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.