Male chickens, Gallus gallus, produce aerial alarm calls in response to a broad range of stimuli moving overhead, including both predators and innocuous objects. Computer-generated animations of raptor-shaped images were presented on an overhead video monitor to explore the stimulus characteristics necessary for eliciting both aerial alarm calls and non-vocal anti-predator behaviour (e.g. crouching and visual fixation). In experiment 1, apparent size (angle subtended at the bird's eye) was varied from 1° to 8°. Stimuli subtending more than 4° elicited qualitatively more alarm calls than smaller stimuli. The magnitude of non-vocal responses also increased significantly with stimulus size. In experiment 2, apparent speed was varied over a range extending from values characteristic of soaring raptors at one end (1.875 lengths/s), to values near the highest recorded (30 lengths/s) at the other. Stimuli moving faster than 7.5 lengths/s evoked significantly more alarm calling than slower-moving stimuli. Increases in apparent speed also caused a small but significant increase in the magnitude of non-vocal responses. Hence, the non-vocal anti-predator behaviour elicited by overhead stimuli is strongly influenced by apparent size. In natural encounters, this cue varies with object altitude and is likely to be a good predictor of the probability of detection and attack by a raptor. The data on vocal responses suggest that, at least when shape is constant, the probability of alarm calling is dependent on whether stimuli exceed threshold values for apparent size and speed. This perceptual strategy has the advantage of requiring minimal processing and may be adaptive for species, like the jungle fowl, that have evolved in habitats where visibility is limited and reaction times must therefore be brief.