The Great Bowerbird is a curious bird. The males spend most of their time building bowers, which are elaborate structures, constructed solely for attracting females. These loveshacks are decorated with stones (and sticks, bones, etc.) in a very specific way: Small stones are put near the entrance of the bower; Larger stones are put further away. When looking out from the inside of the bower, which is were the females stand during courtship, this arrangement leads to a striking distortion of perspective. You can see this in the image below (compare b to c). In a sense, the size gradient of the stones flattens the image, reducing the subjective perception of depth.
Photos from Kelley & Endler (2012) and Anderson (2012)
The male bowerbird is quite picky about this arrangement. If the size gradient is disturbed (by a biologist, for example), the males immediately restore it. But why? Since the purpose of the bower is to seduce females, it is tempting to speculate that the distorted perspective is aesthetically pleasing to female bowerbirds. Who, as mentioned above, tend to stand inside the bower as they watch the male perform his dance of seduction.
But there could be numerous other explanations. For example, the males could simply be too lazy to carry big stones all the way to the bower. Or something like that.
But no, it appears that the distorted perspective really is what matters. In a recent issue of Science, Kelley and Endler investigated what the perfect size gradient is. And they found that the gradient that gives the optimal distortion of perspective (which is not necessarily the steepest gradient) is most successful, in the sense that it leads to the highest number of copulations.
Kelley and Endler describe their finding by saying that the male bowerbirds construct an optical illusion for their prospective partners. And they are right. By arranging the decorations in this particular way, the male bowerbirds confuse the females' visual perception. So, by definition, it is an optical illusion.
And, just like people, female bowerbirds enjoy a good illusion!