Bowerbirds' Architecture

Bowerbirds

Bowerbirds as a group are some of the most fascinating of all birds because of their incredible and elaborate architectural abilities. Well known is the fact that male bowerbirds create the most elaborate displays to attract females for mating, putting untold effort into designing, crafting, and displaying the perfect "love nest" to attract females of the species. But less well known is the amount of thought and calculation put into the creation of the perfect bower. Depending on the species, bowerbirds will get extremely detailed about the colors used, how the colors are displayed and even more interesting, the geometry of the displays.

Male bowerbirds use their intelligence to impress the females, constructing elaborate structures called bowers to attract mates. They are not on master builders, but also accomplished artists. Males of some species decorate their bowers lavishly with flower petals and sparkly manmade objects. The Satin bowerbird even paints the walls of his bower with charcoal or chewed up berries. Male Great bowerbirds are even more remarkable. Their bowers, which are among the most complex of all, are true marvels of avian architecture. But as well as being builders and artists, males of this species are also magicians - the bowers they build are like a house of illusions, with built-in visual tricks that manipulate females' perceptions and increase their likelihood of choosing the builder as their mate.

Bowerbirds are a family of twenty species that are native to Australia and New Guinea that are renowned for their unusually complex mating behavior. The Great bowerbird of northern Australia is the largest species in the family. Males sport brownish-grey plumage build bowers, and spend many months building their bowers. The bowers consist of a thatched twig tunnel forming an avenue of approximately half a meter in length, 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 colorful objects he has collected, one after the other.

This species creates a courtyard with large objects placed toward the back and smaller objects placed toward the front, nearer to where the female approaches. The overall effect is that the courtyard looks smaller overall than it actually is. The male great bowerbird very specifically designs and arranges his bower in such a manner, though why it works to woo the females is still unknown. Even more interesting, this is the only species of animal known to build something that creates perspective. But the talent of bowerbirds doesn't stop with geometry. There is also color coordination.

It is reported that, the males use visual illusions when constructing their bowers. They do so by arranging the objects covering the floor of the court in a particular way, so that they increase in size as the distance from the bower increases. This positive size-distance gradient creates a forced perspective which results in false perceptions of the geometry of the bower, which is visible only to the female when she is standing in the avenue. From her point of view, all of the objects in the court appear to be the same size. Consequently, she may perceive the court as being smaller than it actually is, and the male to be bigger.

Analysis of research data revealed that the geometry of the bower was directly related to the mating success of its builder. The most successful males were the ones that had arranged the objects to form the most regular patterns on the floor of the court. There was also a direct relationship between the regularity of the pattern formed by the objects in the court and the strength of the forced perspective illusion.

As better gradients produce more even patterns when seen from within the avenue,  the males 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. When in the avenue, they also sometimes fix the twigs in the walls, too. The researchers still don't know if this results in an improvement of the illusion.

As well as making the court seem smaller, the illusion created by a regular pattern may make the male's displayed objects stand out more, because a regular background pattern is less distracting than an irregular one. It is noted that, the displayed objects are slightly larger than the perceived size of the objects arranged on the floor of the court. This may produce another illusion of relative size perception, called the Ebbinghaus illusion, whereby an object is perceived as larger when viewed next smaller ones, or smaller when next to larger ones.

Chickens, thrushes, pigeons and parrots have all been shown be sensitive to various illusions, and males of many species display themselves to females at a characteristic angle and distance. This suggests that the use of illusions might be widespread in birds. Females of many bird species are known to prefer males with larger colored patches on their bodies, and it is possible that the Ebbingaus illusion could be used by males of those species to increase the apparent size of their patches. So, too, could the related Wundt-Jastrow illusion, in which an object appears smaller when its shorter edge is next to the longer edge of an object of the same size.