Aerial and terrestrial alarm calls in chickens are acoustically distinct. The aerial call is a single low-frequency descending note, hard to locate; the ground alarm is a repeated, sharply modulated series. Playback experiments show that hens hearing the aerial call crouch and scan the sky, while the ground call produces upright vigilance and horizontal scanning, without any predator present.
This functional referentiality is one of the better-documented cases outside primates. Calling is also audience dependent: a cockerel gives more alarm calls when a hen is present than when alone or with a bird of another species, and gives the food call tidbitting display more often when a hen can see him. Males have been recorded producing food calls when no food is present, in the presence of hens.
The usual claim that chickens are stupid does not survive contact with this literature. Chickens perform transitive inference in social ranking tasks and show some delayed-gratification behaviour. What is genuinely unclear is whether the calls encode a mental category of predator type or simply the level and direction of danger.