Temporal Variation in Activity of Bats and the Design of Echolocation-Monitoring Studies

TLDR

Temporal variation in bat activity can bias estimates if not accounted for in research design. The study aimed to evaluate how temporal variation in bat activity affects sampling by subsampling data over 2–12 nights. Researchers recorded bat echolocation with Anabat II detectors over 195 detector‑nights at two streams and then randomly subsampled 2–12 nights to compute mean activity levels. Bat activity correlated positively with insect biomass and minimum temperature, peaked after sunset and before sunrise, varied widely nightly, and subsampling of seven or more nights captured the overall mean within 20% for over 60% of subsets, while blocked or paired designs increased sampling efficiency by 20%.

Abstract

I used Anabat II bat detectors to monitor echolocation calls of bats over two streams in the Oregon Coast Range for a total of 195 detector-nights. Activity of bats was positively correlated with biomass of insects and minimum nightly temperature, and was negatively correlated with length of night; activity levels at the two streams were positively correlated. Activity of bats was not significantly correlated with either hours of moonlight or with phase of moon. Level of activity within a night generally peaked shortly after sunset with a second, smaller peak in activity shortly before sunrise, but patterns varied substantially among nights. Total nightly activity at a site also varied substantially among nights, sometimes varying several-fold on consecutive nights. 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 for each subset. For subsets with seven or more nights, >60% of the subsets 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 entire dataset for any number of nights subsampled. When comparing activity between sites, use of blocked or paired designs improved sampling efficiency by 20%. Failure to account for temporal variation in activity of bats when designing research projects and monitoring programs could result in biased estimates of activity of bats.