Abstract
Intimidatory behaviour of nestling Bubo bengalensis to human intruders was investigated in five subjects. This disclosed a series of distinct behavioural units- viz., hissing, ruffled plumage, bill clicking and the full intimidatory display - increasing in intensity from a crouched concealing posture and linked by short duration transition phases. Often no reaction to human presence was discernable. This was due to the effects of repeated stimulation/ of the parents' alarm calls/ developmental stage of the young. The rationale for these intensity types and behavioural patterns, with emphasis on motivational and phylogenetic elements has been discussed.