Skuas
| Skuas | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
Arctic Skua ( Stercorarius parasiticus ) |
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| Systematics | ||||||||||
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| Scientific name of the family | ||||||||||
| Stercorariidae | ||||||||||
| GR Gray , 1870 | ||||||||||
| Scientific name of the genus | ||||||||||
| Stercorarius | ||||||||||
| Brisson , 1760 |
The skuas (Stercorariidae) are a bird family in the order of the plover-like birds . They are relatives of the seagulls , which are mainly native to polar regions. They have a wide range of food, are active as bird, lemming or fish hunters or hunt other sea birds for prey.
features
Skuas are large birds with a seagull-like shape. They have a strong beak, the tip of which is curved. The stocky legs end in feet with fully developed webbed feet and sharp claws. The wings are long, narrow and pointed at the end. They enable agile and fast flight, which exceeds that of seagulls in speed and acceleration. The darker plumage, which may serve as camouflage when sneaking up on prey, is also striking in comparison with the seagulls. As a rule, the large skuas are monochrome brown, the small ones are dark gray with lighter chest and head areas. However, the polymorphism is pronounced in almost all species , that is, within a species there are dark and light morphs . These are not subspecies as they are not geographically separated from each other. However, the frequency of morphs changes with geographical distribution; As a rule, the light morphs are more common in the high Arctic regions, the dark ones are more common towards the south. The dark morph seems to be preferred by sexual selection and has a higher breeding success. The dense plumage of the skuas offers effective protection from the cold, but has the disadvantage that excess heat cannot be dissipated easily.
Males and females do not differ in their plumage color, but they do differ in size and weight. On average, females are three percent larger and twelve percent heavier than males. Likewise, males have proportionally longer tails. In some species the middle tail feathers are considerably elongated; this is also more pronounced in the males.
distribution and habitat
Skuas breed in polar and temperate cold regions. Four species are native to the northern hemisphere, three in the southern hemisphere. The breeding areas are along the coasts and on small islands, with the small skuas also far away from the coasts in the tundra . Often the proximity of other bird colonies is sought. A grassy subsoil is preferred, but if there is no vegetation, stony ground is also accepted.
Outside of the breeding season, skuas scatter, while some stay near the coast, others become pronounced deep-sea birds of the open oceans. Significant distances are often covered, and four of the seven species overwinter in the other hemisphere. On such long-distance trains, skuas are sometimes taken far from their routes by storms, so that the great skua has already been sighted in Switzerland and Austria. Not fully grown birds sometimes stay in their wintering area and only return to the breeding grounds a year later.
Among the skuas we meet two remarkable bird world records: The Antarctic skua breeds closer to the South Pole than any other vertebrate. The bird with the furthest known migration route belongs to the same species: ringed on the Antarctic Peninsula , the same bird was later found in Greenland north of the Arctic Circle.
Way of life
activity
Skuas mostly live solitary. Groups can form during the breeding season, but this is more due to a lack of space and not to social interaction between the pairs. Even if several skuas chase a prey, they act as competitors and not together. Unusually for a high-sea bird, skuas are often seen in small freshwater pools and puddles where they bathe and drink. Drinking fresh water consumes less energy than excreting salt after ingesting sea water, and bathing is particularly observed on warm days when the birds have problems with their heat balance.
nutrition
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Skuas are opportunists with a very wide range of food. They are probably best known for kleptoparasitism , but they are also active as fish hunters, egg thieves, scavengers and herbivores and prey on young and adult birds.
Kleptoparasitism is the act of stealing food from other birds. These are attacked so that they drop or strangle their prey. The smaller skuas in the northern hemisphere attack mainly gulls, terns and puffins , while the great skuas also attack gannets and guillemots . The attacks are carried out either by chases or by surprise attacks in a dive.
Often the seabirds are also victims of the skuas themselves. The great skua prey on kittiwakes , puffins and petrels , which are often pushed under the water surface and drowned. On the island of Foula to skuas specialized, black guillemots waylay when leaving their nests and kill which resulted in the destruction of an entire colony. The colonies of terns were visited to feed the young birds, that of fulmars to feed their eggs. The Spatula sku feeds a particularly high proportion of captured birds, for example water treaders . The Shetland Islands and was cannibalism observed here the young of other skuas pairs were eaten. In the southern hemisphere, penguins are also among the victims of skuas. For the Sub-Antarctic skua penguin pups and eggs make up an important part of the food spectrum, for the Antarctic skua only complementary food in times of food shortages.
In addition to birds, fish are the main prey of the skuas. This is especially the case with the Antarctic skua, where the Antarctic silverfish is the main food. The northern species are also at least partially active as fish hunters, for example sand eels , whiting and haddock have been identified as prey fish for the great sku . The species that breed in the interior of the tundra are mainly prey for small mammals such as lemmings , but also for insects, berries, eggs and carrion. Large skuas occasionally eat the afterbirths of sheep or even attack newborn lambs.
Reproduction
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Skuas mostly live in monogamy for life . Exceptions are the Spatula skua, which does not breed locally like the other species and is therefore seasonally monogamous, and the Subantarctic skua, in which one female partners with two or more males ( polyandry ); these groups can remain stable for years, and each male takes care of all the young, including those of the other males. The monogamous species only look for a new partner after losing the old one. Occasionally a bird tries to drive away a placeholder by attacking and fighting; these fights often result in serious injuries.
Where skuas breed in colonies, the older birds have nesting places in the middle, younger and inexperienced ones have to be content with places on the edge, where the brood is mostly unsuccessful. As the years go by, they gradually advance towards the middle. If the ground allows, a shallow nesting trough is scraped into the ground. Usually two eggs are laid, less often one egg. An extremely rare but occasionally observed exception are three eggs, which are then all incubated unsuccessfully because they do not receive enough heat. When brooding, the eggs are placed on the feet and warmed on top by the plumage. The young bird that hatches first attacks the second-born and tries to compete for food. While in the Antarctic kua this always leads to the death of the second born, in the other species the younger one comes through at least occasionally. After a day or two, young leave the nest and wander around. It happens both that lost cubs are adopted by other couples, or that these are killed and eaten.
Ninety percent of the breeding skuas return to their breeding site the following year. Skuas are relatively long-lived. The record is held by a ringed great skua on the Shetland Islands, which has been proven to be 34 years old.
Systematics
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Usually, skuas are treated as a separate family of Stercorariidae. In the Sibley and Monroe system , they were a tribe within the Laridae family, which also included the actual gulls and terns . Skuas are an undoubtedly monophyletic taxon that is likely to have a sister group relationship with the gulls. According to the molecular clock, the separation from the seagulls may have occurred in the Miocene .
Traditionally, the skuas were divided into two genera, which differ significantly in shape: the large, massive and brown skuas of the genus Catharacta , and the small, slender and mostly gray-white skuas of the genus Stercorarius . In German only the former are referred to as "Skuas"; however, this distinction is not common in English, where all skuas are called "skuas". Over time, however, it turned out that the Spatula sku, a typical Stercorarius representative, showed much greater similarities with the Catharacta species in terms of skeletal and behavioral characteristics than with other Stercorarius species. DNA analyzes and examinations of the parasites also confirmed this finding. Possible explanations for this are hybridization between large and small skuas or convergent evolution . The classic genus Stercorarius is paraphyletic with respect to Catharacta , with the Spatula skua being in a sister group relationship to the great skuas. As a consequence, most of the time all skuas are now listed in a single genus Stercorarius .
- Subantarctic skua ( Stercorarius antarcticus ; Syn . : Catharacta antarctica )
- Chileskua ( Stercorarius chilensis ; Syn .: Catharacta chilensis )
- Hawk sku ( Stercorarius longicaudus )
- Antarctic skua ( Stercorarius maccormicki ; Syn .: Catharacta maccormicki )
- Arctic Skua ( Stercorarius parasiticus )
- Spatula skua ( Stercorarius pomarinus )
- Great skua ( Stercorarius skua ; Syn .: Catharacta skua )
Carl von Linné described the skua as Larus parasiticus - although the epithet parasiticus belongs to the arctic skua today, he probably described the hawk skua. Because of the many morphs, a large number of species were subsequently described; 23 scientific names existed in the 19th century for the Arctic Skua alone. The generic name Catharacta was also mistakenly assigned because Aristotle probably used it to describe the gannet .
Skuas and humans
Eggs of skuas have been collected for centuries in northern regions such as Iceland, the Faroe Islands, and Shetland Islands. For a long time, this did not happen within a framework that would have endangered stocks. This changed in the 19th century when hobby hunters came to the islands and wiped out entire colonies. This hunt resulted in only four breeding pairs of great skuas left on the Faroe Islands by 1900. Since then, however, protective measures have ensured that stocks have recovered. Accordingly, none of the seven species is globally endangered.
supporting documents
literature
- Josep del Hoyo et al .: Handbook of the Birds of the World (HBW). Volume 3: Hoatzin to Auks. Lynx Edicions, Barcelona 1996, ISBN 8487334202 .
Web links
Single receipts
- ↑ a b del Hoyo et al .: HBW Volume 3, Morphological Aspects , pp. 557-558, see literature
- ↑ a b c del Hoyo et al .: HBW Volume 3, Habitat , pp. 558-560, see literature
- ↑ a b del Hoyo et al .: HBW Volume 3, Movements , p. 565, see literature
- ↑ del Hoyo et al .: HBW Volume 3, General Habits , pp. 560-561, see literature
- ↑ a b c del Hoyo et al .: HBW Volume 3, Food and Feeding , p. 562, see literature
- ↑ a b c del Hoyo et al .: HBW Volume 3, Breeding , pp. 562-565, see literature
- ↑ Charles Sibley, Jon Ahlquist, Burt Monroe: A classification of the living birds of the world based on DNA-DNA hybridization . In: The Auk 1988, No. 3, pp. 409-423
- ^ A b Philip C. Chu, Sarah K. Eisenschenk, Shao-Tong Zhu: Skeletal morphology and the phylogeny of skuas (Aves: Charadriiformes, Stercorariidae) . In: Zoological Journal of the Linnean Society 2009, Vol. 157, No. 3, pp. 612-621
- ↑ del Hoyo et al .: HBW Volume 3, Systematics , pp. 556-557, see literature
- ↑ BL Cohen et al .: Enigmatic phylogeny of skuas (Aves: Stercorariidae) . In: Proceedings of the Royal Society B 1997, Vol. 264, No. 1379
- ↑ a b del Hoyo et al .: HBW Volume 3, Relationship with man , pp. 565-566, see literature
- ↑ IUCN Red List of Threatened Species , accessed December 7, 2017.