VOCALIZATIONS OF THE CHRISTMAS ISLAND HAWK-OWL NINOX-NATALIS - INDIVIDUAL VARIATION IN ADVERTISEMENT CALLS

Six different calls of the Christmas Island Hawk-Owl Ninox natalis wer e recorded during a two-year study of its biology. These vocalisations showed many structural and functional similarities with those describ ed for the Southern Boobook N. novaeseelandiae. Of 195 advertisement c alls by 11 owls, 91.3% were correctly attributed to an individual usin g a multiple discriminant analysis model comprising five measurements of call frequency and duration. We provide some evidence for the calls of individuals being constant over time. Individual variation in adve rtising calls may have some use in estimating owl densities, especiall y in areas where the terrain makes traditional techniques such as radi o-tracking impractical.