2.3 Data collection
We conducted behavioral observations of black-necked cranes during 09:00-18:00 in the winter 2013-2014. Cranes were firstly detected along a 96 km long line transect by vehicle (Figure 1). Once cranes were located one observer walked to the focal flock and collected behaviour data of the birds with a digital video recorder (Sony HDR-XR260E with 55× optical zoom) set on a tripod. Meanwhile distance to the birds was measured with a laser rangefinder monocular (Newcon Optik LRM 2200SI with a maximum range of 2,200 meters). We aimed to take 10 minutes (mins) film for each family group encountered, because it is sufficiently long to get sufficient vigilance bouts for black-necked cranes (Li et al., 2017). In order to get time series data of each family member, we made sure each individual was in the picture at the same time; film recording ceased as long as one bird was out of sight or flew away (Ge et al., 2011). Vigilant and non-vigilant behavior were extracted in second (s) from videos taken in the field. We defined head-up scanning with neck stretched while standing, walking or even laying down as vigilance (Kong et al., 2020), otherwise as non-vigilant.