Yok Don National Park was divided into three sectors (south, center and north) separated by Yok Don Mountain in the south and Chu Minh Mountain in the north. In each sector we located 2 km line transects (15 in total) along the accessible road system that runs within the park (Fig. 1b). One observer walked each transect twice a day for five consecutive days (five observers in total), giving 10 repeat survey per transect.
The number of detected calling males at each survey was treated as a response variable. The location of each calling bird was defined, after collecting bearing and distance estimated from each surveys, using ArcGIS 9.3. Double count of a single calling bird within the same location was corrected for each survey to estimate a number of bird occurrence at each survey.