4.3 Discovering cooperative vigilance pattern by spectral analysis
Different from a recent vigilance synchronization study on black-necked crane wintering at Lhasa, Tibet, Che et al. (2018) did not detect significant deviation between observed and expected collective vigilance. They concluded that no synchronized vigilance occurred for family members. However, it is difficult to figure out which cooperative anti-predator behavioural strategy group members adopted regardless of vigilance spectrum (Pays et al., 2007a, b). As our study indicated no significant difference between expected and observed collective vigilance of two adults, revealing independent scan of two adults. When compared collective vigilance deviation with zero (when collective vigilance deviation above the value of zero, coordination occurs; in contrast, synchronization occurs) as spectral analysis suggested (Pays et al., 2007a, b; Ge et al., 2011), we found parent cranes cooperating their vigilance bouts in different situations (Figures. 2-4). When the family is facing great threats e.g., more juveniles, higher disturbance and more proximity to human observers, adults shift collective scan from synchronous to asynchronous ways, so as to ensure less scan gap left (Pulliam, 1973; Sirot and Touzalin, 2009). Just as our results proved that families with 2 juveniles performed stronger vigilance than families with 0-1 juvenile.