Effects of Vocal Behaviour on Abundance Estimates of Rainforest Galliforms

Abstract

A comparative study was undertaken to assess the effects of the vocal activity of a target species on its density estimates made using line transect counts and point counts. In Kayan Mentarang National Park (Indonesian Borneo) the Great Argus Pheasant Argusianus argus displayed different levels of calling activity. The study was conducted at a period when the birds were highly vocal, and again, at the same site, when the birds were significantly less vocal. Transect counts during periods of low vocal activity resulted in 13–20% lower density estimates compared to periods of high vocal activity, but these differences were not significant. Estimates derived from five-minute point counts during periods of low vocal activity, however, were 52% lower than those from periods of high vocal activity. Comparison of the two methods shows that density estimates derived from the point count method were consistently lower than those from the line transects. The lack of a fixed calling season in Argus Pheasants makes year-round censuses possible, yet the distinct temporal differences in calling rates necessitates that caution be exercised when results obtained in different years or at different times of the year are compared. It is concluded that there is an increased need for understanding the behavioural plasticity of species if census methods are to be improved.