3. Methods
3.1. Density estimation
Surveys were conducted during two breeding seasons (January 2012 and 2013) when the birds frequently call (Brickle et al., 1998, Johnsgard, 1999 and Ponsena, 1988). Density was defined using line transects. In both protected areas line transects were located on existing non-asphalt roads inside the park where traffic was very limited. Each transect was 2 km long and was marked every 20 m before the survey was conducted. Each transect was walked 10 and 14 times, by different observers, during daily peak calling periods. These times were 06.30–08.30 (at Cat Tien in 2012) and 06:00–08:00 (at Yok Don in 2013) in the morning (time adjusted to reflect difference in the start of calling at each site) and 16.15–18.15 in the evening. The distance to each calling bird was assigned within 50 m distance categories and the farthest detection distance was set at 1 km for both auditory and visual detection from the observer (Brickle et al., 1998 and Indrawan, 1995). Before data collection started, all observers conducted preliminary surveys together to standardize estimates of distance to calling birds in order to minimize errors between observers (Buckland et al., 2001).