2.4 Data processes
All our filmed pictures are sufficiently stable to distinguish each
behavioural state and long enough to meet at least 5 samples of
vigilance/non-vigilance bouts for statistical effectiveness, even for
our shortest video sampling of 2.2 min, as scan and inter-scan duration
for black-necked crane are 4s and 20s on average (Li et al., 2017). So,
all our data were taken into subsequent analysis. Vigilance behaviour
was evaluated by both vigilance proportion and duration for each group
member. Vigilance proportion was calculated as percentage of time spent
on vigilance during the observation secession while vigilance duration
as the average time span (seconds) of each vigilance bout (Li et al.,
2017). Studies documented that group members could coordinate individual
vigilance in order to increase group collective vigilance (Rasa, 1986;
Bednekoff, 2015), and in small crane groups (Ge et al., 2011). We
focused on the collective vigilance time pattern (time serials) of two
adults in family groups, as no vigilance interactions between juveniles
and adults were detected (Ge et al, 2011; Che et al., 2018).
We classified the areas where cranes occurred into low, intermediate and
high disturbance levels according to tourist accessibility. Cranes
distributed along the road to Dahaizi lake and Jigong mountain where
nearly 90% tourist visited facing the highest disturbance, moderate
disturbance in the areas around Tiaodunhe lake with less 30% public
visitation, and lowest disturbance in the other areas. We considered the
observer (data collector) as proxy of potential tourists, the distance
from observer to cranes is another effect variable on vigilance. Only
one observer approached crane without aggressive behaving e.g.,
shouting, chasing, means the lowest disturbance from tourist. Because
inexperience always correlated with high predation vulnerability, a
higher proportion of juveniles in groups usually indicated higher
predation risk (Xu et al., 2013; Beauchamp, 2015). We considered our
study objectives of three family typs with two adults and 0-2 juveniles
as three levels of predation risk; and adults without juveniles endure
the lowest predation risk while families with two juveniles have the
highest predation risk.
We found that our data sets deviated from normality in a one-way
Kolmogorov-Smirnov test, and arcsine square root transformation for
vigilance proportion and logarithmic transformation for vigilance
duration were subsequently conducted in order to get normalized data for
parametric tests. Comparisons of vigilance difference between adults and
juveniles and observed to expected collective vigilance of two adults
were accomplished with t-test. One-way ANOVA was used to test both
individual and collective vigilance difference of adults in three family
types. We selected general linear model to distinguish effects of
disturbance level (categorical variable of three levels), observer
distance (continuous variable) and predation risk (categorical variable
of three family types) on both individual and collective vigilance
deviation of black-necked crane (Ge et al., 2011). Collective vigilance
deviation was used to determine collective vigilance pattern
(synchronization or coordination) by considering the deviation between
expected (independent vigilance) and observed collective vigilance (Pays
et al., 2007a, b). Expected collective vigilance was calculated with the
equation of 1-[(1-p1 ) *
(1-p2 )], where p represents vigilance
proportion of two adults in a family (Pays et al., 2007a, b; Ge et al.,
2011). Statistics were accomplished with IBM SPSS 20.0 software with a
two-tailed significant level of p<0.05.