"Meerkats — those charismatic mongooses that stand on their hind legs to scan for predators — give calls that announce both the general type of predator (coming from the sky, coming from the ground) and how close it is — in other words, how urgently everyone should react. Black-capped chickadees — small songbirds that live in North America — have calls that say whether a predator is flying or resting, and if it is resting, how dangerous it is."