We initiated a 5-year study in mid-summer 2005 to determine patterns of bat activity and evaluate the use of acoustic monitoring to predict fatality of bats at a proposed wind energy facility in south-central Pennsylvania. The primary objectives of this study are to 1) determine level and patterns of activity of different species groups of bats using the proposed wind facility prior to and after construction of turbines; 2) correlate bat activity with weather and other environmental variables; and 3) determine if indices of pre-construction bat activity can be used to predict post-construction bat fatalities at proposed wind facilities.
We recorded echolocation calls of bats with Anabat II zero-crossing ultrasonic detectors programmed to record calls each day from one half-hour before sunset to one half-hour after sunrise each day of the study from 1 August to 1 November 2005. We used meteorological (met) towers and 22 m tall, portable, telescoping towers to vertically array detectors for acoustic sampling during this study. We recorded calls at proposed turbine locations from detectors deployed on 3 met towers (one detector at 1.5, 22, and 44 m high at each tower) and from 6 locations using portable towers (one detector at 1.5 and 22 m high at each tower) from a forested ridge and from 2 met towers and 4 portable tower locations on an open strip-mined ridge.
We recorded a total of 9,162 bat calls from all detectors and tower locations combined from 1 August through 1 November 2005. Bat activity was highly variable throughout the study period, but generally highest from mid-August through mid-September with brief peaks of high activity in October. Bat activity generally was highest immediately after sunset and declined through the night until just before sunrise the following morning. High (>35 kHz, e.g., Myotis species) and low (<35 kHz, e.g., Lasiurus spp.) frequency-emitting echolocating bats tended to fly at different heights on the study area. While the two species groups had approximately equal activity levels at 22 m, activity rate of high frequency-emitting bats was estimated to be 9–59% higher than that of low frequency bats at 1.5 m. This trend was reversed at 44 m where it was estimated that activity rate of low frequency-emitting bats was 17–210% higher than that of high frequency bats. The height at which either species group tended to fly differed in the two habitats. Although activity rates for either species group at 44 m were approximately equal in forest and open habitats, it was estimated that activity rate in the forest habitat was 9–61% higher than in open habitat at 1.5 m. This trend was most extreme at 22 m, where it was estimated that activity rate in forest habitat was 99–229% higher than in open habitat.
The best model and eleven other models in the 95% candidate set all included linear effects of temperature and wind speed, quadratic effect of temperature and the interaction of temperature with height. Total bat activity increased with increasing temperature up to about 19–21o C, after which activity began to decline. While bat activity was positively related to temperature, the effect differed at different heights. For every 1o C increase in temperature, bat activity increased 7–13% at 1.5 m, 0–7% at 22 m, but was unaffected by temperature at 44 m. The optimum temperature for maximum activity was similar for the two species groups in both habitats. Wind speed was less than 6.5 m/s (23.4 km/h) on 80% of the nights and the highest wind speed recorded was 15.7 m/s (56.5 km/h); even at wind speeds above 6.5 m/s, there was still some bat activity in both species groups. The effect of wind speed was the same for both species groups in both habitats and at all three heights. For each 1 m/s (3.6 km/h) increase in wind speed, activity rate was estimated to decrease by 11–39%. Activity patterns of the two species groups were similar in both open and forest habitats at 44 m, but at 22 m was between 2 and 3 times higher over forests than in the open habitat.
This study was conducted at one proposed wind energy facility located on a forested ridge and an open, reclaimed ridge that had been previously strip-mined, so statistical inferences are limited to this site. However, we believe that our findings likely reflect patterns of bat activity on similar forested and open ridges with comparable vegetation composition and topography in this region. We caution that our study only encompasses the late summer-fall period and does not represent a full period when bats are known to be active (generally April through November). Analyses presented in this report are exploratory, in part because so little data exist upon which to develop a priori, confirmatory hypotheses and associated candidate models. The current analysis only estimates activity rates and differences in activity patterns of two species groups (high and low frequency), in forested and open habitat, and at three heights.
We began a second year of pre-construction acoustic monitoring in mid-April 2006 that will continue through the end of October 2006. Turbine construction for this site is tentatively scheduled for summer-fall 2007, after which we will gather two years of post-construction activity and fatality data from April through October in 2008 and 2009.