Male Silky Bowerbird(Ptilonorhynchus violaceus)
The bowerbirds (Ptilonorhynchidae) are a family in the order of passerine birds (Passeriformes) found exclusively in Australia and New Guinea. The family is named after the male’s courtship display, which he uses to try to attract a female. These arbors are built by 17 of the 20 species within the family and are decorated with objects that the males often collect by color. This courtship behavior is so unique in the bird world that ornithologist Ernest Thomas Gilliard noted that the order of birds could actually be divided into two groups: Bowerbirds and all other bird species. Bowerbirds are often counted among the most intelligent species among birds. They have very large brains relative to body weight, live to a comparatively old age, and take several years to reach sexual maturity. Most species are polygamous.
Considered the best-studied and best-known bowerbird, the silk bowerbird is native to eastern Australia. It was already kept as a caged bird in Australia in the second half of the 19th century. Little research has been done, however, on the bowerbirds found in remote mountainous regions of New Guinea. For example, the feeding habits and reproductive details of the red-crowned gardener are still unclear. The Yellow-crowned Gardener was scientifically described in 1895 on the basis of a few bird bellows, but living individuals were not observed until 1982 in the Foja Mountains. Only in 2005 it was possible to photograph some individuals.
There is a close relationship to the birds of paradise. Like them, the bowerbirds are found on New Guinea, the neighboring islands and in Australia.
With a body length between 23 and 36 centimetres, bowerbirds are medium-sized songbirds with very different colourful plumages. In them, the hind toe is always slightly shorter than the middle toe. The males of this family are not as brightly colored as the closely related birds of paradise, but individual species have a feathered cap or elongated head feathers. In a number of species, the males have feather areas that are shiny metallic, with bright highlights in appropriate light. Sexual dimorphism is present in several of the species. In the species of the genus Chlamydera, however, there is no or only a very slight sexual dimorphism.
Numerous bowerbird species are excellent mockers, imitating bird calls but also sounds from their surroundings. The brown-bellied bowerbird has even been described as imitating the human voice. It also mimics sounds of the environment such as fast flowing water, the neighing of horses, and the hoof sounds of shod horses. A three-beaked bowerbird mimicked the sound of water running fast over gravel as well as the squealing of pigs, the sound of a chainsaw, and hammering. Archbold bowerbirds also imitate the rustling sound that dead screw tree leaves make when they rub against each other in the wind and the sound of such a leaf falling from the tree to the ground.
Bowerbirds also imitate the calls of numerous bird species in their environment. In the case of the brown-bellied bowerbird, which occurs on the Cape York Peninsula, among other places, these include noise leatherhead, the courtship sounds of the silky bowerbird, the voice of the Reinward’s cockatoo and the yellow-crested cockatoo. The call repertoire of the pillar warbler is even more extensive. Evidence includes imitation of the calls of white-breasted dove, Banks’ raven cockatoo, yellow-crested cockatoo, king parakeet. Pennant parakeet, Noisy pitta, Yellow-throated sericornis(Sericornis citreogularis) and Pale-fronted sericornis(Sericornis magnirostris), Grey-cheeked gerogyne(Gerygone mouki), Variegated Honeyeater(Lichenostomus ferrands), Grey-backed Dickbird(Colluricincla boweri), Spectacled Monarch, Thick-billed Shrike, Victoria Bird of Paradise, Black-eared Bowerbird and Silky Bowerbird. The Tooth-billed Bowerbird mimics the sounds of fruit bats, frogs, and cicadas, in addition to at least 44 different bird species.
Tooth-billed bluebirds are often opportunistic, responding, for example, to calling passing troops of pennant parakeets or yellow-crested cockatoos by imitating their calls. They also frequently call in alternation with the imitated bird species, that is, they respond, for example, to the calls of the fern flycatcher, which in turn responds to their calls. Neighbouring males also mimic each other’s calls and imitate each other’s bird calls.
Both breeding females of the Gray Pigeon as well as males of this species near their bower mimic the calls of predators. Evidence includes imitation of the calls of Wedge-tailed Kite(Haliastur sphenurus), Papuan Kite(Aviceda subcristata) and Black Kite. A female with two newly hatched young flew up, took cover in the brush, and imitated the calls of the hunter kingfisher and blue-eared honeyeater, which is known to respond to predators by hating when someone approaches the nest. There is also other evidence of the purposeful use of imitated sounds: A gray bowerbird perched on a branch above a sleeping cat watched it and imitated meowing until the cat left the area. Something similar is described for the pillar gardener and the spotted bowerbird. Spotted Bowerbirds then imitate primarily the calls of the Wedge-tailed Kite. However, imitations of the calls of the Wedge-tailed Eagle, Grey-crowned Shrike(Pomatostomus temporalis), New Holland Crow, Flutist, Hooded Lily, Grey-backed Crow Shrike, Black-throated Crow Shrike, and Bullfinch have also been documented, all of which eat young birds and eggs. A female pillar gardener, trying to entice a predator away from her nestling, first mimicked the calls of various nest predators such as the mangrove heron, crested kingfisher, black-backed crow shrike, and later, when the situation relaxed again, let out the calls of the garden fantail.
Bowerbirds spend a lot of time on the ground. They breed on the trees. They look for their food on the ground and on the trees and bushes. They have a strong bill and strong legs. In the plumage colour the females are mainly brown or grey. Among the inconspicuously coloured males, there are some species in which the male has a conspicuous head cap hidden in the head feathers, which is only put up for a short time during courtship.
Instead of the 9-10 arm wings typical of songbirds, including the umbrella feathers, bowerbirds have 11-14. The brain, especially that of the bower-building species, is also larger than in other songbirds of the region. The life expectancy of these species is between 20 and 30 years, though polygamous species take seven years to develop adult plumage.
In 17 species of bowerbirds, males court the favor of a female with elaborately built and decorated courtship displays.
The male does not participate in breeding and rearing the young birds. He also does not defend a territory. The female chooses her mate solely on the basis of the quality of the arbour and the courtship dance shown. The male’s arbors are no more than 100 meters apart in some species, so the female has a choice of several males. Some males succeed in attracting large numbers of females with their bowers. Very successful males in the silky bowerbird mate with twenty to thirty females. Other males, however, remain unsuccessful and do not come to mate. Accordingly, the construction of arbors plays a major role in the lives of males.
Types of arbours
The arbor varies from species to species. The more inconspicuous a male is coloured, the more splendid the arbour turns out to be. Since females mate with the builders of the most beautiful arbors, the ability to build such arbors can be considered a “secondary sexual trait”. As an “alternative strategy” for sexual selection – as opposed to brightly coloured plumage – pergola building has the advantage that males are less conspicuous to predators. The arbours are built on a free, flat place in the thicket.
Jared Diamond, as he explains in his book The Third Chimpanzee, sees the arbor as a very effective trait for sexual selection, as it allows females to assess a great many characteristics of a potential mating partner-including keeping one’s arbor from being destroyed by a rival.
Basically, individuals of a species build the identical type of arbor. In one case an arbour of the grey bowerbird has been found which deviated from the basic principle and in its form was reminiscent of the complex arbours of the three-aisle bowerbird, because the central aisle was short and two further walls were erected at the end of the central aisle. Had the two walls been connected, they would have framed the arbor’s center aisle in a semicircle. This pergola represents the strongest deviation from the respective species-specific construction method found so far in pergolas. The constructions of the bowerbirds can be divided into three types: Yard or threshing floor, maypole or avenue.
Arbour type threshing floor
Among the bowerbirds, the species that first clean a larger area of forest floor and then line it with leaves are called barnbuilders. Among the best-known species that exhibit this behavior is the toothed bowerbird. It lays out leaves on the cleaned area, turning them so that their paler undersides face upward. The tennen are in closer proximity to each other. The biologist Hansell therefore speaks of a lek-like behaviour. In biology, a lek is the term used to describe a courtship area in which several males court a female together. The term is mainly used in ornithology, as this form of courtship occurs in a number of species of different bird families.
Arbour type maypole
A total of five species of bowerbirds build a bower of the maypole type. This is a construction in which small branches are joined around a thinner tree trunk or around a tree fern. The resulting column of branches around this trunk is the essential characteristic of this type of pergola.
The simplest form of this arbor is built by the golden-haired gardener. The maypole is only two or three times the height of the male’s body, and consists of a few hundred finely interlaced little branches in the centre of an otherwise undecorated moss platform. The arbor of the yellow-crowned gardener is similar, but here the surrounding area is decorated with clusters of yellow, green, and blue fruit. The arbour of the columnar gardener is much more elaborate. The most complicated form of this type of arbor, however, is built by the cottage gardener. Above the so-called maypole, the cottage gardener erects a roof that reaches down to the ground and opens into a threshing floor in front of the maypole.
Arbor type Alley
An avenue usually consists of two parallel walls built of small branches. The best known species that builds such arbors is the silk bowerbird. Alley builders also include species of the genus Chlamydera and Sericulus.
The most complex type of such arbors of the “avenue” type is built by the three-aisle arbor. Its arbor has four walls instead of two. The walls of the middle aisle face outward, while other avenue builders among the bowerbirds either incline their walls inward or build them vertically. The base of the middle avenue is a thick platform of branches and grasses that widens at either end of the avenue. At each end of the platform there is another wall at right angles to the main avenue. The arbor thus has a central aisle and two transverse aisles. Completed arbours range from 71 to 97 centimetres in length, are 48 to 66 centimetres wide and 36 to 64 centimetres high. The middle aisle is between 6 and 8 centimeters wide and is 17 to 32 centimeters long. Including the small pebbles and fruits that the males use to decorate the arbors, the structure weighs between 3 and 7.5 kilograms. One particularly large arbour of the three-stranded bowerbird had more than 3000 branches installed, more than 1000 blades of grass laid down and decorated with more than 1000 stones with a total weight of almost 4.5 kilograms.
In the brown-bellied bowerbird, the male can be observed near his bower for about eight months of the year. In experiments in which a male occupying an arbor was removed, the arbor site was immediately occupied by other males. The male that reoccupied an arbor site immediately destroyed the existing arbor and rebuilt. As a rule, the arbor was smaller thereafter. The rapidity with which abandoned arbor sites are reoccupied indicates, according to Clifford and Dawn Frith, that there are more males than suitable sites for the construction of an arbor.
In the silk bowerbirds, which also build avenues, the males steal each other’s blue decorative material. The male’s arbors are about 100 meters apart, putting them out of sight of each other. The ability of males to selectively seek out the arbors of other males is evidence that males have a mental map of their environment. A male seeking out the arbor of a neighboring silky bowerbird will fly silently near it and observe the surroundings from a vantage point. If the arborist is not nearby, it approaches the arbor. Some of the males not only confine themselves to stealing decorative material from the arbours of their competitors, but also destroy their arbours with rapid movements within a few minutes.
Adornment of the arbor
In a majority of species, males decorate their arbors with colored objects. For many species, arbors have also been found painted by the male with fruit or plant pulp in specific locations. When it comes to decorative objects, males show a preference for certain colors. The silk bowerbird, for example, prefers to decorate its arbours with blue objects – a colour that is comparatively rare in the wild – while the drop-bowerbird shows a preference for whitish objects.
Decorative objects often include seed pods, snail shells, flowers, pebbles, or bleached small bone fragments. Decorative objects of human manufacture are also frequently used, such as glass fragments, plastic parts, and even so-called pull tabs from tin cans and ammunition casings. The silky bowerbird with its preference for blue objects, for example, often places blue plastic bottle caps at its pergolas. The number of decorative objects attached can be very high. Arbors are consistently found with more than 1000 decorative objects attached. In one bower of the teardrop bowerbird examined near Alice Springs, the male used 1427 bone objects, 174 snail shells, and numerous pebbles and fragments of glass and metal. In total, the attached decorative material weighed 7.4 kilograms. More than 1350 small bones were found in one arbour of the spotted bowerbird, and about 1900 snail shells in another.
To paint the bower, for example, male yellow-naped bowerbird chews leaves and/or fruits and leaves them on the ground. He then returns to this pile of paint several times and, with the help of his bill, applies the chewed plant material to the inner walls of his pergola by pecking or side-swiping movements. Red and tawny colours are preferred.
Learning the pergola
It could be demonstrated in several species that the construction of an arbour is a learning achievement in which the example of other, older birds is also important: the fact that individually captive-raised males of the yellow-naped bowerbird build only incomplete arbours indicates that they partly copy the arbour construction from older males. Young males of the Silky Bowerbird spend a great deal of time learning to build arbours, the perfection of which is a major determinant of their reproductive success. Young males of this species initially build very inadequate arbors. They typically select small branches for the construction of the pergola walls that are too thick, have too different lengths, and also have not yet mastered the skill that enables them to build symmetrical walls. The first constructions are purely practice arbours, often with several young birds working on them. They do not work on them cooperatively – for example, a single fledgling builds single branches, the next fledgling destroys the existing structure and starts again, a third adds more branches.
Young birds perfect their technique by imitating older birds. They regularly visit the arbors of older silky bowerbirds and help out. 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 practice in front of a watching conspecific.
Bowerbirds and people
Several species of bowerbirds are shown in zoos. More commonly kept birds are the species of the genus Ailuroedus, which are monogamous and do not build bowers. With them, breeding succeeds regularly. The white-eared bowerbird in particular has also been successfully bred from this genus in several zoological gardens. Successful breeders in Germany are Weltvogelpark Walsrode, Wilhelmina, Heidelberg Zoo and Krefeld Zoo. It was also successfully bred at Berlin Zoo from 1998 to 2001 and at Berlin Zoo in 2007. The Chester Zoo in Great Britain, the Hong Kong Zoological-Botanical Garden, the San Diego Zoo and the Los Angeles Zoo have also bred offspring. Important insights into reproductive biology were gained in particular at Krefeld Zoo.
Although the most commonly kept, arbour-building species in captivity build arbours and also court, only a few species have been successfully bred in captivity to date. In the case of the silk bowerbird, captive breeding was successful as early as 1912, and there is evidence that it was regularly kept as a caged bird as early as 1860. It was therefore already clear at the beginning of the 20th century that the male alone builds arbours and that it takes several years for a male to show adult plumage. The Brown-bellied Bowerbird raised offspring at Cologne Zoo in 2008.
The yellow-naped bowerbird was a commonly kept bird in captivity for several decades, both in Australia and outside. Yellow-naped bowerbirds were kept in Britain as early as 1867, with London Zoo keeping this species of bowerbird as early as 1905/1906. The first breeding in captivity succeeded in 1905. In Australian zoos, the yellow-naped bowerbird is currently being regularly bred. Captive bred Yellow-naped Bowerbirds are still occasionally offered for sale in the bird trade in Australia. A pair, both of which were proven to be captive-bred, was offered in Sydney in 2000 for 1500 Australian dollars.
There are numerous reports in both the technical literature and popular science writings that silk bowerbirds and spotted bowerbirds remove jewelry, keys, and the like from homes, vehicles, and tents.
During the winter months, when silky bowerbirds feed predominantly on saplings and occur in larger flocks in more open areas, 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 maturing corn. The green-leaved bird also visits gardens and plantations more frequently to eat fruit grown there. It is still occasionally shot for this reason today.
Bowerbirds as game
The green-leaved bird was hunted for a long time because its meat was considered delicate. It was mostly shot during hunts aimed at pigeons.
Genera and species
- Green Catbird(Ailuroedus crassirostris)
- Black-eared Catbird(Ailuroedus melanotis)
- White-eared Catbird(Ailuroedus buccoides)
- Tooth-billed Catbird (Scenopooetes dentirostris, Syn.: Ailuroedus dentirostris) i
- Gardener’s birds(Amblyornis)
- Yellow-crowned gardener(Amblyornis flavifrons)
- Golden-breasted Gardener(Amblyornis macgregoriae)
- Cottage Gardener(Amblyornis inornata)
- Red-headed Gardener(Amblyornis subalaris)
- Archbold’s Bowerbird(Archboldia papuensis)
- Ruffed Bowerbirds(Chlamydera)
- Spotted Bowerbird(Chlamydera guttata)
- Spotted Bowerbird(Chlamydera maculata)
- Grey Bowerbird(Chlamydera nuchalis)
- Brown-bellied Bowerbird(Chlamydera cerviniventris)
- Three-gang bowerbird(Chlamydera lauterbachi)
- Column gardener(Prionodura newtoniana)
- Silky Bowerbird(Ptilonorhynchus violaceus)
- Golden Birds(Sericulus)
- Yellow-naped Bowerbird(Sericulus chrysocephalus)
- Flame Bowerbird(Sericulus ardens)
- Golden Bowerbird(Sericulus aureus)
- Red-crowned Bowerbird(Sericulus bakeri)
Where the range of individual species overlaps, natural hybrids occasionally occur. For example, the range of the Spotted Bowerbird overlaps to a small extent along the Cape River with the Gray Pigeon. Natural hybrids between the two species have been observed on several occasions.
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