Gray Bat (Myotis grisescens) Thermal Infrared Monitoring in Missouri, 2008-2011

We report the results of three years of thermal infrared (TIR) video monitoring of gray bats, Myotis grisescens, in Missouri. From 2008-2011 we visited 14 caves, including 13 maternity caves and one major hibernaculum, taking 50 videos and 5 guano estimates. The mean outflight was about 42,000 bats, and the largest was more than 311,000. We recorded an aggregate of 2,115,000 gray bats with TIR and the Thermal Target Tracker (T 3 ) program. Comparing three methods we have used, we found that for accuracy and reliability we prefer the methods in this order: TIR/T 3 method over our own near-infrared (NIR) method over guano measurements. Our NIR method is less expensive and reasonably accurate for outflights up to about 100,000. Guano measurements may still be used at some caves as a backup method if the guano piles are distinct and especially if there was poor weather or insufficient time to record TIR at the cave at the right time. We conclude that gray bat numbers in Missouri are generally stable or increasing, and that there are mass migrations in late summer from the Lake of the Ozarks area in central Missouri to Boone County and other areas. We are extending the TIR/T 3 method further to count bats in fall swarms or emerging from hibernation in the spring.