By now, the intelligence of birds is well known. Alex the African gray parrot had great verbal skills. Scrub jays, which hide caches of seeds and other food, have remarkable memories. And New Caledonian crows make and use tools in ways that would put the average home plumber to shame.

Pigeons, it turns out, are no slouches either. It was known that they could count. But all sorts of animals, including bees, can count. Pigeons have now shown that they can learn abstract rules about numbers, an ability that until now had been demonstrated only in primates. In the 1990s scientists trained rhesus monkeys to look at groups of items on a screen and to rank them from the lowest number of items to the highest.

They learned to rank groups of one, two and three items in various sizes and shapes. When tested, they were able to do the task even when unfamiliar numbers of things were introduced. In other words, having learned that two was more than one and three more than two, they could also figure out that five was more than two, or eight more than six.