3.3 Collective vigilance pattern
The deviation of observed and expected collective vigilance did not differ significantly from zero (t =-1.127, df =31,p =0.268); and deviation between expected and observed collective vigilance did not differ significantly among family type (F =1.940, df =2, p =0.340), disturbance level (F =0.699, df =1, p =0.491) or observer distance (F =0.740, df =20, p =0.718).
However, we found, in family group, two adults behaving synchronized collective vigilance in low disturbance areas when juveniles were absent, synchronization decreased with increased disturbance and predation risk (more juveniles in group) to independent vigilance, and then to coordinated collective vigilance at high level of disturbance with two juveniles (Figures. 2, 3).
We also found that collective vigilance deviation varied as a function of observer distance, that crane couples shifted synchronously to coordinate vigilance with observer approach (Figure 4). On average, adults shifted collective vigilance from synchronization to coordination when observers were 400m away for the most vulnerable family with two juveniles, while two adult birds with 0-1 juveniles kept synchronized vigilance (Figure 4). While black-necked cranes could endure human approach to about averagely 100m at population level (Appendix Figure 1).
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