To do this, they presented the pigeons with three images containing one, two, or three objects. All three images appeared at once on a touch screen and the pigeons pecked the screen to make a response. If they correctly accomplished the task -- pecking the images in ascending order -- they received a wheat snack.
"We took steps to ensure things like volume could not control responding in training and testing," he said. "For example, during training and testing, the higher numerosity did not always have the largest surface area/volume and thus the pigeons could not respond based on this stimulus dimension."
The images also came in different colors and shapes, so the pigeons weren't somehow linking those qualities to quantity.
Next, the researchers upped the ante, to see whether or not pigeons had just learned to order 1, 2, and 3, or if they'd learned a more abstract rule. Scarf and his team presented the pigeons with pairs of images containing 1, 2, 3, 4, 5, 6, 7, 8, or 9 objects. The pigeons again had to pair the items in ascending order. For example, if a pigeon saw 8 and 5, it had to peck the objects representing 5 first.