Black axel pair
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Black- axed pair ( Elanus scriptus ) |
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| Gould , 1842 |
The black- axed pair ( Elanus scriptus ) is a bird of prey from the species of gliding-hairs . The species, which is restricted to the Australian continent, was first described scientifically in 1842 by the British ornithologist John Gould . The specific epithet scriptus comes from Latin and roughly means "written". It refers to a drawing on the underside of the wings that, when seen in flight, resembles the letter "M" or "W" in shape.
Description and behavior
When fully grown, black-axed hairs can reach a size of 34 to 37 cm with a wingspan of 84 to 89 cm. The females tend to be slightly larger and heavier, with weights ranging between 290 and 420 g. Male specimens are around 220 to 330 g. A further sexual dimorphism does not exist in the species, however, only the slightly lighter hood of the male can be used to distinguish the sexes optically. The plumage of adult birds is predominantly white, on the back and wings the color turns into a pale gray. On the underside there is a characteristic black line that extends from the armpits over the wings to the wrist and can be used to clearly differentiate between similar species. In flight, this drawing resembles the letter "M" or "W", depending on the attitude of the wings, which earned the species the common name Letter-winged kite , where letter means "letter" and winged means "winged". Another noticeable feature is a black ring around the eyes that widens to a spot towards the beak. The iris of the large, brown eyes is colored red. The physiology of the visual apparatus shows clear but incomplete adaptations to a nocturnal lifestyle. For example, the eyes are generally enlarged and have modified photoreceptors , as are typical for night hunters. However , there are still two visual pits on the retina , which is a clear sign of descent from a diurnal animal. The sharp, curved beak, like the nasal wax skin, has a grayish color. Overall, the face is reminiscent of that of a typical owl . The flight pattern outside of a hunting situation corresponds roughly to that of a typical consecration with calm and rather slow strokes . With their wings extended and raised above their bodies, black-axed hairs can cover long distances in gliding flight. Confusion with the closely related Australian gliding hair ( Elanus axillaris ) can occur, whereby this lacks the easily recognizable wing pattern of the black axillary hair . At night, the species also superficially resembles the barn owls ( Tyto alba ) and eastern grass owls ( Tyto longimembris ), which are then also active , but on closer inspection it can be distinguished relatively easily on the basis of the less rounded head shape, the non-rounded wing tips and less long legs.
The chicks of Schwarzachselaars are after hatching, first of creamy white, soft down covers that after the first moult adopt a more gray-brown color. The boys are then clearly darker in color, especially on the head. Juvenile birds are still predominantly brown in color, especially on the head, chest and back and only gradually take on the lighter, white color of their parents.
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Black-axed hairs are sociable birds that are mainly active at night and spend most of the daytime resting on trees with sufficient foliage to protect them from being discovered. In some cases, large groups of up to 400 individuals can form, which only become more active at dusk and search for food. During the rest phases, however, the birds are easy to alert; if humans come too close, they react quickly with flight and disperse. Due to this susceptibility to disturbances, the social behavior of the species has so far been little researched. In times of severe food shortages, the birds sometimes show unusual behavior, such as gathering in groups on the ground or hunting flights during the day.
Vocalizations
Individual black-axed hairs are considered to be less vocal, but if they gather to form breeding or resting colonies, they can reach a considerable volume. Couples usually contact each other with short, “scratchy” calls that are repeated about six to seven times. This call is particularly often used by brooding females as the male approaches the nest. This rather emits a loud whistle, which is also used by both sexes as an alarm call.
nutrition
Although it is able to hunt successfully during the day, the black- axed pair has specialized in hunting nocturnal rodents, especially the Australian longhair rat ( Rattus villosissimus ). If these are not available in sufficient quantities, other small mammals, reptiles or insects are also accepted. There are also reports of the ingestion of carrion in the form of dead mammals; a behavior that apparently only comes to light in times of need. The hunt mainly takes place in the first two hours after sunset. The birds move in a circle over their hunting area at an altitude of around ten to 20 meters and search the ground for potential prey. If this has been spotted, the birds push down on it with their wings stretched over their bodies and feet first and finally grab the prey with their claws. Black-axed hairs rely on their eyesight to locate their prey and, unlike many owls, rely on the light of the moon to make them out. Because of this restriction of birds, a noticeably reduced activity can be observed in Australian longhair rats when the moon is visible in the sky.
Reproduction
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The breeding areas of the species are around Cooper Creek in Central Australia, which only occasionally has water . The breeding season is highly dependent on the availability of their main food source, the Australian longhair rat. In years in which the rats are present in sufficient quantities, it usually extends quite predictably from spring to autumn, but is sometimes completely absent in years when the rat population is too small. The black-axed pair mostly forms loose breeding colonies of up to 50 - according to some sources even up to 100 - pairs; solitary nests away from a colony are more rarely reported. The nest is built in the branches of a tree at a height of two to 14 meters above the ground. It consists of twigs and grasses that are woven into a platform that is then lined with leaves or cattle dung. During courtship, both birds circle over the designated nesting site. The male rises significantly higher and then drops down to the same level as the female. Afterwards, both partners circle each other with loud shouts, and food is often given to the female.
After mating, the female usually lays four to five, more rarely three or six eggs. The time until the young hatch is an average of 31 days. The young birds are then looked after in the nest for another 30 to 35 days, until they finally fledge and leave the nest. As long as this is still in the nest, the offspring are huddled by the female, while the male adult bird brings food to them at night. Here he approaches the nest with loud shouts, whereupon the female flies up, takes the prey in the air and then returns to the nest, where she feeds it to the offspring. In times of abundance of food, female black ax hairs leave their first brood after a certain period of time and start a second brood. In this case, the males alone take care of the older offspring. In the opposite case, however, when food sources suddenly disappear, it can also happen that the first brood is given up prematurely.
After they have learned to fly, the young birds are only dependent on parental care for a short time. They reach sexual maturity after about a year, which can lead to rapidly growing breeding colonies if the food conditions continue to be good.
Spread and endangerment
The Schwarzachselaar is an endemic inhabitant of Australia. The core of its range is in eastern central Australia and is particularly concentrated on the grassland area of Barkly Tableland in the Northern Territory and the dry Channel Country , which largely belongs to Queensland . As a result of heavy rains and the regularly associated, explosive reproduction of the Australian longhair rats, there is also a considerable increase in the population of the black axillae. In these times of abundance of food, the range of the birds extends over large parts of the continent, only the driest regions are avoided. They then actually colonize atypical habitats such as open coastal areas or islands. Young birds between the ages of one and two years are mainly involved in these seasonal expansions of the range, the majority of which perish when food availability is reduced to a normal level. Survivors, mostly older animals, then withdraw into the core range until another rapid increase in the rat population causes this cycle to start again. Due to these strong, natural population fluctuations, it is not easy to make reliable statements about the threat status of the species. The IUCN lists the Schwarzachselaar as near threatend as of 2016 , so it does not rule out a future threat to the conservation of the species. Nevertheless, the organization assumes a relatively stable core population of a minimum of 670 and a maximum of 6700 adult individuals. There are hardly any natural predators, only soot hawks are known to hunt adult black axillary hairs , while nestlings are occasionally preyed on by black kites . However, feral domestic cats introduced into Australia by humans are serious food competitors and may also loot the birds' clutches. In addition, the expansion of pasture farming and the associated degradation of its habitat represent a threat to the species. Secondary poisoning through ingestion of prey contaminated with rat poison is not uncommon. Furthermore, persistent periods of drought, which are expected more frequently in the future due to the consequences of climate change, cause a decline in the number of the Australian longhair rat and thus also have a negative effect on the black-axed hair population.
Systematics
John Gould first described the Schwarzachselaar in an article in the Proceedings of the Zoological Society of London in 1842 . The scientific name given by Gould Elanus scriptus has been retained until today. The species is considered to be monotypical and there are also no geographical variations. The black- axed pair shares its distribution area with another member of the genus Elanus , the very similar Australian gliding hair.
literature
- Stephen Debus: Birds of Prey of Australia: A Field Guide . CSIRO Publishing, Clayton South 2019, ISBN 978-1-4863-1111-8 , pp. 18-19, 114-116 .
- Stephen Marchant, Peter Jeffrey Higgins: Raptors to Lapwings . In: Handbook of Australian, New Zealand and Antarctic Birds . tape 2 . Oxford University Press, Melbourne 1993, ISBN 0-19-553069-1 , pp. 45-52 .
Web links
Individual evidence
- ^ Penny Olsen: Australian Predators of the Sky . Ed .: National Library of Australia. NLA Publishing, Canberra 2015, ISBN 978-0-642-27856-2 , pp. 41 .
- ^ A b Penny Olsen, Leo Joseph: Stray Feathers: Reflections on the Structure, Behavior and Evolution of Birds . CSIRO Publishing, Collingwood 2011, ISBN 978-0-643-09493-2 , pp. 39-40 .
- ↑ Debus, p. 18
- ↑ Marchant & Higgins, p. 46
- ↑ a b c d Marchant & Higgins, p. 49
- ↑ a b c d e f Debus, p. 115
- ↑ a b Marchant & Higgins, p. 48
- ↑ Letter-winged kite. In: birdlife.org.au. BirdLife Australia, accessed December 17, 2019 .
- ↑ a b Marchant & Higgins, p. 51
- ↑ Debus, pp. 114–115
- ↑ Letter-winged Kite Elanus scriptus. In: iucnredlist.org. BirdLife International, 2016, accessed December 13, 2019 .
- ^ Debus, p. 116
- ^ John Gould: Elanus scriptus . In: Proceedings of the Zoological Society of London . tape 9 , 1842, pp. 80 .
- ↑ Marchant & Higgins, p. 52