Julius Schnorr von Carolsfeld (1794 – 1872)
Chickens, thrushes, pigeons and parrots have all been shown to be sensitive to various illusions, and–lacking the peacock’s tail–males of many species display themselves to females at a particular angle and distance with deliberate intention, because–for instance–the females may prefer males with larger coloured patches on their bodies.
Male Great bowerbirds spend many months building true marvels of complex architecture which consist of a thatched twig tunnel forming an avenue approximately half a metre long, opening out onto a court whose floor is covered with bones, shells and stones.
When a potential mate steps into the avenue, the male stands in the court just by the avenue’s exit, displaying to her the colourful flotsam and jetsam he has collected, one piece after the other.
They are magicians – the bowers they build are like a house of illusions, with visual tricks that manipulate females’ perceptions.
The objects covering the floor of the court are arranged so that they increase in size as the distance from the bower increases. Thus, when the female is standing in the avenue all of the objects in the court appear to be the same size from her point of view, so she may perceive the court as being smaller than it actually is, and the male to be bigger.
Scientists reversed the objects by placing the larger objects closest to the bower and the smaller ones further away, and found that the birds corrected the disarray very quickly.
In all cases the pattern was almost identical to the original within two weeks.
The birds go to great lengths examining their work and rearranging objects to make the pattern as even as possible.
“Males spend most of their time on the bower going into the avenue and looking out, then moving objects, going back into the avenue, and so on. They sometimes fix the twigs in the walls, too.”
During his courtship display, the male waves his treasures towards the female, causing their apparent size to increase.
The more time a female spends in a bower, the more likely she is to mate with its builder so holding her attention longer is important.
After carefully constructing a twig avenue on the forest floor; the Satin bowerbird chooses decorations, arranging them around the sunny northern entrance.
He favors blue, but may use yellowish-green ornaments, like the female’s plumage.
If anything outside his color scheme (such as a white flower) falls onto the bower, he’ll quickly remove it.
The male bowerbird is one of the few birds known to use tools. He forms soft bark or other plant fiber into a sponge to absorb a mixture of saliva and bushfire charcoal, holding the sponge in his bill to daub the bower’s inner walls. He also paints the twigs by rubbing them with the juice of pulped blueberries.
The Vogelkop gardener bowerbird of New Guinea builds an astonishing courting place — a hut up to 5′ wide with a moss front garden on which he arranges flowers and fruits. MacGregor’s bowerbird builds a 2′-high twig maypole ringed by a circular dance floor because the brightly colored male dances around his bower to entice a female to enter.
Rival males steal trinkets from an unguarded bower and may even demolish it if the owner doesn’t return in time.
The bowerbird is an accomplished mimic whose repertoire has been known to include the mew of a cat.
All male bowerbirds decorate their bowers lavishly with flower petals and sparkly manmade trash: plastic bottle caps, straws, paper, jewelry, teaspoons …
Chief threats to the bowerbird’s future are forest clearance and shooting by fruit growers, which has led to extermination in some areas.
Reference: Kelley, L. A & Endler, J. A. (2012). Illusions Promote Mating Success in Great Bowerbirds. Science, DOI: 10.1126/science.121