Effect of roost size on the emergence behaviour of pipistrelle bats

We refined established methods for examining the temporal organization of b ehavioural events and applied them to the study of bats emerging from roost sites. Previous studies have shown that as roosting numbers of bats increa se, temporal patterning (clustering) in their emergence behaviour, as measu red by chi(2) or G statistics, also increases. Expressing the extent of tem poral clustering using chi(2) or G, however, confounds two effects. The sta tistic 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. U sing this method, we examined intra- and inter-roost variation in the tempo ral patterning of emergences of the pipistrelle bat, Pipistrellus pipistrel lus, from four roosts in northeast Scotland. The 'corrected' expression of clustering decreased as roost size increased, which was opposite to the eff ect found with the uncorrected expression and also opposite that predicted a priori from the bottleneck hypothesis. This novel result suggests bottlen ecks may disrupt clustering rather than promote it. This latter interpretat ion 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 o n clustering behaviour. (C) 1999 The Association for the Study of Animal Be haviour.