Male bower birds boast an architectural prowess, it is true. They also have a discerning eye when it comes to the color palette for their homes. It turns out that, if that weren’t enough, these birds also use forced perspective, arranging stones in their court in size order to create an optical illusion for the female who is shopping around for a mate.
This is a bower from the one of the avenue species of bower birds — those who build a long avenue out of sticks, with a court at the end made of stones, shells, bones and bits of colored plastic. The female stands outside the avenue (where the photographer was lying to take this picture) and looks through it to the male bower bird who is dancing around on the stones at the back. The funny thing about this picture is that to us the stones look like they are all a similar size, but they are actually arranged with the largest ones in the back, and the smallest ones in the front. If you switch the positions of the stones, as Endler, Endler and Door (Current Biology, 2010) did in this photograph…
… the males will move them back into the opposite size gradient within three days.
The males are creating variation of an Ames room, sort of like this one:
The trick in this picture is that the room is actually much deeper and taller on the left side, and so the leftmost suited guy looks really tiny, whereas the suit on the right is standing closer downstage, on the smaller side of the irregularly shaped room, which makes him look huge. One caveat of this illusion is that it only works if the viewer is standing at one particular point, but this is guaranteed of the female bower bird because she has to look down the long narrow avenue of twigs to see the court. One of the possible reasons the male bower bird creates this Ames court might be to make himself look bigger in the front of the court when compared to other objects placed in the back next to the bigger stones, like the suit on the right.
The bower photographs are from the research of Endler et al, which can be found in this paper from September. The Ames room photograph was grabbed from this blog. More on bower birds and female chosiness can be found in an earlier video of ours on picky females.