Visual illusions in great bowerbirds

Great bowerbird bower

Male bowerbirds build structures called bowers that are used purely to attract a mate. The specific design of the bower is unique to each species, but in every species females will visit a handful of bowers and then select a mate based on the attractiveness of his bower. Male great bowerbirds (Ptilonorhynchus nuchalis) build avenue-type bowers with large piles of grey stones and white bones at either end called courts as seen in the photos on the right and  below. These objects increase in size as distance from the bower avenue increases.  This creates the visual illusion of forced perspective when a female views the male display court from inside the bower.

Forced perspective occurs when unconscious assumptions about the sizes and distances of objects are violated. For example, in architecture a building can be made to look taller if the windows get smaller with each consecutive storey. Our brains assume that the windows in the building are all the same size and therefore when standing on the ground looking up at the building we get the impression that the building is taller than it really is. Bowerbirds do the reverse: the objects on their courts increase in size as they get further away.

Males that create high quality illusions mate with more females than males that have lower quality illusions, and this provides evidence that not only can non-human animals create visual illusions, but also that they can be used to promote mating success. Watch a video here! Interestingly, each male constructs his visual illusion to his own specifications so there is a lot of variation in illusion quality. These differences among males are also consistent over time, but males seem to improve slightly year on year when constructing their illusions.

PhD researcher Menno is currently investigating whether the quality of illusion construction is a reflection of male cognitive ability, and testing how these illusions affect female perception. He is currently running an online experiment to test what the visual effects of the illusion are – if you want to see what it’s like being a female bowerbird you can contribute to his experiments:

Experiment 1: Bigger or smaller?

Experiment 2: Quick draw: can you find the decoration?

Experiment 3: How big is the decoration?

Experiment 4: Just how big is this court anyway?

Each experiment only takes a few minutes to do and you’ll be contributing to our knowledge of how female bowerbirds perceive the forced perspective illusion. Thanks very much for your time!