Female silkworm bird ( Ptilonorhynchus violaceus )
|Scientific name of the genus|
|Kuhl , 1820|
|Scientific name of the species|
|( Vieillot , 1816)|
The silk arborebird ( Ptilonorhynchus violaceus ) is a songbird that lives in eastern and southeastern Australia and is also called the satin arborebird . It is one of the 17 species within the 20 species of bower birds that compete for the favor of a female with elaborately built and decorated courtship areas.
The male does not take part in the breeding or rearing of the young birds. Nor does he defend any territory. The female chooses her partner solely on the basis of the quality of the arbor and the mating dance shown. The arbors of the males are usually 100 meters apart, so that the female has a choice of several males. Some males manage to attract a large number of females with their arbor construction. Very successful males mate with twenty to thirty females. Other males, however, remain unsuccessful and do not mate.
Silkworm birds are very long-lived and take several years to reach sexual maturity. Due to the intelligence they show when building their arbors, they are counted among the most intelligent of the birds.
The IUCN classifies the silkworm bird population as safe ( least concern ). There are two subspecies.
The male of the silk arbor bird reaches a body length of 32 centimeters, of which 10.3 to 12.5 centimeters are on the tail. Females become slightly larger and reach a body length of up to 33 centimeters, of which 10.8 to 12.8 centimeters are on the tail. The beak measures 3.1 to 4 centimeters in males, 3.1 to 3.8 centimeters in females. Males weigh between 173 and 290 grams. Females weigh between 170 and 258 grams.
The subspecies P. v. minor is slightly smaller than the nominate form. The tail plumage measures between 9.2 and 10 centimeters in the male and the beak measures 3.2 to 3.4 centimeters. In females, the tail plumage is between 9.8 and 10.6 centimeters long, the beak measures 3.1 to 3.2 centimeters. Both subspecies show a conspicuous sexual dimorphism .
Appearance of the male
Males show their full adult plumage from their sixth moult, when they are seven years old.
The male then has a metallic shimmering blue-black plumage which, when exposed to light, has violet highlights, but appears black when exposed to other light. In the feathers of the small and large elytra, only the feather tips and seams have a metallic sheen. The wings of the hand are blackish. The tail plumage is blackish on the upper side with indigo-colored tips and feather edges. They are brown-black on the underside. The lower abdomen and the flanks have little sheen and appear overall a bit blacker than the rest of the plumage.
The male's beak is yellow-green in color. The legs are pale dirty white to light yellow brown. The iris is purple.
Appearance of the female
The slightly larger female is gray-green on the upper side of the body and the wing coverts; in certain light conditions the feather edges also shimmer blue-green. The crown of the head and the reins are a little darker and brownish than the rest of the upper part of the body.
The ear covers are green-brown with gray-white feather shafts. The feathers of the large wing covers are lined with gray to a different extent. The hand and arm wings are cinnamon brown, the respective outer flag is washed over green. The control springs are cinnamon brown, the middle control spring pair has a slightly gray-green tone, while the spring shaft of all control springs is lighter than the respective spring flags.
The chin and throat are grayish white with darker flecks. The basic color of the chest and belly is a pale yellow, with the individual feathers lined with dark brown so that the underside of the body looks scaly. The beak is dark jester, the iris is dark blue and the legs are pale dirty white to light yellow-brown like the male.
Appearance of the young birds
It takes several years for the males of the silk arboreal birds to show the plumage of the adult males. In the first plumage they resemble the females, with the feathers of the wing covers having bright tips. In the males, the ear covers are a little less dashed. The irises of both sexes are blue-violet, the legs are silvery gray. With increasing age, the ear covers of the males become darker and brown. In the second year of life, the throat of the males is less spotted and greener. By the third year of life, the males have a green throat and chest with no further color markings. From the age of three onwards, females can no longer be distinguished from older females.
In the fourth year of life the male has an increasingly yellowish-greenish bill. The color of the legs gradually changes from gray to yellow. In the fifth year of life, the males have a beak color like the adult males. The legs are now bright yellow. In the sixth year of life, the now subadult males show plumage in which either some blue-black feathers can be seen or the blue-black coloration of adult males already dominates. However, there are still some greenish feathers in the plumage.
distribution and habitat
The distribution area of the silk foliage bird extends from southeastern Australia to the so-called Wet Tropics of Queensland in the northeast of the Australian continent.
The two subspecies occur in the following regions:
- P. v. minor - AJ Campbell, 1912 - The range of this subspecies is limited to the Wet Tropics of Queensland. The northern limit of distribution is Cooktown . The distribution area of this subspecies does not extend beyond the city of Paluma in the east of Queensland. The altitude distribution of this subspecies ranges from 640 meters in altitude on the Atherton Tablelands to at altitudes of 1220 meters in the Carine Uplands, about 80 kilometers northwest of Cairns .
- P. v. violaceus - Vieillot, 1816 - The nominate form occurs from Rockhampton in the southeast of Queensland to west of Melbourne in the south of the Australian state of Victoria. The distribution area still includes the Great Otway National Park as the westernmost distribution region . The height distribution of the nominate form ranges from the lowlands to altitudes of 1100 meters in the McPherson Range .
The distribution area of the silkworm bird thus covers a strip along the south and east coast of Australia, which does not extend further than 250 kilometers inland.
The silk arbor bird colonizes rainforests in its range. It occurs preferentially along the edges of the forest and prefers areas with a tall, hard- leaf vegetation with dense undergrowth. In the winter months in particular, it can also be observed regularly in more open regions. It then also occurs in parks, orchards and orchards as well as in gardens.
Silkworm birds are omnivores and feed on berries and fruits as well as flower nectar, leaves and insects. Fruits play a particularly important role in their diet, and they eat a wide range of them.
The feeding habits of silkworm birds have been examined in more detail in different Australian areas and have shown that the nutritional composition is shaped by the respective habitat. In the vicinity of Laura, a small town in the Blue Mountains of the Australian state of New South Wales , it has been observed that the food composition of silkworm birds varies greatly with the season. In late summer and autumn, nectar plays a huge role in their diet. When absorbing nectar, it helps that the tongue is forked at its front end. They prefer especially nectar of banksia , a plant genus within the proteaceae , which is found only on the Australian continent. During the winter months they mainly ate fruits, while in summer insects dominated the diet.
In another study area, which lies further north in the McPherson Range , differences in the food composition were found between the silkworm birds that live in the closed subtropical rainforest there and those that inhabit more open forest areas. The silkworm birds of the rainforest eat more fruit, while those in open forest areas make up 40 percent of their diet. Figs play a bigger role in both regions.
Animal protein covered between 12 and 16 percent of the nutritional requirement in the studies in the McPherson Range. Beetles, singing cicadas and ghosts and other invertebrates were eaten . Most of the insects eaten are pecked by leaves. Silkworm birds also examine tree trunks, branches and twigs for insects. They rarely go to the ground to look for insects. Song cicadas, which play a particularly important role in nutrition, are sometimes caught in flight and silkworm birds actively chase them. Caterpillars, insect eggs, spiders and millipedes play a lesser role. Occasionally silkworms eat eggs and nestlings of other small bird species.
Way of life
Way of life outside the breeding season
Outside the breeding season, silky-bowerbirds form large flocks of up to 100 individuals. During this time, they prefer to eat the saplings of grass and herbs, but also those of trees and bushes, including those of various eucalyptus species. They can then be observed on cattle pastures, among other things, during the early morning and late afternoon hours. The troops are occasionally socialized with thick-billed strangler crows . The winter troops are usually separated according to sex.
Troops seek common resting places, but keep a distance from each other during the night, so that there are about 50 resting silkworm birds per 1000 square meters. Some populations undertake high-altitude hikes during the winter months. Populations of tropical mountain rainforests then reside at altitudes of around 600 meters. The nominate form is then mainly to be found in more open terrain.
Sunbaths and rugs
Silkworm birds regularly use ants to care for their plumage. They belong to the species of birds that show an active one . They take several ants in their beak and stick them in their plumage. Silkworm birds, which stick ants in the plumage of their wings, lift them up slightly and then tremble slightly with their wings.
Silk arborebirds sunbathe by dropping their wings and raising their tail plumage slightly.
Arbor and courtship
Construction of the arbor
The male clears part of the forest floor and sets up a mating ground, a kind of arbor made of twigs, the arches of which are approx. 35 cm high and 45 cm long and are always built in a north-south direction. The incidence of light seems to play a role in the choice of the courtship area. Individual males were observed to selectively remove leaves from the overhanging vegetation in order to improve this incidence of light.
The male built hundreds of branches when building the arbor. Experienced males choose thin, straight branches, all of which are of similar length, and build them into two thick, slightly inclined walls. The walls are symmetrical. To achieve this, the silky arborebird uses a specific technique. Holding a branch in its beak, it positions itself in the middle of the arcade. It first moves from the center to one side of its arbor and then moves with exactly the same movements to the other side of the arbor in order to block the branch there. The two walls of the arbors are created simultaneously in this way. However, it has been shown in experiments that silky-bower birds are able to adapt their construction. In experiments only one side of the arbor was destroyed. Silk arborebirds then only build the missing side of the arcade.
Decorating the arbor
This arbor is laid out by the male with blue, blue-green and yellow objects such as flowers, feathers, insects, berries, snail shells, broken glass or civilization garbage such as ballpoint pens and plastic toys. These are colors that stand out particularly intensely against the twilight background of the forest floor. Blue is also the color that is rarely found in nature in the form of flowers or berries. While decorating the arbor, the males repeatedly take the position from which a female would approach the arbor and then decorate their arbor accordingly.
The males steal the blue decoration material from each other. A richly decorated arbor is therefore also an indication that the male is able to prevail against the neighboring males, to outsmart them and steal material from their arbors and at the same time protect his own arbor from theft. The arbors of the males are about 100 meters apart and are therefore out of sight of the individual males. The ability of the males to seek out the arbors of other males is evidence that the males have a mental map of their surroundings. A male looking for the arbor of a neighboring silky arborebird flies noiselessly near it and observes the surroundings from a viewing point. If the arbor owner is not nearby, he will then approach the arbor. Some of the males not only limit themselves to stealing decorative material from the arbors of their competitors, but also destroy their arbors with quick movements within a few minutes.
The male also uses blue berries - a color preferred by this species of bird - and distributes the pulp with his beak or with a twig on the branches that are erected to form an arbor. Again, this is an essential element in attracting females. Significantly fewer females returned to the arbors from which this painting was experimentally removed. Females always choose their partner according to how much the arbor is decorated. If the decoration of the arbor was experimentally removed, the chances of a male to mate with a female decreased very significantly.
Aversion to red
Red flowers or objects that are experimentally placed in the arbors are immediately removed by the males. It is believed that the combination of blue (flowers and objects) and shades of yellow (the dry branches of the arbor) sends a clear and unmistakable signal to the females. The color combination is otherwise not to be found in the habitat of the silk arborebird. Any color tones that interfere with the clarity of the signal are therefore removed immediately.
Learning to build arbours
The arbor construction that the silk arbor bird shows is learned to a large extent as a young bird. Similar to the perfection of the courtship song of many songbirds, the elaborate arbor structure shown by a fully grown male silky arborist seems to give the female clues about her cognitive abilities. The male juveniles spend a correspondingly large amount of time learning this ability, which largely determines their reproductive success.
Young males initially build very inadequate arbors. they typically select twigs for building the arbor walls that are too thick, too varied in length, and have not yet mastered the skill that enables them to build symmetrical walls. The first buildings are pure practice arbors, on which several young birds are often working. They don't work cooperatively on this - a single young bird, for example, builds individual branches, the next young bird destroys the existing structure and starts over, a third adds further branches.
Young birds perfect their technique by imitating older birds. They regularly visit the arbors of older silk arboretists and help out there. They also learn courtship behavior by approaching the arbor similar to a female and watching the male perform his courtship dance and song. The older males tolerate this because they also benefit from the exercise in front of a fellow watching.
Courtship for the female
In the finished arbor, the males wait for a female to show interest in their arbor. As soon as a female approaches the arbor entrance, she begins a complicated courtship dance, which consists of hopping movements, fast running and abrupt, rapid movements of the wings and spreading of the tail feathers. The males imitate the calls of a number of other bird species. The calls of the hunter's nest , the golden-eared honey eater , the yellow-hooded cockatoo , an Australian crow species and the black cockatoo were described for individual males .
Choice of partner by the female
Females seek out the arbors of several males in their respective area. They judge the males according to their “architectural skills” and their courtship behavior. A complete and decorated arbor not only signals to the female that the male is sufficiently experienced to manage the construction, but that he is also able to defend it against other males. Females stay in the arbor of a single male for anywhere from a few seconds to half an hour. Until the final decision, she visits individual arbours several times.
Ackermann points out that the choice of partner also demands a considerable level of intelligence from the female. During the courtship season, she has to sift through and sort out the number of potential candidates, whose arbors are hidden under bushes and sometimes kilometers apart. If she encounters a new potential partner, she has to compare him with those she has previously visited and place him in a ranking. In addition to the construction work that a male builds, she also has to evaluate his singing and courtship dance.
Jared Diamond sees, as he explains in his book The Third Chimpanzee , the building of arbours as a very effective feature for sexual selection , as this enables females to assess many characteristics of the potential mating partner.
The breeding season lasts from October to January. A nest is built from twigs on forest trees at a height of 4 to 10 m. The female lays 1 to 3 cream- to buff-colored, dark-colored eggs. The male participates neither in the breeding nor in the rearing of the young birds.
Investigations on the silk arborebird
The aversion of the male silkworm bird to red objects in its arbor was used in studies to investigate the problem solving ability of silkworm birds. For this purpose, three red objects were placed in the arbor and each covered with a transparent plastic container. Few birds were unable to remove the plastic container and thus reach the red object. Very skilled birds completed the task in less than 20 seconds. Most pecked at the plastic container until it fell over.
In an aggravated problem-solving situation, a piece of red brick was placed in the arbor and fastened to the ground with a screw in such a way that the silky arborebird could no longer remove it. This represented a task for the birds that they do not encounter in their natural environment. A number of males very quickly began to cover this red with leaves or other material so that it was no longer visible. Male silkworm birds, which found solutions to both tasks very quickly, were also the ones that mated with females more often than their conspecifics. From these experiments it was concluded that the construction of the arbor reveals something to the female about the cognitive abilities of the potential father of her offspring.
Reaction to the behavior of the females
Females generally seem to prefer a courtship dance and courtship song of the males, both of which are powerful and intense. Too aggressive behavior leads to females leaving the arbor and not coming back.
In order to make the reaction of the males to the behavior of the females measurable more objectively, a robot was built that was stuck in a prepared hide of a female silkworm. Various motors enabled the device, like a real female bird, to approach the arbor in a crouched position, to look around and to spread its wings in a manner similar to that of females, thus signaling readiness for mating. The movement sequences were consciously standardized in order to make the reaction of the males to the individual actions of the female more measurable and the behavior of 23 different males during their courtship dance was examined. It was found that some males hardly respond to the reactions of the female in courtship dance and song. Others, however, react when the female appears to be alarmed and moderate the intensity of their behavior. The males who react to the females have a higher mating rate.
Position in the bowerbird family
The silkworm bird is most closely related to the genus Sericulus as well as to the genus Chlamydera . All species that belong to these genera have in common with the silk arborist that they build arbors of the avenue type. The skull has basic features that are typical of the family.
The length of the tail plumage in the females corresponds to the average within the family, in the males it is short compared to other species of bowerbirds. The number of arm swing is 13 to 14, as is typical for bower birds. Songbirds usually have nine or ten arm wings. The length of the run corresponds to 28 percent of the wing length, so the length of the runs in the silky arborebird is slightly longer than the average of all species in the family. Basically both sexes are of a similar size, the male tends to have a slightly shorter tail plumage, slightly longer legs and weighs slightly more than the male.
The species Ptilonorhynchus rawnsleyi was described on the basis of a single specimen that was collected as a type specimen on July 14, 1867 near Brisbane. The typhoid specimen basically had the appearance of a fully grown male silkworm bird, but had blue-green eyes and a large yellow mirror on the wings. This type specimen is now generally classified as a random natural hybrid between a silk arborebird and a yellow-naped bowerbird. No other hybrids have been described so far.
Arborebirds and humans
The silk arborebird is one of the species of arborebird that is shown on various occasions in zoos. In Australia they can basically be found in all zoos that also show birds. The silkworm bird was bred in captivity as early as 1912.
The first reports of silkworm birds being kept in captivity come from the year 1860. The naturalist G. Bennett noted in that year that this species is often kept in captivity in Sydney and that for a couple between 3 and 5 guineas at that time . From captivity it was already known at the beginning of the 20th century that only males build arbors and that it took several years for males to show the plumage of adult birds.
There are numerous reports, both in the specialist literature and in popular scientific writings, that silkworm birds remove jewelry, keys, and the like from houses, vehicles, and tents.
During the winter months, when silkworm birds mainly feed on saplings and occur in larger flocks also in more open terrain, they pose a problem for fruit and vegetable growers, as they eat both cultivated fruit and young leafy vegetables during this time. They also have a particular fondness for maize as it ripens .
- Jennifer Ackerman: The Genius of Birds . Corsair, London 2016, ISBN 978-1-4721-1437-2 .
- Clifford B. Frith, Dawn. W. Frith: The Bowerbirds - Ptilonorhynchidae . Oxford University Press, Oxford 2004, ISBN 0-19-854844-3 .
- Ptilonorhynchus violaceus in the endangered Red List species the IUCN 2008. Posted by: BirdLife International, 2008. Accessed December 19, 2016th
- Videos, photos and sound recordings for Ptilonorhynchus violaceus in the Internet Bird Collection
- English website about silkworm birds ( Memento from November 13, 2011 in the Internet Archive )
- Pictures of silky arborebirds
- Discussion of the bowerbirds with beautiful pictures of them.
- Ackerman: The Genius of Birds . P. 159
- Ackerman: The Genius of Birds . P. 165.
- Ptilonorhynchus violaceus in the endangered Red List species the IUCN 2008. Posted by: BirdLife International, 2008. Accessed December 19, 2016th
- Handbook of the Birds of the World zum Goldaubenvogel , accessed April 1, 2017
- Frith: The Bowerbirds - Ptilonorhynchidae . P. 364.
- Frith: The Bowerbirds - Ptilonorhynchidae . P. 365.
- Frith: The Bowerbirds - Ptilonorhynchidae . P. 362.
- Frith: The Bowerbirds - Ptilonorhynchidae . P. 367.
- Frith: The Bowerbirds - Ptilonorhynchidae . P. 368.
- Frith: The Bowerbirds - Ptilonorhynchidae. P. 366.
- Ackerman: The Genius of Birds . P. 161.
- Ackerman: The Genius of Birds . P. 162.
- Ackerman: The Genius of Birds . P. 164.
- Ackerman: The Genius of Birds . P. 168.
- Ackerman: The Genius of Birds . P. 170.
- Ackerman: The Genius of Birds . P. 169.
- Ackerman: The Genius of Birds . P. 160.
- Jared Diamond: The Third Chimpanzee . Fischer Taschenbuch Verlag, Frankfurt am Main 2000, ISBN 3-10-013912-7 , Chapter 9 “How art arose from the animal kingdom” p. 217 ff., P. 223.
- Ackerman: The Genius of Birds . P. 166.
- Frith: The Bowerbirds - Ptilonorhynchidae . P. 361.
- Frith: The Bowerbirds - Ptilonorhynchidae . P. 387.
- W. Grummt, H. Strehlow (Ed.): Zoo animal keeping birds . Verlag Harri Deutsch, Frankfurt am Main 2009, ISBN 978-3-8171-1636-2 , p. 750.
- Frith: The Bowerbirds - Ptilonorhynchidae . P. 386.