Australasian Science Magazine

the bower of a great bowerbird with one display court visible in the foreground, the other court is visible through the avenue. Note the green decorations to the side of the court.

By Laura Kelley & John Endler

Male great bowerbirds construct visual illusions that enhance mating success by altering female perception of their displays.

Male animals produce a dizzying array of courtship displays when attempting to attract a mate, but perhaps the most impressive of all are the courtship rituals of the bowerbirds. Of the 20 species of bowerbird that are endemic to Australia and New Guinea, 17 of these decorate cleared areas or construct bowers.

It is a common misconception that bowers are nests. Their sole purpose is to lure females so that the males can mate with them.

Many males never succeed in attracting a mate to their bower and only a few males get to mate with the majority of females. The potential costs and rewards are therefore very high, and males do everything they can to attract females such as building elaborate bowers, decorating their bowers with brightly coloured objects and mimicking the vocalisations of other species.

The breeding season for most bowerbirds lasts for several months, so males have to invest a lot of time and effort in keeping their bowers and decorations looking perfect in case a female should drop by. However, males cannot be at their bowers all day every day, and rival males take advantage of this by marauding unattended bowers. Upon arrival at an unattended bower, the marauding male often spends several minutes pulling down the walls of the bower and then steals as many decorations as he can fit in his beak. Often the bower is left almost completely destroyed (Fig.1.) Marauding gives the male a two-fold advantage: he has made a rival less attractive as a female will not visit a destroyed bower, and he has made himself more attractive to females as he has stolen extra decorations.

Great bowerbirds build avenue bowers that consist of two parallel walls approximately 60 cm long made of a tight thatch of sticks that create an avenue down the middle. The male then creates two display courts by placing a variety of grey and white objects such as stones, shells and bones (collectively called gesso) at either end of the avenue entrance. He decorates his bower with ornaments by placing green and red coloured decorations on either side of the avenue entrance (Fig. 2).

During display, the female stands in the centre of the avenue and watches the male displaying over his court. The male stands to the side of the avenue entrance and the narrow avenue walls restrict the female’s view so that she can usually only see the male’s head during most of the display. Using his beak, the male picks up coloured decorations and shakes and tosses them over the display court, and he also intermittently shakes the lilac crest of the back of his head.

Females may visit several males to assess their suitability as potential mates, during which time they scrutinise many aspects of the male’s display. If she likes what she sees, she will mate with the male in the avenue.

Great bowerbird males also have an extra trick up their sleeve to woo females – they produce a visual illusion known as forced perspective. Forced perspective is commonly used in architecture, ornamental gardens, photography and film-making to give false impressions of size or distance.

One famous example of this is Cinderella’s Castle at Disney theme parks – the windows decrease in size as the height of the building increases. Our brains assume that all of the windows are the same size so we are tricked into thinking that the building is much taller than it really is. Conversely, a building will look smaller than it really is if window sizes increase as the building height increases.

Bowerbirds construct their display courts using a similar principle. They arrange the grey and white gesso objects on their display courts so that they increase in size as the distance from the bower increases, created a positive size–distance gradient (Fig. 3). When viewed by the female within the bower avenue, the pattern created by this arrangement creates the illusion of forced perspective for the female looking at the court.

All males arrange their gesso objects to create the patterns required to produce forced perspective, but some males are more skilful in creating patterns than others. So why do males create these patterns? We suspected that it might be related to mating success and that the display courts were another aspect of the fabulous displays of male bowerbirds.

To test this, we set up motion-sensitive cameras at the bowers of a population of great bowerbirds in Queensland. These cameras monitored the courtship and mating of males during the course of the field season.

We found that the quality of the forced perspective was an excellent indicator of male mating success. Females preferentially mated with males that produced high quality patterns.

It’s not clear exactly how the visual illusion affects the female’s perception to make her more likely to mate. The female may be directly assessing the quality of the pattern and choosing the male with the best pattern.

The arrangement of the gesso also creates a smooth pattern for the viewing female. This uniform background is likely to make objects much easier to see than if the background was more random.

The visual effect of the forced perspective probably makes the display court appear smaller than it is in reality, so the coloured objects that the male displays over the court appear relatively larger. Further visual illusions may also occur as the male moves decorations and the female moves her head.

The ability to construct the patterns required to produce forced perspective may show the female how clever the male is. There is some evidence in other species of bowerbird that males that perform well on certain cognitive tests also gain more mates, but we first need to determine whether forced perspective is cognitively demanding to produce. Males may create illusions using trial and error or a simple rule of thumb, neither of which would provide any indicator of intelligence to the female.

However, if the male has to learn how to produce the illusion then it may give the female some clues about the male’s intelligence or age. Male bowerbirds take up to 7 years to learn how to build bowers, so the quality of a male’s bower may reflect the male’s age. The quality of the size–distance gradient may also be used by females in a similar manner if it is a skill that requires practice.

Alternatively, females may not be aware that the gesso objects are arranged in a way that creates a visual illusion. Females may simply choose males with the most conspicuous display or those that display larger objects, and males use forced perspective to trick the female into perceiving that he possesses these traits. We need to better understand how females perceive the male’s display before we can establish the effects of the visual illusion.

Great bowerbirds are the first example of a non-human animal constructing a visual illusion. Birds appear to see many visual illusions in the same manner as humans, so we know that they are susceptible to them. This is not surprising given that animals use size and depth cues of objects in a scene as visual shortcuts to get information about where they are, where they are going and how they can get there. Visual illusions are simply misleading cues that trick the brain into perceiving a scene that is in fact different to the actual scene.

It is unknown whether other animals use visual illusions. Many species tend to display towards females with a characteristic orientation and distance, which is a prerequisite of many visual illusions. Given that the displays of many species fulfil the requirements for visual illusions, do they use them?