Discrimination of shape and size sues by day-old chicks in two one-trial learning tasks

•Ability of day-old chicks to classify shape and size was studied in two tasks.•Chicks could discriminate sizes, but not shapes, in the taste avoidance task.•Chicks could discriminate shape, but not size in sickness-conditioned learning.•Chicks rely on different cues for different learning tasks.

The ability of day-old chicks (Gallus gallus domesticus) to discriminate between the shape and size of beads was investigated in two one-trial tasks, taste avoidance and sickness-conditioned learning. Previous studies determined that color is a critical classification cue for conditioned stimuli in these tasks. In taste avoidance learning, a chick pecks a bead coated with a bitter substance. In sickness-conditioned learning, chicks peck a dry bead and are injected 30 min later with lithium chloride. Chicks could discriminate beads of different sizes, but not different shapes, when trained in the taste avoidance task, whereas in the sickness-conditioned learning task, chicks could discriminate shape, but not size. These results suggest that chicks use a number of classificatory cues to remember an avoidance response, and, in the absence of color cues, chicks rely on different cues for different learning tasks.