Birds in the infraorder Corvida  (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 . Great bowerbird males construct bowers with a long avenue from within which females view the male displaying over his bower court . 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.
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