Razorbill
| Razorbill | ||||||||||
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Razorbill ( Alca torda ) |
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| Systematics | ||||||||||
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| Scientific name of the genus | ||||||||||
| Alca | ||||||||||
| Linnaeus , 1758 | ||||||||||
| Scientific name of the species | ||||||||||
| Alca torda | ||||||||||
| Linnaeus , 1758 |
The Razorbill [ tɔʁtʔalk ] ( Alca torda ) is a bird from the family of the Auks and the only member of the genus Alca . Two subspecies are distinguished, which differ slightly in size, but are genetically only slightly differentiated.
The razorbill is a strongly built alkenvogel with a high and laterally compressed beak and an unusually long tail for an alkenvogel. On the water and in flight it is reminiscent of guillemots , from which it differs, however, by its more powerful beak and the longer tail. On land, the razorbill walks and crouches in an upright posture. In Central Europe , the razorbill is a local breeding and summer bird. Winter guests also stay regularly on the East and North Sea coasts.
The razorbill has been rated as near threatened by the IUCN since 2015 .
description
Adults are 38 to 43 cm long and have a wingspan of 60 to 69 cm. They weigh between 600 and 800 grams and are around 22% lighter than the guillemots that look like them - at least for inexperienced bird watchers . The sexual dimorphism is only slightly pronounced. The back and head are black, the belly and the underside of the wings are white. A thin white line runs from the eye to the upper beak. The beak is black with a conspicuous, white, vertical line just before the nostrils.
The plain dress largely resembles the magnificent plumage, but the throat, the sides of the neck and the face behind the eyes are white. The white lines on the head and beak are less noticeable or completely absent. Young birds have a plumage that resembles the plain dress. Their beak, however, is shorter, narrower and pure black.
The flight of these diurnal and crepuscular alkenbirds is straight with a rapid, whirring wing beat. Razorbills go hunting individually. The diving depth is usually only a few meters. Similar to the puffin , fish are transported across the beak for the nestlings.
distribution and habitat
The breeding area of the razorbills includes boreal and subarctic coasts of the North Atlantic between 43 ° and 73 ° north latitude. The distribution area overlaps very strongly with that of the guillemot. Unlike this species , the razorbill occurs only in the North Atlantic. The main area of distribution is on Iceland, where about 65 to 70% of the world population breed. Another twenty percent breed in the British Isles and ten percent of the global population is spread over small colonies mainly in Canada and Norway . The southernmost occurrences are in Europe off the coast of northern France and in America near the state of Maine . In Germany there are some breeding pairs on Heligoland after the breeding population there initially died out in the 1960s. Razorbills, which belong to the subspecies Alca torda islandica , have been breeding there since the late 1980s . The birds that can be observed in winter, on the other hand, belong to the nominate form Alca torda torda. The nominate form has a very large breeding colony with up to a thousand breeding pairs on an island near Bornholm.
In winter, European birds migrate to the western Mediterranean and American birds to Newfoundland and New England . The migratory movements are therefore more pronounced than in other alken birds breeding in the Atlantic. They return to their breeding grounds in March.
Food and diet
The razorbills catch their food while diving underwater. Dense schools of diving razorbills are extremely rare. Unlike the other large alkenbirds, razorbills also look for food in estuaries with a reduced salinity. The diet consists mainly of fish such as herrings , sand eels and sprats , but also crustaceans and sea worms. In the North Sea, most of the foraging areas that the razorbills visit during the breeding season are close to the breeding colony. On Skomer , an island off the Welsh coast, razorbills find their food up to 15 kilometers away from their breeding site. The diving depth can be up to 120 meters, but mostly only reaches 40 meters. On average, razorbills dive for less than sixty seconds.
Common terns , herring gulls , little gulls, and kittiwakes can often be found near the diving razorbills while foraging. The kittiwake in particular seems to benefit from prey that is driven to the surface by the razorbills dives.
Reproduction
Social behavior
Razorbills return to their breeding colonies between late February and May, depending on their latitude. The loyalty to the breeding site is very high and amounts to 91.5%.
Razorbills often form large colonies with other species of the alkene family. First brooding razorbills are usually between four and six years old. Typically three-year-old razorbills appear near the colony at the end of the breeding season. Before the beginning of the breeding season, the number of birds within a colony fluctuates very strongly. Every four to six days almost all breeding pairs in the colony are present, then the number of breeding birds present drops to almost zero. Pronounced social behavior can be observed during this time both in water and on land. Social behavior includes, among other things, extensive clawing of the partner bird.
Nesting site and clutch
The razorbill breeds on the coast on cliffs or on flat beaches covered with boulders. Razorbills rarely build their nests on rock ledges. Usually the nest is in a rock cavity or niche. It is occasionally built between or under boulders, and razorbills sometimes use puffin nesting holes. In principle, however, the breeding success in covered and thus protected breeding sites is significantly higher than that in exposed areas. Material is usually not brought in, but there is occasionally a cluster of small stones around the nesting trough. The distance to the next nest is usually one bird's length.
The oviposition is strongly synchronized within a colony. 80% of all eggs are laid within a period of ten days. The time of oviposition is at the beginning of May in the south and beginning of June in the north of the distribution area. The female usually only lays a fairly large egg. It weighs around 90 grams, making it around 17% lighter than that of the guillemot. The egg is spindle-shaped to oblong oval, the basic color usually white, but occasionally also brownish or greenish. The eggs have a finely grained, roughened surface and have a very variable pattern of brown and black speckles, dots, blobs, scribbles and ribbons. The breeding period is very variable and lasts between 28 and 43 days, which is probably due to the different breeding intensity at the beginning of the breeding season as well as to weather influences. Both parent birds are involved in the incubation of the clutch and the care of the young birds. Brood detachment takes place after 12 to 24 hours.
Fledglings
Young birds of the razorbills are place stools. Shortly after hatching, they have short and dense downs with silky tips. The head and stomach are light grayish white to pale cinnamon. The top of the body and the flanks are brown-black. The sides of the neck, the goiter region and the wings appear brightened by the lighter dune tips. The large, black and laterally flattened beak is already striking in the nestlings. The throat is pale yellow. They are fed by the adult birds between two and five times a day. The parent birds typically bring several fish across their beaks at the same time. The daily amount of food that a single chick receives is between 20 and 22 grams.
Immediately after hatching, the nestlings weigh an average of 63 grams, they gain around 8.3 grams a day until they are 14 days old and then weigh around 180 grams. Your weight will then remain constant or even decrease slightly. The nestlings are able to regulate their body temperature themselves when they are nine to ten days old. In unprotected breeding grounds, however, they are guarded and flocked by a parent bird during their entire nestling period . In protected nesting locations, they are left alone by their parent birds for hours.
The young birds leave the nest after an average of 17 to 18 days. They then wear their second downy dress, the so-called mesoptile , which is made up of intermediate feathers with a thicker shaft and downy rays. They are already colored black and white and usually have a white throat. Tail and wing feathers are not yet developed, so that they are not yet capable of flight at the time of fledgling. The so-called alkene jump, in which the young birds jump from the breeding rock, usually takes place from late July to early August. They either land immediately on the sea or on the grass and scree slope below the breeding site. When jumping, they are usually accompanied by one of the parent birds. Usually it is the male who tries to slow down his flight to the fall speed of the young bird with outstretched wings, fanned tail and spread webbed webs. The adult birds then lead the chicks out to sea. The breeding success in Great Britain is 71 young birds per 100 breeding pairs. Studies in other regions have shown similar results. The predators of eggs and young birds include ravens , carrion crows , jackdaws and gulls. Out of 100 young birds of this type, only about 11 to 18 reach the fifth year of life. The mortality rate of adult razorbills is around 9%.
Existence and causes of danger
The global population is estimated at 0.6 to 1.0 million breeding pairs at the beginning of the 21st century. The European breeding population makes up about 430,000 to 770,000 breeding pairs. The largest populations are in Iceland and the UK. Herds of more than 10,000 breeding pairs also exist in Ireland, Norway and Sweden. The breeding colony on Heligoland went out for some time in the 1950s. There was a permanent resettlement from 1975. Between 2001 and 2003 between 12 and 17 breeding pairs brooded there.
In the past, as with many alken birds, the sources of danger included direct hunting by humans and collecting eggs. However, hunting is currently only of local importance. There are currently high losses mainly due to increasing oil pollution of the sea. Razorbills also drown in fishing nets and suffer from high levels of pollution.
Subspecies
Two subspecies can be distinguished based on their body size:
- Alca torda islandica is found in Iceland, the Faroe Islands, the British Isles and the North Sea. This subspecies therefore lives in slightly warmer waters and is slightly smaller than the nominate form.
- Alca torda torda represents the nominate form.
The sister group of the razorbill is the giant aalk, which was eradicated in historical times and which was also widespread in the North Atlantic.
literature
- Hans-Günther Bauer, Einhard Bezzel , Wolfgang Fiedler (Hrsg.): The compendium of birds of Central Europe. Everything about biology, endangerment and protection. Volume 1: Nonpasseriformes - non-sparrow birds. 2nd, completely revised edition. Aula-Verlag, Wiebelsheim 2005, ISBN 3-89104-647-2 .
- 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 .
- Collin Harrison, Peter Castell: Fledglings, Eggs and Nests of Birds in Europe, North Africa and the Middle East. 2nd, revised edition, German licensed edition. Aula-Verlag, Wiebelsheim 2004, ISBN 3-89104-685-5 .
- Renate Kostrzewa: The Alken of the North Atlantic. Comparative breeding ecology of a group of sea birds. Aula-Verlag, Wiesbaden 1998, ISBN 3-89104-619-7 .
Web links
- Alca torda in the endangered Red List species the IUCN 2008. Posted by: BirdLife International, 2008. Accessed January 31 of 2009.
- Videos, photos and sound recordings of Alca torda in the Internet Bird Collection
- Feathers of the razorbill
Single receipts
- ↑ a b c Bauer et al., P. 563
- ↑ IUCN Redlist on Razorbills, accessed on March 24, 2017
- ↑ a b c d Kostrzewa, p. 46
- ↑ Gaston et al., P. 126
- ↑ Gaston et al., P. 126 and p. 127
- ↑ a b Gaston et al., P. 129
- ↑ a b c d Kostrzewa, p. 49
- ↑ a b c Kostrzewa, p. 48
- ↑ Gaston et al., P. 130
- ↑ a b Harrison et al., P. 172
- ↑ Kostrzewa, p. 51
- ↑ Gaston et al., P. 131
- ↑ a b Bauer et al., P. 565
- ↑ a b Harrison et al., P. 173
- ↑ a b Kostrzewa, p. 50
- ↑ Gaston et al., P. 133
- ↑ a b Bauer et al., P. 564