Water sources are critical for desert bats and receive heavy use, especially when these water sources are in close proximity to established roosts. Lactating bats, in particular, need large amounts of water during early summer. Since the late 1990s, the Department has been renovating older wildlife water catchments, some of which were greater than 40 years old. Many older catchments are being replaced with new designs that have deeper drinking troughs in which water levels fluctuate as water is depleted or recharged. During summer months, water levels can vary considerably and the ability of different bat species to access these new trough designs, especially at low water levels, is unknown.
We are conducting the study in southwestern Arizona. The study site is at an elevation of 980 feet and located within the Lower Colorado River Valley subdivision of the Sonoran Desert Biome. Average annual precipitation is less than 5 inches and is received during winter frontal systems and summer monsoon thunderstorms. A wildlife water catchment has been operational at this site for greater than 40 years. It was recently renovated to a contemporary design with increased storage capacity, lower evaporation loss, and underground components less vulnerable to vandalism or mechanical failure.
In 2006, we installed 2 additional experimental troughs at the study site. Along with the existing system, these represent 3 of the most commonly used trough designs for Arizona wildlife water catchments. From 2006–2007, we observed bat activity during four consecutive nights each month from May–July, the period of peak use by bats. Each night represented a different experimental trial with troughs filled to 100% water level on night 1, 75% on night 2, 50% on night 3, and 25% on night 4. Each evening was divided into 6 15-minute sampling periods, during which 1 trough was open and the others covered to prevent bat access. Each trough was sampled twice per night. We used night vision goggles along with supplemental infrared illumination to count the number of bats that visited an open trough; we classified each visit as a “pass” or “successful drink.” Because bats cannot be reliably identified to species during counts, we are conducting concurrent acoustic sampling with ultrasonic detectors.
Results from the first phase of the study showed similar use of all 3 trough designs at medium-high water levels. However, we observed reduced bat use of the 2 largest troughs at the 25% fill level. Because bats are relatively long-lived and repeatedly visit the same water sources, we suspect they may adapt to these conditions given additional time. We are testing this hypothesis during 2008-2009 field seasons, where we will collect data from a single trough (at low water level) for the entire 2-hour sampling period.
The results of study will help guide selection of catchment designs where providing water for bats is a management objective. It may also be useful in determining minimum water levels at which supplemental water hauling may be needed during drought years.
For more information contact:
Steve Rosenstock, Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086