Great Bowerbirds Create Theaters with Forced Perspective When Seen by Their Audience

Abstract

Birds in the infraorder Corvida [1] (ravens, jays, bowerbirds) are renowned for their cognitive abilities [2-4], which include advanced problem solving with spatial inference [4-8], tool use and complex constructions [7-10], and bowerbird cognitive ability is associated with mating success [11]. Great bowerbird males construct bowers with a long avenue from within which females view the male displaying over his bower court [10]. This predictable audience viewpoint is a prerequisite for forced (altered) visual perspective [12-14]. Males make courts with gray and white objects that increase in size with distance from the avenue entrance. This gradient creates forced visual perspective for the audience; court object visual angles subtended on the female viewer's eye are more uniform than if the objects were placed at random. Forced perspective can yield false perception of size and distance [12, 15]. After experimental reversal of their size-distance gradient, males recovered their gradients within 3 days, and there was little difference from the original after 2 wks. Variation among males in their forced-perspective quality as seen by their female audience indicates that visual perspective is available for use in mate choice, perhaps as an indicator of cognitive ability. Regardless of function, the creation and maintenance of forced visual perspective is clearly important to great bowerbirds and suggests the possibility of a previously unknown dimension of bird cognition.

Visual effects in great bowerbird sexual displays and their implications for signal design

“…Both locations are far from urban centres and therefore have a distribution of available bower objects more similar to those that they evolved with compared with those in cities. We measured the female's FOV and the location, type and colour of all objects at each bower each bower using our previous methods [18,19,21]. The data for both courts of a given bower were pooled before analysis; both courts are used for male displays and they typically do not differ significantly [19].…”

“…The courts are areas of cleared ground covered with uncoloured objects, primarily stones, bleached bones and bleached snail shells [18,19]. During courtship, the female stands in the middle of the avenue and looks out onto a court.…”

“…The female's predetermined viewpoint from within the bower is important for the forced perspective illusion [18], but the importance of this view for other aspects of the male's display has not been investigated. For example, the female's restricted view may allow males to control what the female sees, when she sees it, and her visual state during the display.…”

What is stereoscopic vision good for?

“…Even perspective cues can be manipulated, e.g. by bowerbirds who fake projection geometry to make their bowers appear larger than they really are 36 . However, I cannot think of any example where a non-human animal is able to manipulate stereo depth cues in order to make its size, distance or structure appear other than it really is.…”

Response to Comment on “Illusions Promote Mating Success in Great Bowerbirds”

“…Therefore, stable footing is unlikely to explain the presence of object gradients in this population. We considered courtship traits that are important to great bowerbirds (2,7,8), not traits that are important in all other species. Different bowerbird species are known to produce different sexual displays; for example, Borgia et al (1) cite vocal mimicry, which is not part of the sexual display in great bowerbirds.…”