4.2 Collective vigilance and affecting factors
Our study indicated that cooperative vigilance shift was disturbance and predation risk dependent. Many studies demonstrated that cooperative sentinel activity (one form of coordinated vigilance) increase when pups are present (Santema and Clutton-Brock, 2013) and when predation risk becomes greater (Ridley et al., 2010; Rauber and Manser, 2017). These findings support our findings that cranes coordinate vigilance pattern when more young birds are present, in high disturbance areas and when humans show proximity. In our study, we considered the number of juveniles in a family as predation vulnerability or risk (Xu et al., 2013), as predators prefer hunting those left behind (Sirot and Touzalin, 2009), more juveniles cost more individuals and collective alert time of adults (Table 1). Leading parents to adopt time-consuming coordinated vigilance strategy (Figure 2) as our study indicated. Significant interactive effects of family type and observer distance on adult collective vigilance indicate that cranes with juveniles are more vulnerable to human disturbance. Our study also emphasizes that coordinated vigilance or sentinel behaviour may actually exist in relatively vast systems in small family groups (Ward, 1985; Wickler, 1985), under circumstances with significant disturbance and predation risk (Rauber and Manser, 2017), even for monogamous breeding systems (Mainwaring and Griffith, 2013) as in black-necked cranes.