Effect of roost size on the emergence behaviour of pipistrelle bats

We refined established methods for examining the temporal organization of behavioural events and applied them to the study of bats emerging from roost sites. Previous studies have shown that as roosting numbers of bats increase, temporal patterning (clustering) in their emergence behaviour, as measured by chi(2)or G statistics, also increases. Expressing the extent of temporal clustering using chi(2)or G, however, confounds two effects. The statistic may increase because clustering increases. However, an increase will occur, even if the amount of clustering stays constant, if the sample size of observations increases. A positive relation between emergence size and chi(2)or G may not necessarily reflect increased clustering. We developed a method to separate these effects using computer-modelled event streams. Using this method, we examined intra- and inter-roost variation in the temporal patterning of emergences of the pipistrelle bat, Pipistrellus pipistrellus, from four roosts in northeast Scotland. The 'corrected' expression of clustering decreased as roost size increased, which was opposite to the effect found with the uncorrected expression and also opposite that predicted a priori from the bottleneck hypothesis. This novel result suggests bottlenecks may disrupt clustering rather than promote it. This latter interpretation was supported by observations that clustering was significantly reduced in the middle of large emergences. Variation in clustering between roosts was significant, suggesting exit topology may have an important influence on clustering behaviour. Copyright 1999 The Association for the Study of Animal Behaviour.