Darksawers
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Darksawer ( Mergus octosetaceus ) |
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| Scientific name | ||||||||||||
| Mergus octosetaceus | ||||||||||||
| Vieillot , 1817 |
The Darksawer ( Mergus octosetaceus ) is a species from the duck family and is one of the sawsmen . The species occurs in South America and is one of the rarest duck birds in the world. Presumably the world population is less than 250 specimens.
Appearance
The dark saw reaches a body length of 49 to 51 centimeters and weighs between 600 and 700 grams. Overall, it is a bit smaller than the Eurasian middle saw . In its area of distribution, the darksaw can only be confused with the cormorant species Phalacrocorax olivaceous , as both have a similar dark plumage and are of the same height. The dark saw, however, has the characteristic saw shape. It flies very quickly and usually has a low altitude. In flight, the white wing mirrors that are missing in the cormorants are noticeable. The two sexes are largely the same. The females are only slightly smaller and their feather bonnets are less pronounced.
Darksawers wear a year-round dress. The head, neck and torso have dark brown plumage that shimmers slightly green. The individual feathers hang down to the shoulders. The chest and the lower part of the body are pale brown and have very fine gray markings. The beak is black, the legs are red, and the iris is brown. In juveniles, the breast is gray and the feather bonnet is missing. The cheeks and neck are whitish. They also have a white eye ring. The upper bill is dark in them, while the lower bill is reddish. Chicks are black on the upper side of the body and have white spots on the flanks, trunk and feathers. The face shows the black and white markings typical of Säger. However, the red-brown color transitions that are typical for sawdaw chicks are missing. The beak is black. Feet and legs are dark gray, the webbed feet are black. The iris is gray.
Distribution and existence
The Darksaw is an endemic species of tropical and subtropical South America. Its original distribution area includes the south and the middle of Brazil , the northeast of Argentina and parts of Paraguay . Today the species only occurs in the extreme north of Argentina and in some places in Brazil. In Argentina, however, despite an intensive search, only a single dark sawsaw has been sighted since 1992 and it seems that this species is now extinct there. The distribution area is always sparsely populated, as breeding pairs are territorial and occupy an area that covers an average of nine kilometers of a river. The species was considered extinct by the end of the first half of the 20th century. In 1947 it was rediscovered by chance. An Argentine research team therefore undertook corresponding investigations in 1948 and 1954, which showed that the species was nowhere common, but that the middle and upper reaches of the eastern reaches of the Río Paraná were inhabited by dark saws.
Overall, the number of individuals is probably less than 250, even if the population size is very difficult to determine. The Darksaw is under protection in both Brazil and Argentina. In Argentina the population size is very small. The species was observed there at Arroyo Uruzú in 2002 after no dark saws had been detected in Argentina in the previous 10 years. Extinction here seems inevitable. In Brazil, the Darksaw is found in three national parks. The Serra da Canastra National Park is one of the most important distribution areas , where six breeding pairs were counted in 1996. In 2002 the number of birds there was 81. Another population was discovered on the Rio São Francisco in the west of Bahia . However, this area is not under protection. The species was last observed in Paraguay in 1984; now there are no longer any suitable habitats for them.
The main reason for the decline in stocks is presumably a clouding of the water due to the extraction of raw materials on the upper reaches of the river. The deforestation of the gallery forests and logging in the forests also contributes to the fact that the habitat of the dark sawsman changes in such a way that it no longer offers them suitable living conditions. Hydroelectric power plants are another reason for the decline in this species. In the Serra da Canastra National Park there is also a risk of diamond mining.
habitat
The Dark Saw is an inhabitant of clear rivers that flow through subtropical forest and grassland with gallery forest . Observations suggest that it prefers to colonize the upper reaches . The height distribution ranges from 200 meters to 1400 meters above sea level. The rivers that he inhabits meander and have rapids and small waterfalls that alternate with barrages and calm pools.
Like many ducks that breed on riverbanks, the darksaw is distinctly territorial. According to previous knowledge, a single couple seems to claim at least eight kilometers of riverbank as a territory. Depending on the exact nature, some couples even occupy areas with a shore length of 14 kilometers. The size of the territory appears to be related to the number of waterfalls, rapids and the general speed of water flow. There are also speculations that the Darksawer is particularly present in rivers because there are fewer large fish species that can be dangerous to the chicks.
Food and diet
The darksaw feeds almost exclusively on fish. He looks for these mainly in rapids. In deeper water it also dives for food and then stays under water for between nine and 27 seconds. On the basis of stomach analyzes carried out in the 1950s, it mainly eats fish that are six to 19 centimeters long. In contrast to this, one individual in this study had predominantly fly larvae in the stomach and, to a small extent, snail larvae as well. This is taken as an indication that this species is at least temporarily dependent on invertebrates as its main food source.
The search for food takes place mainly in the morning and in the evening. For most of the rest of his time, the Darksaw sits in the shade of rocks, trunks, or branches. Due to the dark plumage, the Darksaw is difficult to make out against the flowing water.
Reproduction
Courtship
So far, only insufficient data are available on the reproductive behavior of the dark slayer. Chases on the water have been observed in which one Darksawer followed another. The birds swam in a circle and with violent wing movements. This is very likely part of courtship behavior. A copulation between two dark saws was also observed, in which both sexes moved their heads up and down violently beforehand. During the actual act, the male Darksaw bit into the female's hood.
The couple bond lasts for a long time. The pairs stay in their breeding ground all year round and are very territorial. Occasionally there are fights with pairs of dark saws whose territory is adjacent.
Breeding season and rearing of the chicks
The breeding season begins in June. The breeding season falls in July and chicks can be seen from late July to August. The only nest described so far was in a tree hollow 25 meters above the ground. The nesting material consisted of only fine, rotting wood. While the female was breeding, the male stayed nearby by the river. The female only interrupted incubation once a day and then left the nest for one to 1.5 hours. Most of the time she spent eating and cleaning the plumage. The shell remains found later were light cream-colored.
The clutch size and incubation period are unknown. Four, five and six chicks were counted in the family associations observed. The male is involved in guiding the chicks. The chicks eat water insects, which they pick from the surface of the water. In rapids they have been seen looking for food with their heads submerged. The predators include the also threatened magpie eagle , the South American otter ( Lontra longicaudis ) and very likely Salminus maxillosus , a South American species of fish from the family of the true tetra .
Under ideal conditions, the reproductive success can be very high. In the Serra da Canastra National Park, four breeding pairs have demonstrably raised a total of seventy young birds in just five years.
Keeping in human care
An attempt at breeding in human care was made at the beginning of the 20th century. However, it was not possible to acclimate the captured adult birds. Ornithologists are urgently calling for a breeding program for this species to prevent it from becoming extinct.
supporting documents
literature
- Dominic Couzens : Rare Birds - Survivors, Evolution Losers and the Lost. Haupt Verlag, Bern 2011, ISBN 978-3-258-07629-4 .
- Janet Kear (Ed.): Ducks, Geese and Swans. Oxford University Press, 2005, ISBN 0-19-854645-9 .
- Hartmut Kolbe; Die Entenvögel der Welt , Ulmer Verlag 1999, ISBN 3-8001-7442-1
Individual evidence
- ↑ a b Kear, p. 749
- ↑ a b c Kolbe, p. 317
- ↑ Couzon, p. 133
- ↑ Mergus octosetaceus on the IUCN Red List of Threatened Species . Retrieved January 24, 2009.
- ↑ Couzon, p. 130
- ↑ Couzon, p. 130
- ^ Couzon, p. 131
- ↑ a b c Kear, p. 751
- ↑ a b Kear, p. 750
- ↑ Couzon, pp. 132-133
- ↑ Couzon, p. 132
- ↑ Couzon, p. 132