Abstract
We estimated detection, dispersion and density of 10 hornbill, one pheasant and nine diurnal raptor species at Bala forest, southern Thailand, throughout 2004. We conducted 620 1-h lookout watches over the forest canopy from 14 elevated locations along 13.1 km of paved road through Bala, during which we recorded all detections and movements of target species. For a further 619 h, we also recorded all in-forest detections of target species during excursions along and adjacent to the road. Our records covered all months of the year and hours of the day, enabling us to estimate seasonal and diurnal patterns of detection for each target species. We found significant seasonal and diurnal variation for all commoner species in rates of overall detection, visual versus aural detection, and vociferousness. We related most monthly variation to the timing of nesting and most hourly variation to aspects of the behavioural ecology of each species. We used three techniques to estimate dispersion and density for each species, two from lookouts based on our counts of individuals or breeding units per unit area observed, and one from spot maps of core breeding units within 500 m of the road. Differences in our estimates of density varied within and between species, but were mostly lowest for the mean number of individuals detected from lookouts and highest for the maximum of breeding units, whether from lookout detections or core area counts from spot maps. These ranges of densities and their variance indicate upper and lower estimates for each species, and the differences between them and other survey techniques were explicable partly by the behavioural ecology of each species or apparent changes in density. Our density estimates never exceeded the highest comparative estimates for target species or near relatives in other forest habitats, although few were available for diurnal raptors. We suggest that 1-h lookout watches over forest, where feasible, offer alternative and efficient estimates of detection, dispersion and density to conventional distance-sampling techniques conducted from within forest. They will achieve optimum accuracy when conducted during the most detectable periods for target species, particularly for obvious and sparsely dispersed bird species.
