Spatial and temporal patterns of bat use of water developments in southern Arizona

Abstract

I studied bat activity at 13 artificial and 4 natural water sources in southern Arizona from May 1995 through August 1996 using mist-nets and ultrasonic detectors. I used 2 indices of bat activity, capture rates, calculated from mist-netting data, and bat passes/30 minutes, calculated from ultrasound data. Both methods provided similar information on temporal patterns of bat activity. Indices of bat activity were highest during the reproductive season (May through August) compared to the non-reproductive season (September through April). Bat activity did not differ between any of the 2-week time periods within the reproductive season. During the reproductive season, bat activity was bimodal with peaks during the first 2 hours after sunset and the 2 hours preceding sunrise. During the non-reproductive season, bat activity was unimodal, with the majority of activity occurred within 2 hours of sunset. I captured six bat species within the study area. Sex ratios tended to be male biased during the non-reproductive season but close to 1:1 during the reproductive season. Sex ratios did not differ between natural and artificial water sources for any of the bat species compared. Age ratios did differ significantly between natural and artificial water sites. A higher percentage of the total captures at natural water sources were juveniles compared to artificial water sources, possibly indicating higher reproductive output in areas surrounding natural sites. Indices of activity during the reproductive season were significantly higher at natural compared to artificial water sources. However, this difference was not due to higher insect biomass at natural water sources. I developed stepwise multiple regression models to explain variation in bat activity and species richness at water sources. Elevation, type of water source and distance to the nearest cliff or rock outcrop were significant variables in explaining variation in overall capture rates at water sources. Type of water source was the only significant variable explaining variation in ultrasonic activity (passes/30 min), while distance to the nearest source of permanent water and size of the water source explained 33% of the variation in species richness.