Supplementary material from "How do great bowerbirds construct perspective illusions?"

Great bowerbird ( Ptilinorhynchus nuchalis ) males build and maintain display courts by placing tens to hundreds of objects in a positive size–distance gradient. The visual angles created by the gradient create a forced perspective illusion that females can use to choose a mate.

Although the quality of illusion is consistent within males, it varies among males, which may reflect differences in how individuals reconstruct their courts. We moved all objects off display courts to determine how males reconstructed the visual illusion. We found that all individuals rapidly created the positive size–distance gradient required for forced perspective within the first 10 objects placed.