Bat Conservation and Management Workshop

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BCI Bat Conservation and Management Workshop – Arizona cover was 100% in most areas, and branches of alders interdigitated, creating the appearance of a tunnel of air space over the stream. I monitored activity of bats for 94 nights at Bark Creek and for 101 nights at Buttermilk Creek between 29 June 1993 and 12 October 1994 using the Anabat II bat detector system according to methods described by Hayes and Hounihan (1994). Echolocation calls were recorded on audio tape as bats flew over or near a monitoring station. I defined each sequence of one or more echolocation pulses with < 1 s between sequential pulses as a pass by a bat (Fenton, 1970). Calls were recorded along with the time of day and a calibration tone to aid in later analysis. Bat detectors were set at a sensitivity of six to minimize stream and insect noises and to eliminate detections of bats flying in adjacent habitats. Each monitoring station was within 3 m of the edge of the stream with the microphone of the bat detector facing parallel to the main axis of the stream. I sampled populations of insects for 89 nights at Bark Creek and for 87 nights at Buttermilk Creek between June 1993 and October 1994 using 10-watt black light traps (Bioquip, Santa Monica, CA) powered by 12- volt gel cells. Traps were set to operate for a 3-h period beginning 30 min after legal sunset using a 12- volt timer (Real Goods, Ukiah, CA). Insects were collected in alcohol, oven-dried $ 24 h, and weighed. Preservation of invertebrates in alcohol decreases their dry weight biomass (Leuven et al., 1985), and, thus, estimates of dry mass may be biased and only should be considered as indices. I monitored minimum nightly temperatures at the bat-monitoring stations using Hobo-Temp monitors (Onset Instruments, Pocasset, MA). Analysis of activity levels and environmental correlates. An analysis of 1,879 passes recorded during 10 randomly selected nights at Bark Creek and at Buttermilk Creek revealed that > 99% of identifiable calls had characteristics typical of species of Myotis (Hayes and Adam, 1996). Because of similarities in characteristics of echolocation calls among species of Myotis in this geographical area, I did not attempt to categorize calls to species in this study. I used an index of activity (IA) as a measure of activity levels. For nights when bats were successfully recorded throughout the night, IA is the total number of passes recorded. During nights with highest levels of activity, audio tapes were filled with calls of bats before the end of the night. To determine the IA for these nights, I assumed that the best estimate of total activity was a function of the number of passes recorded, the proportion of the night elapsed when the tape was filled, and the proportion of the total number of passes expected to occur during that portion of the night. This approach is not ideal, as patterns of activity can vary among nights. However, this approach should provide a general index of activity that is acceptable for use of rank-order statistical procedures. To determine patterns of activity within nights for use in calculating IA and for assessment of temporal pattern, I determined each night at Bark Creek for which audio tape were not completely filled with calls before the end of the night and for which at least 175 passes were recorded (n = 24 nights). I restricted the analysis to these night because I assumed that the pattern of activity in these nights with relatively high levels of activity ($ 175 calls) would most closely reflect the pattern of activity on nights for which the tapes were filled with calls prior to the end of the night. To account for differences in length of night, activity was partitioned into 20 equal-time intervals from sunset to sunrise these intervals varied from 26 to 41 min (0 = 29.7 min). The proportion of passes recorded in each interval was determined and the mean proportion for all these nights was calculated. For nights when the tape was filled with calls of bats before the end of the night, the IA was calculated by dividing the number of passes recorded by the mean proportion of passes recorded in that proportion of the night. I tested for correlations between nightly IA at Bark and Buttermilk creeks, and between IA at each site and length of night, hours of moonlight, phase of moon (expressed as a percentage of full moon), dry mass of insects captured at the site, and minimum nightly temperature at the site using Spearman‘s p. I also examined correlations between dry mass of insects and minimum nightly temperature. I determined statistical power of tests that resulted in nonsignificant results using tables in Kraemer and Thiemann (1987). Because of significant correlations among variables, I examined partial correlations of activity of bats with dry mass of insects and with minimum nightly temperature. Effect of number of nights sampled. As activity of bats generally is greatest and most sampling typically occurs during the summer months, I examined data from June, July, and August at Bark Creek (1993, n = 24 nights; 1994, n = 22) and Buttermilk Creek (1993, n = 28; 1994, n = 18) to determine the influence of number of nights sampled on estimates of activity of bats. I randomly © 2011 – Bat Conservation International Page 159
BCI Bat Conservation and Management Workshop – Arizona sampled from 2- to 12-nights subsets 100 times each from each of the four datasets and determined the mean nightly IA for each random sample. I then determined the proportion of each 100 random samples that had means within 10, 20, 30, 40, and 50% of the mean computed using the corresponding complete dataset. All sampling and analyses were performed using the SAS statistical software package for personal computers (SAS Institute, Inc., 1985). This pattern is typical of many species of insectivorous bats (Erkert, 1982; Kunz, 1973; Maier, 1992; Taylor and O'Neill, 1988) and probably results from a period of initial foraging and drinking after emerging from day roosts, reduced activity during the middle of the night when bats are at night roosts, and a final bout of foraging and commuting activity before returning to day roosts (Kunz, 1974; Kunz et al., 1995). Comparison of paired and independent sample designs. The number of samples necessary to achieve the same statistical power with paired and unpaired designs is a function of the variance estimates for the two designs. An unbiased estimate of the variance for the paired design is the sample variance for the paired data, S 2 D. An unbiased estimate of the variance for the unpaired design is: 2S 2 p- (S 2 p-S 2 D)/(2n-1). where, S 2 p = (S 2 1 + S 2 2)/2, S 2 p is the pooled sample variance of the two populations, n is the sample size of each population, and S 2 1 and S 2 2 are the sample variance of the two populations (Snedecor and Cochran, 1980). I compared the relative efficiency of paired and independent sampling designs by examining the ratio of variances for the two designs. To account for differences in degrees of freedom in the two experimental designs, I compared variances adjusted for degrees of freedom by multiplying the variances by (v + 3)/(v + I ), where v is the number of degrees of freedom of the experimental design (Snedecor and Cochran, 1980). I determined S 2 p and S 2 D using the IA-values for all nights when activity of bats was recorded at both sites. To eliminate the potential influence of low levels of activity in winter months on estimates of variance, this analysis was repeated omitting data collected from November through April. Results And Discussion Temporal patterns of activity. Levels and patterns of activity varied substantially within nights, among nights within seasons, among seasons, and between sites. Mean activity within a night had a bimodal distribution with a peak of activity shortly after sunset and a second, smaller peak just before sunrise (Fig. 1). Figure 1 Although distribution of nightly activity tended to be bimodal, patterns of activity varied substantially among nights. Distributions of activity were bimodal on some nights (Fig. 2a), but frequently the second peak of activity was missing (Fig. 2b). On other nights, activity persisted at moderate levels throughout the night with multiple peaks (Fig. 2c), and occasionally there was little activity early in the evening, with increased activity later in the night (Fig. 2d). The reasons for this variability are not clear, and may be related to changes in abundance of insects, meteorological conditions, social factors, energetic needs of the bats, or some other factor. Variability in nightly patterns suggests that caution should be employed when interpreting data collected during small portions of the night. Variability due to changes in distributions of activity will increase sample variance, requiring larger samples for precise estimates, and increases the probability that incorrect inferences will be drawn if sites are inadequately sampled. Total activity also varied substantially among nights (Fig. 3). Levels of activity varied seasonally, but sometimes levels of activity on consecutive nights also differed by several-fold. Bats were active throughout the year, but activity during winter months was uniformly low (Fig. 3). Low levels of activity during winter is typical for bats (Arlettaz, 1990; Avery, 1985; Brack and Twente, 1985; Hays et al., 1992; Speakman and Racey, 1989; Whitaker and Rissler, 1992). Reasons for activity in winter are controversial, and probably vary with species and conditions, but include © 2011 – Bat Conservation International Page 160
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Figure 6 (top). Typical bat gate (n
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Page 113 and 114: BCI Bat Conservation and Management
Page 163: BCI Bat Conservation and Management
Page 201 and 202: ACOUSTIC INVENTORY DATA SHEET Locat
Page 203 and 204: INSTRUCTIONS FOR FILLING OUT BAT CA
Page 205 and 206: BAT CAPTURE DATA FORM Additional Da
Page 207 and 208: Capture Survey Passive Active Scien
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Bat Conservation and Management Workshop

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