Many nocturnal species are known to modify their behavior in response to changes in the lunar cycle/moon illuminance, including bats. Previous work in the tropics has shown that bats may change their activity as a function of lunar cycles to avoid heightened predation risk from owls during the full moon, and reduced capacity to see their own insect prey during moonless nights. It is unclear if bats in temperate regions of the American West show similar patterns of activity in relation to the lunar cycle. In this study, we observed how bat activity changes in response to the lunar cycle and percent moon illuminance in temperate environments by monitoring their ultrasonic calls. We used paired Pettersson D500X acoustic monitoring devices from November of 2018 to September of 2019 to monitor nightly bat activity in the foothills of the Central Valley.
Results/Conclusions:
We detected a total of 14,292 bat echolocation calls over the course of the study. We binned the average number of calls into three lunar categories: new moon, full moon, and half-moon, which included both the first and third quarter. The average number of calls per cycle were 59 for moonless nights, 62 for full moon, and 154 for half-moon. Similar to previous work in tropics, bats in our study show elevated echolocation activity during the half moon phase, presumably as they balance the risk of predation by owls during bright nights with the increased insect prey capture that half-moon nights provide over moonless nights. We also analyzed activity as a function of percent moon illuminance and had peak call activity at 30%, 60%, and 90% illuminance in a remarkably consistent pattern between our paired sites, suggested cycles of activity strongly dependent on lunar illuminance (or some other co-occurring factor).