January 19, 2012
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Male great bowerbird. Image by algaedoc via Flickr
Male bowerbirds are virtuoso architects. To woo females they construct an intricate structure (a bower) from twigs that they meticulously decorate with a variety of found objects. The result is the ultimate avian bachelor pad. Biologists have long marveled at the male bowerbirds’ elaborate courtship scheme. Now new findings add to a growing body of evidence that it is even more complex than previously thought.
Among great bowerbirds, which live in Australia, the bower takes the form of an “avenue” made of twigs that opens out onto a “court” assembled from bones, stones, shells and other gray items that are together referred to as gesso. When a female pays a visit, she stands in the avenue and looks out onto the court, where the male proceeds to pick up and display a variety if brightly colored objects. If she likes what she sees, she will mate with him.
Great bowerbird bower. Image courtesy of L. A. Kelley
Female view of the court. Image courtesy of L. A. Kelley
Previous research revealed that the males create visual illusions in building their bowers, carefully arranging the gesso according to a size gradient in which smaller objects are placed closest to the avenue and larger ones farther away. Because objects generally appear smaller with distance, this arrangement tricks the eye into thinking the gray gesso components are uniform in size, and that the court is smaller than it actually is. This “forced perspective” may somehow make the brightly colored objects that the male displays look more appealing to the female.
Great bowerbird bower court viewed from above reveals a size gradient in the gesso objects, with smaller items placed closer to the "avenue" where the female is standing (left side) and larger items placed farther away (right side). Image courtesy of L. A. Kelley
In the January 20 Science, Laura A. Kelley and John A. Endler of Deakin University in Australia report that female great bowerbirds do indeed tend to choose the males that produce the best forced-perspective illusions in their courts. In fact, they write, “illusions may be widespread in other animals because males of most species display to females with characteristic orientation and distance, providing excellent conditions for illusions.”
But in a commentary accompanying the Science report, Barton L. Anderson of the University of Sydney cautions that whether the male bowerbirds are intentionally creating these illusions to attract mates, and whether females are specifically choosing this behavior or some other, as-yet-unknown behavior that is correlated with the production of forced-perspective displays remain unknown.
In the videos below, a male great bowerbird woos a female in his bower (top) and another male succeeds in his courtship (bottom).
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Seems to me that the objects further away from the avenue are more likely to be bones and other objects that the female may know the basic size of, while closer in are stones that could be of any size. By placing objects of known, large size further away, the other objects will seem larger when in fact they are just closer. The objects picked up and thrown by the male are closest, and seem largest. Ergo, he seems to be inflating his size and power through the art of illusion.
What male doesn’t?
Link to thisHad we been able to hear those magnificent calls that Bower birds make, everyone would have been over the moon for them. In the late 80′s I spent over 6 months in Australia around an assortment of Bower Birds we were filming for The Nature of Australia. They have the ability to over lap their calls and their mimicry is unsurpassed. They can do horses, plane engines, generators, other birds, domestic animals, water flowing or dripping and the Satin Bower bird, which is all black with purple eyes sounds like the Star Wars robot R2D2.
Link to thisWhirling and gurgling like an expert. The Satin bower bird collects only blue colored items for his bower gesso and he also makes an alley for mating. He plants the twigs in the ground and then smears his saliva all over them.
The natural colored blue items the males bring in are usually fruits, berries, stones, feathers, etc. Though in the past few years the young males have taken to using short cuts and are decorating their bowers with blue writing pens, blue throw away razor blades, blue straws, blue plastic from broken toys and a number of man-made objects they are retrieving from the trash. In the long run it’s detrimental to the species as a whole as the young males rather than being clever actually are defending poor territories with low food resources and will eventually fail in mating or producing viable offspring.
Most importantly, if anyone has the chance to listen to the huge variety of Bower Bird & Birds of Paradise mating calls they are not to be missed.