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).
INSERT FIG. 2-5 HERE