TEMPORAL VARIATION IN ACTIVITY OF BATS AND THE DESIGN OF ECHOLOCATION-MONITORING STUDIES

I used Anabat II bat detectors to monitor echolocation calls of bats o ver two streams in the Oregon Coast Range for a total of 195 detector- nights. Activity of bats was positively correlated with biomass of ins ects and minimum nightly temperature, and was negatively correlated wi th length of night: activity levels at the two streams were positively correlated. Activity of bats was not significantly correlated with ei ther hours of moonlight or with phase of moon. Level of activity withi n a night generally peaked shortly after sunset with a second, smaller peak in activity shortly before sunrise, but patterns varied substant ially among nights. Total nightly activity at a site also varied subst antially among nights, sometimes varying several-fold on consecutive n ights. To assess the implications of temporal variation in activity of bats on sampling, I randomly sampled subsets of the data using from 2 - to 12-night sample periods and calculated mean levels of activity fo r each subset. For subsets with seven or more nights, >60% of the subs ets had means that were within 20% of the mean of the entire dataset. Less than 50% of the subsets had means within 10% of the mean of the e ntire dataset for any number of nights subsampled. When comparing acti vity between sites. use of blocked or paired designs improved sampling efficiency by 20%. Failure to account for temporal variation in activ ity of bats when designing research projects and monitoring programs c ould result in biased estimates of activity of bats.