The dataset consisted of 80 focal observations in 22 different groups ranging in size from 3 to 104. The two nearest neighbours of focal sleeping gulls also slept in 10 focal observations (12.5%). The two nearest neighbours were not generally sleeping and included at least one alert gull in 56 per cent of the observations. Observations were never interrupted by external causes and alert gulls were simply scanning their surroundings.
In the mixed linear model, age and focal species identity were not significant and dropped from further analyses (p>0.30). Group identity accounted for 12 per cent of the total variance in percentage time spent sleeping, which indicates a weak correlation in percentage time spent sleeping among birds of the same group.
Percentage time spent sleeping increased with group size (β(s.e.)=0.39 (0.12), F1,50=10.3, p=0.002). However, a nonlinear trend was apparent and percentage time spent sleeping actually decreased in the larger groups (; β(s.e.)=−0.009 (0.003), F1,50=8.0, p=0.007). Percentage time spent sleeping decreased with increasing average neighbour distance (β(s.e.)=−1.47 (0.53), F1,50=7.7, p=0.008). The behavioural state of the two nearest neighbour influenced percentage time spent sleeping in focal gulls (; F3,50=4.8, p=0.005). Post hoc tests, with the Bonferroni sequential correction, indicated that mean percentage time spent sleeping in focal gulls was significantly lower when the two nearest neighbours were alert rather than either preening or sleeping, and marginally lower (p=0.04 before correction) than when the two neighbours showed non-synchronized behavioural states.
Changes in the percentage time spent sleeping by focal gulls (Larus sp.) as a function of group size. The simple polynomial regression line is drawn to illustrate the nonlinear relationship between sleeping and group size.
Figure 2 Changes in least square means of the percentage time spent sleeping by focal gulls (Larus sp.) as a function of the behaviour of the two nearest neighbours. The mixed category consists of cases where the two nearest neighbours showed different behavioural (more ...)
In loafing groups, not all gulls synchronized their activities, offering an opportunity to compare the behaviour of focal gulls within the same flock when their neighbours were alert to different extents. In groups with more than two focal observations, I matched a focal observation where one or two neighbours were alert to a focal observation where neither neighbours were alert. I then compared percentage time spent sleeping in these matched observations using a paired t-test. Results indicated that focal gulls with more alert neighbours spent on average 22 per cent (s.d.=30%, n=17) less time sleeping than focal gulls from the same flocks with less alert neighbours (t=3.0, d.f.=16, p=0.009).