Various theories suggest functions for the casques of hornbills, including feeding, fighting and sex and species recognition. This study tests whether these structures, most of which are predominantly hollow, could act as resonating chambers, amplifying or otherwise modifying the loud calls of these birds. Fibreglass replicas were made of the casque cavities of 15 hornbills of eight species, ... [Show full abstract] using museum skins and x-rays as guides. Analyses of the resonance characteristics of the replicas were then compared with analyses of the calls of each of the eight species. Within each species the range of resonance frequencies was found to correspond closely to that of the fundamental frequencies of the calls but not with that of the dominant frequencies of the calls, except in one instance. There was, therefore, no evidence that the casque specifically amplifies the dominant notes in the call. We suggest that there may be an amplification effect operating through a feedback mechanism from the casque to the syrinx, along with reasons why some species possess hollow casques while others do not.