Great gray owl

from Wikipedia, the free encyclopedia
Great gray owl
Great gray owl (Strix nebulosa nebulosa)

Great gray owl ( Strix nebulosa nebulosa )

Systematics
Class : Birds (aves)
Order : Owls (Strigiformes)
Family : Real owls (Strigidae)
Genre : Strix
Type : Great gray owl
Scientific name
Strix nebulosa
Forster , 1772

The Bartkauz ( Strix nebulosa ) is a bird art from the genus Strix within the family of authentics owl (Strigidae). It occurs in two subspecies in the boreal zone of the Holarctic . Its German name is derived from a black plumage region under the beak that looks like a small beard. In many other European languages, the great gray owl is called the Lapland owl after its European distribution area . Its English name is Great Gray Owl , so Big Graueule . It is the largest species in the genusStrix and the only one with a Holarctic distribution area. Also within the family Strigidae the great gray owl is one of the largest, but not one of the heaviest species.

features

Appearance

The great gray owl is a strikingly large, round-headed and long-tailed owl. In the overall impression, it appears gray-brown with little contrast. As in all other species of this genus, feather ears are not developed. If the observation conditions are sufficient, the species cannot be confused with any other owl.

The top is on a whitish, gray-white, sometimes slightly cream-colored background, brownish, longitudinally striped and finely brownish banded or speckled. The head and neck plumage is finer and more densely patterned and looks like a sweater. The upper tail-coverts are only banded, the longitudinal lines are missing in this area.

Head portrait of a great gray owl
S. n. Lapponica

The face veil is approximately circular, gray-white and clearly zonally structured with up to nine concentric dark-gray rings. On the outside it is separated from the rest of the head plumage by a dark brownish speckled feather edge. Two distinctive crescent-shaped, white feather edges partially surround the relatively small yellow eyes and widen to the side of the yellow beak. They form a striking mark in the form of an X, which is additionally emphasized by a fine black median line above the beak and by the black beard-like feather region below the beak. The lower border of the face veil is formed by a white feather border of different widths. The chest and abdomen are often a bit lighter than the top and also striped black-brown lengthways. The transverse ligament is particularly evident in the chest area. The broad, rounded wings and the control springs are lightly banded on a dark brown background. The under wing coverts are very light except for the dark shaft lines, sometimes almost white. The dense, almost furry-looking gray leg and toe feathering shows indistinct brown markings. The not very strong claws are dark brown-gray. The sexes do not differ in color. Young birds can usually be recognized by the remains of the intermediate plumage in the head and neck area. The two subspecies differ quite clearly: in the nearctic bearded owl the transverse banding predominates, in the palearctic owls the longitudinal stripes. However, especially in East Asia, individuals of the Nearctic type are not uncommon.

The flight pattern is characterized by the very large, board-like, deep-fingered wings and the long tail. Despite its size and wingspan, the great gray owl is able to maneuver neatly in dense trees. It flies with soft, slow, little sweeping wing beats and glides over longer distances with slightly upwards held wings angled in the wing bend . In gliding the ends of the are primaries significantly bent upward.

Biometric data

Bearded owls grow up to 67 centimeters long and weigh up to 1900 grams, so despite their comparable size, they only reach about half the weight of an eagle owl ( Bubo bubo ), which in large, heavy females can be over 4000 grams. The reverse sex dimorphism is quite evident in this species, especially in relation to weight; Males are on average 300 grams lighter, but less than 3 centimeters smaller than females. In Finland, the average weight of male bearded owls when they were 64.6 centimeters in length was 884 grams, but females weighed 1186 grams and were 67 centimeters tall. Individuals of the Nearctic nominate form appear to be slightly larger and heavier than Palearctic bearded owls, but the data available so far are not representative.

Acoustic expressions

The call repertoire of the great gray owl is made up of a few basic elements, which, however, are individually varied so that there is an abundance of different vocalizations. The male's territorial song is a dull, hollow sounding (B) muu ... that is perceptible up to a maximum of 400–500 meters away and which is usually strung with 8–10 syllables. The singing starts softly, increases in volume and tempo and fades away towards the end of the verse. The intervals are about 0.6 seconds. The highest singing intensity is in the evenings and at night. Similarly, but significantly faster, the male's contact calls to which the female replies in a throaty-rough way, for example chro… chro… chro . In addition, other calls that have been modified from the basic pattern, such as the monotonous, machine- gun- like nesting sound, can be heard. A soft, stretched mooing serves as a universal vocal feeling sound , which is also modified in a variety of ways and adapted to the situation, so that it can hardly be transcribed. How many species of owls scarce Bartkäuze, especially in aggression or defense reactions, persistent and loud with the beak. Wing claps can be heard occasionally during courtship .

Mauser

Only a few specific information is available on moulting of this species.

Chicks carry the thinning egg dunes until about the fifth day, after growing Halbdunen the intermediate plumage ( Mesoptil ) and to reach the flying, the Grand plumage developed. In the inter-moult after becoming independent, which is completed by the 5th month of life at the latest, the immature bearded owls moult into their first adult plumage: The half-downs of the mesoptil plumage are largely lost, but the first large plumage is not changed. In the first annual moult, which begins in late spring of the following year and ends in autumn, parts of the small and large plumage are changed, as in the subsequent annual moults.

distribution

Distribution area of ​​the great gray owl. Dark green: breeding areas; light green: known invasion areas

The great gray owl is a resident of the boreal coniferous forest belt of the entire Holarctic . He is a character bird of the central boreal zone; Both north and south of it, the settlement density decreases sharply.

The northern limit of its distribution lies in the transition area between the tall boreal coniferous forest and the forest tundra . In the Palearctic this is a little north of the Arctic Circle , in the Nearctic it is only crossed in northern Alaska ; to the east this vegetation boundary runs southeast to about the south coast of Hudson Bay .

The Palearctic deposits extend from central Sweden and Finland in a belt of varying widths to the catchment area of ​​the Anadyr and south-east to Sakhalin . The southern border is inconsistent, as the species also penetrates into boreal mountain forests of the Central Asian mountains. In Europe, the southern limit of distribution is in the northern Baltic . Isolated and probably only temporary occurrences exist in the Polish-Belarusian border area, in Belarus and in northern Ukraine .

In the Nearctic , the main area of ​​distribution is also in the central boreal belt. The breeding areas range from central Alaska southeast to central Ontario , possibly to southeast Québec . In the west, the species penetrates far south in the Rocky Mountains and reaches northern California and Nevada in the Cascade Mountains and the Sierra Nevada .

No information is available on the vertical distribution of the Palearctic populations. In North America, the highest evidence is at 2800 meters.

The great gray owl does not occur naturally in Germany. The only specimen that has been reliably proven on formerly German territory was shot on March 10, 1832 in East Prussia, prepared and taken to the Museum für Naturkunde in Berlin, where it is still located today.

hikes

The great gray owl's willingness to migrate is pronounced. The factors that trigger migratory movements of this type are controversial in contrast to owls that also largely roam, such as the snowy owl or the hawk owl . Recent studies do not consider the lack of prey animals after a successful year of reproduction to be at least the most important trigger of the nomadic behavior of this species, although it may play a role. Young birds try to settle again in close proximity to their place of birth. 51 Bearded owls ringed when they were nestling in Sweden were checked again or found again on average only 37 km from their place of birth. In years of invasion , large numbers of bearded owls can be found in regions where they have not been seen for a long time. Some then stay in these areas and breed there too. The distances covered are considerable and in individual cases can be 900 km. In addition to these distance migrations, bearded owls migrate vertically both from the higher altitudes down into the valley and, if the snow cover in the valleys is too high, to higher areas where wind drifts or erosions on southern slopes have created areas free of snow.

habitat

Strix nebulosa nebulosa in a natural winter habitat near Ottawa

In general, bearded owls nest in old stands of spruce and pine in the Palearctic, and occasionally in birch trees and other tree communities. Dense forest stands are not avoided, but open areas without dense undergrowth such as cleared areas, clearcuts , windthrow or fire areas must be available. Forest areas bordering bogs or heather areas can offer the great gray owl just as good living conditions as light, often overgrown larch stands with swamp porst or peat moss . The species also colonizes similar forest structures in the distribution areas in the USA. There it is mainly stocks of Douglas fir and coastal pine bordering mountain meadows that are preferred as territories. Overall, the great gray owl is quite flexible with regard to the tree species composition of its breeding ground. The availability of nesting opportunities and an adequate supply of food are essential for a brood settlement.

The winter territories partly correspond to the breeding grounds; during the winter months this owl often seeks out more open landscapes with only individual tall trees. Areas with low snow cover are preferred. In winter, the great gray owl often appears in man-made landscape structures, such as in the vicinity of farms, on the edge of settlements or on golf courses.

No precise studies are available on the space required by the species. In good mouse years, couples can breed in close proximity to one another without any aggressive behavior being observed. An unmated male claimed an area of ​​800 meters in diameter. Mikkola was one in which he examined about 100 square kilometers of territory in Oulu in Gradationsjahr 1970 eight nests, one of nine nests almost the same number in a likewise 100 square kilometer study area identified a Swedish study for the year 1973. Jagende Bartkäuze were during the breeding season more Found kilometers from the nest.

Food and loot

food

Despite its size, the great gray owl feeds almost exclusively on small mammals. Above all, these are field mice , which can account for up to 90% of the total hive weight. In Europe this is mainly the earth vole , in North America the meadow vole . In addition, other vole species such as the bank vole and the swamp mouse play a certain role, in North America also pocket rats and various types of lemmings . Shrew species are more common in winter. Overall, in an extensive study of the great gray owl's diet, in which 5177 prey animals could be identified, 98.4% of the prey animals were small mammals. The largest regularly beaten animals are muskrats , brown rats and squirrels . Bull & Duncan list relatively heavy species such as snowshoe hares and pine grouse on their list of prey . Such statements are doubted by both Mikkola and Mebs & Scherzinger. All other prey animals play no role in terms of weight. Remains of birds, especially thrushes and tits , are regularly found in the great gray owl . The largest prey among the birds are the jay and the hazel grouse . Occasionally, the remains of amphibians and insects were found in the ridges.

Loot acquisition

The earth vole is by far the most important prey for European bearded owls.
Hunting great gray owl immediately after taking off from the hide

The hunting methods of the great owl are essentially the same as those of other great owls. He is primarily a high seat fighter and - to a lesser extent - search aircraft fighter. Bearded owls look for the surroundings visually or acoustically from an elevated, usually not too high-lying control room, the optimal hunting distance being in a range of less than 50 meters. According to Mikkola, at least two thirds of the prey flights are unsuccessful.

The hearing of this species seems to be exceptionally highly specialized for owls , similar to that of the rough-owl and the Ural owl. The asymmetry of the ear canals found in most owls is not limited to the soft tissues, but continues in the skull: the left ear opening is lower and is noticeably larger than the right. In this way, the owl is likely to be able to locate visually imperceptible prey very precisely acoustically and to hit them successfully. If an animal is discovered, the owl drops in a steep flight from the control room and in the last section falls almost vertically on the prey. The relatively long legs are stretched out, the toes spread. Bearded owls can still hunt successfully in 30 centimeters of snow and are also able to break through harsh, crusted snow surfaces. When the snow cover is high, they shake their prey and sometimes plunge head first into the snow. Bearded owls kill their prey by biting the head or neck; smaller ones are swallowed whole on the spot, larger ones carried in the beak to a feeding place and cut up there. In search flight hunting, which is rarely practiced, the owl flies its hunting area in a slow, low flight interrupted by gliding distances, pauses over a discovered prey and then drops onto it from a low height.

Feeding experiments showed a daily food requirement of about four earth mice, which corresponds to a live weight of 162 grams. Excess food is kept in the nest during the breeding season; the dumping of prey animals with a large food supply has been observed, but should not be practiced by this species to the same extent as by other owls.

Vaults

Bearded owls usually give off bulges twice within 24 hours , once at the start of the day and once at night at the digestive and feeding area. Since bearded owls keep their resting places for a long time, the arches are relatively easy to find. They are firm, compact and measure an average of 77 × 31 × 26 millimeters.

behavior

Activity, rest and comfort behavior

Due to the very different lengths of day and night in the distribution area of ​​this species, the activity patterns of different populations differ greatly. In addition, they are dependent on the availability of food, the weather conditions and the breeding status. During the brood, bearded owls can be found active at any hour of the day, a fact that led to the fact that the daytime activity of this owl was generally somewhat overrated.

In the southern distribution areas of the Nearctic and Palearctic Oceans, the great gray owl shows pronounced two-peaked activity behavior with peaks at dusk and the first hours of the night as well as at the beginning of dawn. The singing intensity is greatest at dusk and the first half of the night. In the long Nordic summer days, the activity peaks are in the twilight hours; The great gray owl spends the short, dark night hours and the bright daytime hours resting or dozing or busy with comfort behavior . In the very short Nordic winter days, the total activity merges into one unit that extends over the entire day from dawn to dusk. During this time, bearded owls are not active at night.

The great gray owl spends its hours of rest on daily stalls, which it often maintains over longer periods of time. These are often completely uncovered on short, broken stumps close to the trunk. The hours of rest are interrupted by caring for the plumage, sometimes also by small-scale changes of location. During the breeding season, the male rests within sight of the nest. In extreme cold to dig for Bartkäuze kind of grouse into loose snow. Bearded owls bathe often and extensively, especially during the breeding season, sometimes soaking their plumage until it is unable to fly.

Social, territorial and enemy behavior

Young great gray owl of the nominate form

Bearded owls live largely solitary animals outside of the breeding season. Overall, however, their social behavior is characterized by great intraspecific tolerance, so that outside of the breeding season, small groups and large numbers of individuals can be formed in a relatively small area without this leading to aggressive behavior. On the other hand, no social interactions were observed even within larger great gray owl societies. During the breeding season, breeding partners can show an intimate closeness, scratch each other's plumage and rest in body contact.

During the breeding season, bearded owls behave territorially, in that they defend a relatively narrow area around the nest from conspecifics and potential enemies, but tolerate or ignore them in the wider hunting area. The only weak territorial behavior of this species means that breeding pairs can breed successfully at a distance of only 100 meters from each other and food competitors such as Buzzards or potential enemies such as hawks are tolerated at a short distance from their own nesting site. At the beginning of the breeding season, the species behaves extremely shy and withdrawn. When checking the nest, the female usually remains sitting on the clutch without any defense reaction. After the chicks hatch, the female in particular defends them with great aggression and forcefulness, whereby the intruder is targeted with the wings and claws and can be seriously injured. Great gray owl pairs are most aggressive in the early stage of their young. Overall, there are different, partly contradicting information on the territorial and aggressive behavior of the species. This is likely to be due to the fact that the intensity of this behavior depends very strongly on the local conditions, in particular on the availability of food.

An attentive great gray owl has hardly any natural enemies, so that the species shows little fear of humans. In threatening situations, bearded owls assume a high, upright, slim frightened position. However, clutches and broods as well as dormant bearded owls are endangered by a number of soil enemies such as wolves, foxes and martens. Bearded owls flee excitedly and almost in panic when they notice such enemies of the soil. Many species of birds hate the great gray owl . He ignores smaller ones like finches or thrushes ; However, if the birds of prey or corvids are bullying for a long time and intensively, a resting great gray owl changes its daily start.

Reproduction

Courtship, pair formation and nesting place

Bearded owls become sexually mature towards the end of the first year of life, but it is unknown what the percentage of annual breeding birds is. A sufficient supply of small mammals, especially voles, is crucial for egg-laying and the start of breeding. The species leads a monogamous seasonal partnership. Polygyny was occasionally noted. Bearded owls only breed once a year, additional clutches should occur when the clutch is lost or, in good mice years, are part of the rule.

The male's first courtship calls can already be heard in late autumn in some regions. However, the main courtship does not begin until mid-February, and later in parts of the large distribution area. The main elements of the main courtship are the rapid call sequences during the nesting, followed by the shrill answering calls of the female. Often the male shows some potential nest locations and carries out trough movements there or digs up the bottom of the nest, which often leads to their destruction in the case of rice nests. The female first observes the male and participates in these actions as they become more familiar. During this courtship phase, the first handovers of food can be observed, and a short time later the first copulations take place, mostly in constant places near the nest.

Most often, bearded owls use the nests of birds of prey or ravens. Osprey , hawk , rough-legged buzzard and raven are the most common nest supplier. In addition, the species breeds on broken, massive tree stumps, in contrast to the Ural Owl on those that have a relatively flat, only slightly rotten and deepened surface. That is why artificial nesting aids for the great gray owl are simple platforms made of brushwood. Ground broods, as well as broods on rock ledges, are rare. He does not generally avoid being close to humans, breeds have been observed on buildings a few times. Like most owl species, the great gray owl does not enter nesting material; decaying, desolate nests are also not repaired, which sometimes leads to a loss of clutch.

Clutch and brood

Egg, Museum Wiesbaden collection

The first clutches are found in the entire distribution area at the beginning of April, usually the female does not begin to lay eggs until mid-April. A full clutch consists of 4–5 (3–6) initially pure white eggs with average dimensions of 53.2 × 42.4 millimeters. At 50 grams, they weigh about as much as a small chicken egg. The eggs of the Nearctic nominate form are on average somewhat larger and heavier. The laying interval is 1–3 days. Since the female broods firmly from the first egg, the young hatch at intervals of several days. Sibling cannibalism has been observed but appears to be limited to emergency situations. The eggs are only incubated by the female, who is supplied with food by the male during this period and in the first nestling period. The chicks hatch after an average of 28–30 days. They grow very quickly and, with a birth weight of around 40 grams, already weigh more than ten times after two weeks. The young leave the nest at around four weeks of age, and a few days earlier in the event of disturbances. You can safely walk, jump and flutter at this point, but not fly yet. With the help of the claws, the beak and supported by wing flapping they climb to a protected place. You can fly short distances with around 40 days. Both parents provide the branchlings with food for at least another 12 weeks. Young bearded owls are not independent of their parents until they are 20 weeks old and leave the parenting area.

Breeding success and life expectancy

The breeding success fluctuates considerably with the food supply. A Swedish study found 2.7 fledged owls in the first year of a vole gradation and 3.9 in the following culmination year. A total of 174 eggs were counted in 42 controlled great gray owls in a Finnish observation area, from which 101 great owls fledged. Bull & Duncan give comparable figures for nearctic populations.

A female was at least 17 years old when she was re-checked alive 120 km from the place of birth; the ring of a 16-year-old male could be read again in the vicinity of the place of birth. An aviary bird was 27 years old. As with many owl species, the death rate is highest in the first winter and only drops significantly after they are two years old.

Systematics

Strix nebulosa lapponica , illustration

The great gray owl is the largest representative of the genus Strix , which is distributed worldwide in 23 species except for Madagascar , Australis and Oceania . 3 subspecies have been described, 2 of which are generally recognized.

  • Strix nebulosa nebulosa J. R. Forster , 1772 inhabits the Nearctic . The nominate form is somewhat larger and darker and overall grayer than S. n. Lapponica . Particularly on the ventral side, the longitudinal stripes are less pronounced than speckles and fine transverse bands.
  • Strix nebulosa lapponica C. P. Thunberg , 1798 is widespread in the Palearctic . It is more gray-brown, with pronounced longitudinal dots on the underside. The white feather regions of the facial veil are more prominent than in S. n. Nebulosa.

The often mentioned subspecies Strix nebulosa elisabethae from the Central Asian mountains does not find general recognition. Phylogenetic studies of the allopatric population in the California Sierra Nevada mountain range showed that it is a sufficiently independent population to assume a subspecies S. n. Yosemitensis .

Persistence and Threat

The stock situation and the stock development are difficult to determine in a species that lives largely vagabond. The IUCN sees the global population of this species as safe, Birdlife Europe classifies it as stable. However, there is a lack of evaluable supra-regional data for large parts of the distribution area. The reasons for the stock recovery or stock stabilization, which began in Scandinavia after a significant decrease in the late 1970s, lie in the decline in direct pursuit through shooting and egg collecting and in the installation of nesting aids.

In addition to the natural, population-limiting factors, modern forest management, road traffic and various obstacles such as pasture fences or power lines have a population-reducing effect. Large-scale campaigns against “pests”, as they are often still carried out, lead to regional inventory losses.

literature

Web links

Commons : Great gray owl  - album with pictures, videos and audio files

Individual evidence

  1. ↑ HBV (1994) vol. 9 p. 630
  2. ↑ a b König & Weick (2008) p. 383
  3. ↑ Mikkola (1995) p. 7
  4. ↑ a b Mikkola (1995) p. 8
  5. ↑ Bull & Duncan (1993) Table 3
  6. ↑ Mebs & Scherzinger (2000) p. 193
  7. ↑ Mebs & Scherzinger (2000) p. 194
  8. ↑ Mikkola (1995) p. 11
  9. ↑ a b HBV (1994) Vol. 9 p. 632
  10. ↑ a b Mebs & Scherzinger (2000) p. 187
  11. ^ Bull & Duncan (1993) Distribution
  12. ↑ Bull & Duncan (1993) Breeding Range
  13. ↑ Mebs, owls and owls
  14. ↑ a b c Mebs & Scherzinger (2000) p. 203
  15. ^ Bull & Duncan (1993) Migration
  16. ↑ Mikkola (1995) p. 30
  17. ↑ HBV (1994) Vol. 9 p. 634
  18. ↑ Mikkola (1995) p. 31
  19. ↑ a b c Bull & Duncan (1993) Demography and Populations
  20. ↑ Mikkola (1995) p. 45
  21. ↑ Bull & Duncan (1993) Food Habits
  22. ↑ Mebs & Scherzinger (2000) p. 195
  23. ↑ Mikkola (1995) p. 41
  24. ↑ Mikkola (1995) p. 15
  25. ↑ Mebs & Scherzinger (2000) p. 197
  26. ↑ Bull & Duncan (1993) Food Selection and Storage
  27. ↑ Mikkola (1995) p. 42
  28. ↑ Mebs & Scherzinger (2000) p. 190
  29. ↑ Mebs & Scherzinger (2000) p. 188
  30. ↑ Mebs & Scherzinger (2000) p. 192
  31. ↑ Mikkola (1995) p. 103
  32. ↑ Mikkola (1995) p. 66
  33. ↑ Mikkola (1995) p. 73
  34. ↑ Mebs & Scherzinger (2000) p. 198
  35. ↑ Mikkola (1995) p. 100
  36. ↑ König & Weick (2008) p. 384
  37. ↑ Mikkola (1995) p. 80
  38. ↑ Mebs & Scherzinger (2000) pp. 202f.
  39. ↑ König & Weick (2008) p. 354
  40. ↑ Mebs & Scherzinger (2000) p. 185
  41. ↑ Eric P. Jepsen, Rick Gerhardt et al .: Range-wide genetic differentiation among North American great gray owls (Strix nebulosa) reveals a distinct lineage restricted to the Sierra Nevada, California . In: Molecular Phylogenetics and Evolution . Volume 56, Issue 1, July 2010, Pages 212–221, doi : 10.1016 / j.ympev.2010.02.027
  42. ↑ Data sheet Birdlife international
  43. ↑ Data sheet Birdlife europe
This version was added to the list of articles worth reading on September 9, 2009 .