons prefer partners with the largest possible spring hood. The plumage smells strongly of citrus during the courtship period, which may play a role in courtship. The beak is bright orange. During courtship it is significantly enlarged by four to five horn-like plates. Males then have a noticeably stronger bill than the females. The plates are thrown off by breeding birds when they start rearing their young; non-breeding adult crested hawks keep them until the end of the breeding season. Crested hawks then have a significantly smaller and less conspicuous colored beak.
The striking beak color is missing in the simple dress , the feather bonnet is significantly reduced or even missing entirely. Some crested falcons, probably not yet sexually mature, birds one to two years old, also show this simple dress during the summer. Fledglings resemble adults in winter plumage, but have gray eyes, no feathers and a small black beak.
voice
Similar to the Aleutian and Red- beaked falcon , the crested falcon is a very call-happy bird. The voice, however, differs significantly from that of other alken birds. Their short, rough call is reminiscent of the bark of a small dog. This “bark” is most commonly heard when the crested hawks are in the breeding colonies or when they are gathering at sea in front of the breeding colonies.
Spreading and migrations
The distribution area of the crested falcon is the Bering Sea and the Sea of Okhotsk in the northwestern Pacific region. Crested hawks prefer waters with a surface temperature of two to ten degrees in summer and two to five degrees in winter.
Crested falcons come ashore only during the breeding season. Breeding colonies of the crested hawk are found on the Kuril Islands and islands of the Sea of Okhotsk such as Sakhalin and the Shantar Islands . In the western Bering Sea, crested hawks breed on the Commander Islands , on the southeastern coast of the Chukchi Peninsula and Ratmanov Island . On the Alaskan coast they breed on the remote islands of the Aleutian Islands , the Pribilof Islands , St. Matthew Island , St. Lawrence Island , King Island and the Shumagin Islands , an archipelago in the Gulf of Alaska .
In the winter months, crested hawks usually stay south of the drift ice border, but some birds also stay in the sea of Okhotsk, which is rich in drift ice. In Asia, the species can be seen in large numbers near the Kuriles, Sakhalins and in the north of Hokkaidō . To the east, there are many crested hawks near Kodiak Island . Stray visitors even reach Wrangel Island or swim up the Yukon River , where they have been observed up to 600 kilometers in interior Alaska. In the south they occasionally reach the coastal waters of British Columbia and Baja California . They have even been seen in the North Atlantic off Iceland.
food
Crested hawks forage for food both in coastal waters and on the high seas. They also look for regions that are more than 100 kilometers away from their breeding colony. The decisive factor is that these waters have a high density of zooplankton . The food spectrum includes small fish and octopus, crustaceans and other invertebrates.
Reproduction
Social behavior
One of the typical behavioral characteristics of the crested hawk is the circling of flocks of crested hawks at heights of sometimes more than five hundred meters above the breeding colonies. Basically, they are extremely sociable birds that stay together in large flocks all year round. Individuals who lose their closeness to their conspecifics immediately look for connections. This sometimes means that the crested hawks in the first row of a flock slow down, while birds at the end of the flock become faster and thus fly in a long swarm formation that condenses again and again and then breaks up into several individual flocks.
The number of crested hawks present in a colony varies greatly from day to day. Typically there are two activity peaks, in between the colony seems almost deserted. The only visible activity is limited to birds leaving their nest holes or bringing in food for juveniles. Crested hawks compete for suitable nesting sites within the colonies, especially males react aggressively in the breeding colonies to too close proximity to other males and young birds. Fighting crested hawks chop with their beaks one after the other, regularly aiming for the eyes. They grab each other by the neck or beak and beat each other with their wings. Males often have plucked feather bonnets as a result of these fights. Basically, the hood size correlates with the dominance, the winner of such attacks usually has the larger spring hood. Aggressive interactions between males can also be observed on the high seas. This is especially true when a mated male defends his female against other males who approach it. Females are less likely to show aggressive behavior. Crested falcons show aggressive behavior towards other alken birds. The little sparrow-sized dwarf balk , which belongs to the same genus, is regularly defeated by the crested balk in duels for nesting sites. It is even assumed that the dead dwarf galks, which can occasionally be found in breeding colonies, are victims of confrontations with the crested lalk.
Nesting site
Crested falcons, which breed in the south of the range, stay in the range from August to early May. The crested falcons, which breed in the northernmost part of the distribution area, remain on the high seas from September to early June. Nests can be found from sea level to heights of 500 meters. Breeding colonies are often found on rubble heaps . Where these heaps of rubble are very extensive, several breeding pairs occasionally use a large gap to get to their burrows, which are then up to ten meters below the surface of the rubble. The density can be very high on a suitable substrate. On Buldir Island , Alaska, 1,500 to 2,000 breeding pairs can be found on 100 square meters. The rubble heaps here are ten meters deep, 1.5 to 2 breeding pairs breed per cubic meter of rubble. Some breeding pairs breed so deep in the rubble that their burrows are completely dark even during the day. The actual nest is a shallow hollow, often lined with small pebbles. However, some breeding pairs also lay their eggs on bare rocks or on a hollow that has just been dug out.
Clutches and young birds
The peak of egg-laying usually falls in the Aleutian Islands in late May, in the Pribilof Islands in early June, in the St. Matthew Islands in mid-June and on the St. Lawrence Island in mid-July. In the northern regions of the distribution area, the time when the snow melts has a considerable influence on the start of breeding. As with many alken birds, the clutch consists of only one egg. Additional clutches in the event of egg loss are very rare. The egg is oval and has a smooth surface. It is white at the beginning and turns dirty brown as the brood progresses from the surroundings. A freshly laid egg usually weighs 36.3 grams, which is about 14 percent of the body weight of a female. The breeding season averages 34 days, both parent birds are equally involved in the brood and usually separate at intervals of 24 hours.
At first, the young birds are constantly huddled, but are increasingly left alone by the parent birds during the day until they are left alone for the whole day from the 20th day of life. By the time they fledge, juveniles weigh about 250 grams. Before they finally leave their nest cavity, they occasionally come to the exit of the building and train their flight muscles by flapping their wings violently for 15 to 30 seconds. When the young birds leave the breeding colony, they climb onto the nearest rock and take off without being accompanied or supported by the parent birds. Many fly out to sea, but others land at the foot of the rubble, injure themselves there or are caught by gulls, hawks and foxes. The parent birds do not take care of the young birds on the high seas.
Breeding success and life expectancy
The reproductive success of crested hawks is difficult to determine because the breeding birds react sensitively to disturbances in their breeding caves. It is estimated that for every 100 breeding pairs, 50 to 55 young birds fledge. The predators include arctic fox and red fox, both of which beat adult birds. Brown bears dig up burrows on the rubble mounds to eat parent birds, nestlings and eggs. Voles eat both eggs and nestlings. Rats also kill adult birds in the breeding caves and also eat eggs and young birds. Ice gull , Bering gull and Kamchatka gull as well as snowy owl, bald eagle and giant sea eagle, gyrfalcon and peregrine falcon are other predators of this species.
The mortality rate of adult crested falcons is around eleven percent per year. They reach an average age of 9.6 years.
Duration
The exact population of the crested hawk is not known. It is estimated that at least two million birds live on the Russian coast and three million on the coast of North America. Breeding colonies with more than 100,000 individuals occur on Buldir Island , Kiska Island , St. Lawrence Island, St. Matthew Island, Ratmanov Island , Chirinkotan and Ushishir , among others .
There is no evidence of global population trends, but the introduction of foxes to some of the Aleutian Islands, like other alken birds, has resulted in a drastic decline in population numbers. Introduced rats have done similar things on some other islands. Oil-spilled carcasses have been found along the coasts of some islands with breeding colonies and close to shipping routes, suggesting that ocean oil pollution may be a major cause of death. Crested hawks also regularly drown in drift nets. The lights of ships attract crested hawks, especially on foggy nights, which then regularly die in collisions.
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 et al. 1998, ISBN 0-19-854032-9 .
Single receipts
- ↑ BirdLife Factsheet on the Schopfalk , accessed on October 23, 2010
- ↑ Gaston et al., P. 244 and p. 245
- ↑ Gaston et al., P. 242
- ↑ a b Gaston et al., P. 243
- ↑ Gaston et al., P. 249
- ↑ a b Gaston et al., P. 247
- ↑ a b Gaston et al., P. 245
- ↑ a b c Gaston et al., P. 248
- ↑ Gaston et al., P. 248 and p. 249
- ↑ Gaston et al., P. 258
- ↑ a b c d e Gaston et al., P. 250
- ↑ a b Gaston et al., P. 251
- ↑ Gaston et al., P. 252
Web links
- Aethia cristatella in the endangered Red List species the IUCN 2008. Posted by: BirdLife International, 2009. Retrieved on October 25 of 2010.
- Videos, photos and sound recordings of Aethia cristatella in the Internet Bird Collection
- Photos of Aethia cristatella in the Oriental Bird Club image database , accessed October 25, 2010