Bowerbirds

Male Satin Bowerbird
Ptilonorhynchus violaceus

Bowerbirds are several species of songbirds found in Australia and New Guinea. To attract female mates, males of the species build elaborate bowers on the ground, decorate them with bright, shiny objects, and then display and sing loudly above them. Females observe the courtship displays, inspect the quality of the bowers, and select a mate. After mating, the female builds a simple nest away from the bower in which to lay her eggs.

Bowerbirds (sg. pronounced /ˈbaʊərbɜrd/) and catbirds make up the bird family Ptilonorhynchidae. The family has 20 species in eight genera. These are medium-sized passerines, ranging from the Golden Bowerbird (22 cm and 70 grams) to the Great Bowerbird (40 cm and 230 grams). Their diet consists mainly of fruit but may also include insects (fed to young), flowers, nectar and leaves in some species.

The bowerbirds have an Austro-Papuan distribution, with ten species endemic to New Guinea, eight endemic to Australia and two found in both. Although their distribution is centered around the tropical regions of New Guinea and northern Australia, some species extend into central, western and southeastern Australia. They occupy a range of different habitats, including rainforest, eucalyptus and acacia forest, and shrublands.

Bowerbirds are most known for their unique courtship behaviour, where males build a structure and decorate it with sticks and brightly coloured objects in an attempt to attract a mate.

Mating behavior

The catbirds are monogamous and raise chicks with their mate, but all other bowerbirds are polygynous, with the female building the nest and raising the young alone. These latter species are commonly sexually dimorphic, with the female being more drab in color. Female bowerbirds build a nest by laying soft materials, such as leaves, ferns, and vine tendrils, on top of a loose foundation of sticks. They lay one or two eggs, which hatch after 19 to 24 days, depending on the species.

Bower of a Satin Bowerbird

The most notable characteristic of bowerbirds is their extraordinarily complex courtship and mating behaviour, where males build a bower to attract mates. There are two main types of bowers. One clade of bowerbirds build so-called maypole bowers that are constructed by placing sticks around a sapling, in some species these bowers have a hut-like roof. The other major bowerbuilding clade builds an avenue type bower made of two walls of vertically placed sticks. In and around the bower the male places a variety of brightly colored objects he has collected.

These objects — usually different among each species — may include hundreds of shells, leaves, flowers, feathers, stones, berries, and even discarded plastic items, coins, nails, rifle shells, or pieces of glass. The males spend hours arranging this collection. Bowers within a species share a general form but do show significant variation, and the collection of objects reflects the biases of males of each species and its ability to procure items from the habitat, often stealing them from neighboring bowers. Several studies of different species have shown that colors of decorations males use on their bowers match the preferences of females.

Uy and collaborators have shown that mate searching females commonly visit multiple bowers, often returning to the male several times, watching as the male owner's elaborate courtship displays and inspecting the quality of the bower and tasting the paint the male has placed on the bower walls. Many females end up selecting the same male, and many under-performing males are left without copulations. Females mated with top mating males tend to return to the male the next year and search less.

Female Satin Bowerbird

Gilliard has suggested[citation needed] the "transfer effect", in which he claimed that bowerbird species that build the most elaborate bowers are dull in color and show little variation between male and female, whereas in bowerbird species with less elaborate bowers the males have bright plumage. This hypothesis is not well supported because species with vastly different bower types have similar plumage. Borgia has suggested that the bower functioned initially as a device that benefits females by protecting them from forced copulations and thus giving them enhanced opportunity to choose males, and benefits males by enhancing female willingness to visit the bower. Evidence supporting this hypothesis comes from observations of Archbold's bowerbirds that have no true bower and have greatly modified their courtship so that the male is limited in his ability to mount the female without her cooperation.

In toothbilled bowerbirds that have no bowers males may capture females out of the air and forcibly copulate with them. Once this initial function was established bowers were then co-opted by females for other functions such as use in assessing males based on the quality of bower construction. Recent studies with robot female bowerbirds by Patricelli and collaborators have shown that males react to female signals of discomfort during courtship by reducing the intensity of their potentially threatening courtship. Coleman and colleagues found that young females tend to be more easily threatened by intense male courtship, and these females tend to choose males based on traits not dependent on male courtship intensity. The high degree of effort directed at mate choice by females and the large skews in mating success directed at males with quality displays suggests that females gain important benefits from mate choice.

Since males have no role in parental care, give nothing to females except sperm it is suggested that females gain genetic benefits from their mate choice but this has not been established, in part, because of the difficulty of following offspring performance because males take seven years to reach sexual maturity.

This complex mating behaviour with its highly valued types and colors of decorations has led some researchers to regard the bowerbirds as among the most behaviorally complex species of bird. It also provides some of the most compelling evidence that the extended phenotype of a species can play a role in sexual selection and indeed act as a powerful mechanism to shape its evolution, as seems to be the case for humans.

In addition, many species of bowerbird are superb vocal mimics. Macgregor's Bowerbird, for example, has been observed imitating pigs, waterfalls, and human chatter. Satin bowerbirds commonly mimic other local species as part of their courtship display.

Relationships

Golden Bowerbird male (top) and female

Though bowerbirds have traditionally been regarded as closely related to the birds of paradise, recent molecular studies suggest that while both families are part of the great corvid radiation that took place in or near Australia-New Guinea, the bowerbirds are more distant from the birds of paradise than was once thought. Sibley's DNA-DNA hybridization studies placed them close to the lyrebirds; however, anatomical evidence appears to contradict this placement and the true relationship remains unclear. Note that the Gray Catbird (Dumetella carolinensis) and Black Catbird (Melanoptila glabrirostris) from the Americas and the Abyssinian Catbird (Parophasma galinieri) from Africa are unrelated birds that belong to different families.