Hens use occlusion to judge depth in a two-dimensional picture

Hens (Gallus gallus domesticus) were taught to peck at a touch screen. On the screen was a grid on which a square and a circle were depicted. The square and the circle were given different positions at random for each trial, but were never overlapping. The hens were rewarded for pecking at the symbol that was higher up on the grid/screen, i.e. at the one that to a human observer was seen as being further away. Every tenth trial was a probe trial in which the animals were presented with either the circle overlapping the square or vice versa. The hens were never rewarded during the probe trials. As mentioned, the hens had learned to peck at the symbol that appeared to be further away during the nonprobe trials. During the probe trials the hens pecked at the symbol that was occluded, i.e. in the absence of any other cues they used occlusion to determine which of the two symbols was further away. The results suggest that not only can nonhuman animals use image height as a cue but that they can generalise this to situations in which occlusion is the only depth cue present.