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.