Data Analysis

We provide simple descriptive summary statistics, percentages, and community indices based on capture data. To compare proportions among categorical data sets, we provide 95% confidence intervals (CI) and assess degree of overlap. For contrasting sex ratios, we expected CIs to overlap 0.5 if sex ratios of males to females were equal. We also used logistic regression (PROC GENMOD, SAS Institute, Inc. 2003) coupled with an information-theoretic approach to select between competing explanatory models (Burnham and Anderson 2002) and test the biological hypotheses that reproductive rates varied between 2006 and 2007; the 2 years differed greatly in amount of spring precipitation and community productivity.

We compared the proportions of reproductive females in each year using 2 models. Under the general model, the probability of a female being reproductive was allowed to vary between the 2 years; whereas under the constrained model, the probability of a female being reproductive was assumed to be constant between the 2 years. Models were ranked using Akaike's information criterion corrected for sample size (AIC_c) (Burnham and Anderson 2002). We also calculated AIC_c differences (Δ_i = difference in AIC_c score between the ith and the topranked model) and Akaike weights (w_i = probability that the ith model is the best approximating model among candidate models). The model with the lowest AIC_c score was assumed to be the best-fitting model (Burnham and Anderson 2002).

Reproductive rate is the life history trait of bats most likely to vary with environmental conditions (Barclay et al. 2004). Reproductive rates were calculated as the proportion of adult females captured that were pregnant, lactating, or postlactating within a time period defined by the recognition of the first pregnant and last lactating female of any species. Limiting rates to this period should reduce incorrect field diagnoses (e.g., pregnant bats with small fetuses not detectable by palpation earlier in the season or postlactating bats with regression of teats miscategorized as nonreproductive later in the season). Spring precipitation data were obtained for the Mesa Verde station from the National Climatic Data Center ( http://cdo.ncdc.noaa.gov/ancsum/ACS). Comparisons of reproduction, relative abundances, and sex ratios of bats between 2006 and 2007 are limited to data from sites that were sampled in both years.

We compare the bat fauna at Mesa Verde with other regional bat faunas reported in the literature. We limit such comparisons to other studies based on captures of bats in mist nets over water or similar high-use areas but not to those studies based on captures at colonies or on echolocation recordings. To conduct these comparisons, we calculated 4 community indices: species richness (the number of species detected), Jaccard's index of community similarity, Simpson's diversity index, and evenness of distribution of individuals among species based on Simpson's index. We calculated Jaccard's index as J = C/(A + B - C), where C is the number of species in common between the faunas of 2 regions, A is the number of species in one region, and B is the number of species in the other region (Magurran 1988). We express Simpson's index as 1 - D, where D = [Σ n(n 1)]/[N(N - 1)], n is the number of individuals in each species, and N is the total number of individuals captured (Magurran 1988). Evenness of the distribution of individuals among species is expressed by the formula (1/D)/S, where S is the number of species in the bat fauna. A number of alternative metrics of species richness, diversity, and evenness are available in the ecological literature, each with its own advantages and biases. Their properties are discussed by Magurran (1988, 2004), and their application to bat studies is reviewed by Kingston (2009).

We used a comparative approach to search for evidence of major impacts of recent extensive fires on bat fauna. We compared the relative abundances of species and the aforementioned community indices with those of other surveys using similar methods elsewhere in the Four Corners states within about 375 km of Mesa Verde. These comparisons did not present major difficulties in determining species' taxonomic identity. One possible exception was distinguishing between M. occultus and Myotis lucifugns (little brown bat) on the basis of geographic distribution. However, only 2 of these other studies reported either species. In one study (Coconino Plateau, Arizona; Morrell et al. 1999), identity of M. occultus is supported by the literature on distribution (Hoffmeister 1986), and in the other (Moffat County, Colorado, 330 km north of Mesa Verde; Freeman 1984), identity of M. lucifugus is likewise supported by the literature (Armstrong 1972, Armstrong et al. 1994, Fitzgerald et al. 1994).

We also compared our results with those of a previous survey at Mesa Verde in 1989–1994 (Chung-MacCoubrey and Bogan 2003) conducted before the most extensive burns. Our ability to make comparisons with the 1989–1994 survey was limited because those results were primarily from different locations and emphasized small pools in canyons (perhaps biased towards maneuverable species that are more likely to drink at sources with small surface areas) sampled mostly in late July through August. These small pools have since filled with silt from postfire runoff or dried due to prolonged drought, and we were thus unable to repeat sampling at most of the same sites. However, we were able to make comparisons with the 1989–1994 results from netting at or within 2.5 km of the Morefield Canyon site. Additionally, in 2007, we sampled a permanent seepage pool in Soda Canyon on 6 nights; the configuration for netting at this site was similar to the configuration at sites sampled in 1989–1994. We used Jaccard's index and rankings of the most abundant species to make qualitative comparisons with findings of the 1989–1994 surveys of bats at these locations. Original data from the 1989–1994 surveys reported by Chung-MacCoubrey and Bogan (2003) are available through the MSB ( http://arctos.database.museum/home.cfm).

In describing relative abundances of bats at Mesa Verde, we arbitrarily use the term “common” for species that account for ≥10% of total captures, “uncommon” for species that account for 2%–9% of total captures, and “rare” for species that account for ≤1% of total captures. To our knowledge, standardized uses for these terms in bat studies are not available in the literature.

Faunal Composition and Adult Sex Ratios

We captured 1996 bats of 15 species at Mesa Verde (Table 1) during 120 nights of sampling (58 nights in 2006 and 62 nights in 2007, for a total of 1082 12-m net-nights). The same 15 species were detected in both years, with similar relative abundances (other than a shift in proportions of silver-haired bats; Table 1). Relative abundances of species at sites sampled in common in both years were similar to relative abundances at all sites (including sites sampled in only one year) in either year, suggesting no strong site differences that would bias annual results. Differences in monthly level of effort between years were also minor, at 1–3 nights per site per month in May–July each year. The long-legged myotis (Myotis volans) was the most frequently captured bat and accounted for about one-third of the total bats captured (Table 1). Other common species (≥10% of captures) were long-eared myotis (Myotis evotis), silverhaired bats, big brown bats, and western small-footed myotis (Myotis ciliolabrum). Five species were rarely captured, each accounting for < 1% of the total (Table 1): pallid bats (Antrozous pallidus), Townsend's big-eared bats, spotted bats, Yuma myotis (Myotis yumanensis), and Brazilian free-tailed bats (Tadarida brasiliensis). Other species were uncommon, including migratory hoary bats (Lasiurus cinereus), taken primarily in late spring and early summer; 86% of silver-haired bats and 67% of hoary bats were captured by 30 June in both years, although only 47% of nights sampled were before 30 June. We recorded 15,389 echolocation call files in 2006 and identified 12,888 of them to species or species group. All calls that we identified were attributed to species also captured in mist nets, with the exception of the big free-tailed bat (Nyctinmnops macrotis). We recorded 698 passes of big free-tailed bats in 2006 during the months of June (389 passes in 8 nights), July (96 passes in 7 nights), and August (213 passes in 8 nights), but no bats of this species were captured.

TABLE 1.

Species and numbers of bats captured at Mesa Verde National Park, Colorado, in 2006 and 2007. Percentages of column totals are given in parentheses. Captures at sites in common in 2006–2007 do not include bats that were captured at sites sampled in only 1 of the 2 years.

Disproportionate adult sex ratios occurred in some species of bats. These patterns included more males than females in big brown bats, hoary bats, silver-haired bats, western small-footed myotis, canyon bats, and Brazilian freetailed bats, but more females than males in long-legged myotis (Table 2). Patterns in sex ratios were consistent between the 2 years at sites sampled in common, except for long-eared myotis, which shifted from more males than females in 2006 to more females than males in 2007 (Table 2). With the exception of long-eared myotis, the proportions of sexes within species in the 2 elevational categories (≤2165 m or ≥2311 m) did not differ strongly between years at Mesa Verde (Table 3). Sex ratios within species also were the same, with broadly overlapping CIs in both elevational categories. The one exception was for long-eared myotis, which had a preponderance of males at higher elevations and females at lower elevations (Table 3). A seeming tendency toward more females at lower elevations was also indicated at pools in Rock Canyon and Soda Canyon (elevations 1899– 2103 m), sampled only in 2007. At Rock Canyon we captured 2 lactating female pallid bats (the only pallid bats captured at Mesa Verde). We also captured 9 female canyon bats at Rock Canyon (8 lactating) and only 2 males. In contrast, we captured 64 adult canyon bats over both years at all other sites, and 56 were male. At Soda Canyon, we caught 6 adult female fringed myotis (Myotis thysanodes), 4 of which were reproductive, whereas all other sites combined yielded only 4 reproductive female fringed myotis in 2007; other sites with reproductive female fringed myotis were also at lower elevations.

TABLE 2.

Comparison of adult sex ratios and proportions of adults that were male in 2006 and 2007 (May–August both years) at selected sites at Mesa Verde National Park, Colorado. Only bats at locations sampled in common in the 2 years are included. M = males, F = females, CI = 95% confidence interval.

Comparisons with Other Bat Faunal Surveys

The 4 most abundant species at Morefield Canyon in 2006–2007 were long-eared myotis (25% of captures, n = 78), long-legged myotis (24%, n = 75), occult myotis (19%, n = 61), and silver-haired bats (16%, n = 50). Rankings at Morefield Canyon in 1989–1994 were similar (but proportions differed), with long-eared myotis (48%, n = 41), long-legged myotis (27%, n = 23), silver-haired bats (6%, n = 5), and California myotis (Myotis californicus; 5%, n = 4) as the most abundant species. Relative abundance was greater for occult myotis at Morefield Canyon in our study (1%, n = 1 in 1989–1994). Only 4 of the 61 occult myotis captured at Morefield Canyon in 2006–2007 were caught during the month of July. The rank order at Morefield Canyon is different than at Mesa Verde in that long-legged myotis was not the most abundant species either in 2006–2007 or in 1989–1994.

Sampling over small pools in the earlier work and sampling over larger ponds in our study resulted in different biases in rank orders of relative abundance. In 2007, we sampled over the small pool in Soda Canyon on 6 nights and obtained relative rankings of the various species that were similar to rankings from the park-wide 1989–1994 surveys: long-eared myotis (61%, n = 50), long-legged myotis (24%, n = 20), fringed myotis (7%, n = 6), and Townsend's big-eared bats (2%, n = 2). Park-wide rankings in the 1989–1994 surveys were as follows: long-eared myotis (40%, n = 71), long-legged myotis (30%, n = 53), Townsend's big-eared bats (11%, n = 20), and fringed myotis (5%, n = 7). Based on Jaccard's index of similarity, the bat fauna at Soda Canyon was more similar to that of the 1989–1994 surveys (J = 0.73) than to the overall bat fauna at Mesa Verde in 2006–2007 (J = 0.40).

Five surveys in the Four Corners states conducted within 375 km of Mesa Verde captured bats in mist nets over water at comparable elevations, most including piñon-juniper woodlands (Table 4). Species composition of the bat fauna at Mesa Verde is very similar to the composition at all of these areas. Species richness at our study area is identical to species richness at 3 other areas and higher than species richness at 2 areas (both primarily in piñon-juniper woodlands). Simpson indices indicate that Mesa Verde is intermediate in diversity and evenness. Two of three species not documented at Mesa Verde have distributions that do not include Colorado (Idionycteris phyllotis in Utah and Arizona, and Myotis auriculus in Arizona); a third species (little brown bat, Myotis lucifugus) was found only at the Moffat County, Colorado, comparison site. Based on Jaccard's index, the bat fauna of Mesa Verde is most similar to the bat fauna of the 2 geographically closest areas previously surveyed: the Henry Mountains in Utah and the Jemez Mountains in New Mexico (Table 4). If migratory tree bats (hoary and silver-haired bats) are removed from consideration because of irregular capture biases during migration peaks, the 3 most common species in most study areas are long-legged myotis, long-eared myotis, and big brown bats. This is true in 4 of the 6 study areas, including Mesa Verde (Table 4), and perhaps in a fifth (the California myotis and western smallfooted myotis were treated as a single species in one study). In the remaining 2 study areas, at least 2 of the 3 most abundant species at Mesa Verde (long-legged myotis, long-eared myotis, and big brown bats) were among the top 3 most abundant species (exclusive of migratory tree bats; Table 4). The proportions of California myotis and western small-footed myotis among the other Four Corners study areas each varied from <1% to 13% (Table 4); at Mesa Verde, the relative abundances of these 2 species were similar: 5% (California myotis) and 10% (western small-footed myotis). Three other species uncommon at Mesa Verde (occult myotis, canyon bats, and fringed myotis) were also typically uncommon at most other study areas. The occult myotis was only present at one other area (Coconino Plateau, relative abundance 13%), the canyon bat was present at 3 of the 6 areas at relative abundances of <1% to 11%, and the fringed myotis was present at all areas but always at a low relative abundance (<1% to 8%, Table 4). Rare species at Mesa Verde were similarly rare or uncommon in most of these other faunas (Table 4). Big free-tailed bats were only captured in one of the other studies and constituted only 1% of the total bats. Townsend's big-eared bat and the spotted bat always made up < 1% to 3% of the total bats in all studies. Pallid bats made up 4% or less in all other regional studies, and Brazilian free-tailed bats made up ≤1% in 5 of the 6 studies; the Yuma myotis represented 2% or less of the total captures in all 6 studies.

TABLE 3.

Numbers of adult bats captured and proportion that were male at capture sites lower (≤2165 m) or higher (≥2311 m) in elevation at Mesa Verde National Park, Colorado, 2006 and 2007. Data from all sites and dates are included. CI = 95% confidence interval.

Bat Reproduction and Spring Precipitation

Spring precipitation differed markedly between the 2 years, with higher precipitation in 2007 (Table 5). Much more precipitation fell in April and May 2007 than in April and May 2006. In April 2007 total precipitation was 1.6 times as high as in April 2006, and it was slightly higher than the monthly median and mean for April 1949–2005 (but within the 95% confidence limits). Total precipitation in May 2007 was more than 25 times that in May 2006 and much higher than typical for May in the 1949–2005 data set (Table 5). Precipitation in June varied less between the 2 study years.

TABLE 4.

Comparative characteristics and results of 6 surveys of bat faunas within 375 km of Mesa Verde National Park, Colorado. Surveys sampled bats by capturing them in mist nets (Freeman [1984] includes 74 based on observation). All study areas were within similar elevation ranges, and most included piñon-juniper woodlands. The value of Jaccard's index was based on similarity with the Mesa Verde bat fauna. ND = not determined.

TABLE 5.

Monthly precipitation totals (cm; April–August, 2006 and 2007) and summary statistics for spring and summer precipitation (cm) at Mesa Verde National Park, Colorado, 1949–2005. Data for 1977 were unavailable.

Thirteen species of bats at Mesa Verde showed evidence of reproduction, with no evidence of female reproduction detected in hoary bats or silver-haired bats. We first detected pregnancy in bats on 22 June in 2006 and 18 June in 2007; we noted the last lactating bat on 22 July in 2006 and 25 July in 2007; and we caught the first volant juvenile on 15 July 2006 and 22 July 2007. Therefore, we limited comparisons of reproductive rates between years to bats captured from 18 June to 26 July in either year to minimize incorrect field diagnoses, and we limited comparisons to species with sample sizes ≥5 females each year. Reproductive rates in 3 species of bats (western smallfooted myotis, long-eared myotis, and long-legged myotis) were higher in 2007 than in 2006, with little or no overlap in CIs and high support by AIC_C for the general logistic regression models for between-year differences (Tables 6, 7). Models wherein proportions were constrained to be equal had greater support in California myotis, fringed myotis, and occult myotis (Table 7). Occult myotis had high reproductive rates in both years. We also captured fewer volant juvenile bats in 2006 than in 2007. In 2006, we captured 6 juveniles of 3 species on 16 nights between 15 July and 15 August (0.4 juveniles per night). In 2007, we captured 68 juveniles of 8 species on 23 nights between the same dates (3.0 juveniles per night).

Roosts of Bats

We radio-tagged 73 bats of 7 species and were successful in finding general roost locations of 46 individuals of 5 species, including long-eared myotis, occult myotis, fringed myotis, long-legged myotis, and spotted bats (Table 8). We were unable to find any daytime roosts of the Yuma myotis or Townsend's big-eared bats that we tagged, but we occasionally detected radio signals from them at night while they were in flight over the mesa tops.

Most bats roosted in rock crevices, but a few also used other structures. Long-legged myotis, fringed myotis, and spotted bats roosted exclusively in rock crevices within steep canyon slopes and cliff faces. Two female long-eared myotis roosted in trees: one nonreproductive female roosted in juniper snags, downed logs, and live trees north of Mesa Verde; and the other roosted for one day in a juniper snag on an unburned part of Chapin Mesa. The latter incident was our only observation of a reproductive female bat roosting in a tree in piñon-juniper woodlands of Mesa Verde. All other female long-eared myotis (n = 17) roosted in rock crevices in canyons, often close to the ground (more detailed analyses of roosts used by longeared myotis are reported by Snider [2009]). The inaccessibility of roosts used by other species precluded our ability to gather detailed information on their characteristics. In most cases, we tracked radio signals to cliff faces but could not determine the exact locations of roosting bats. Rock crevices used by long-legged myotis tended to be in canyon walls between 2 and 15 m in height. On one occasion, we were able to observe an emergence flight of 131 individuals, presumably members of a maternity colony. Signals of spotted bats always were detected from > 10 m to 15 m high on sheer cliff faces, but by watching and listening to emerging bats at dusk, we determined the precise location and characteristics of 2 widely separated roosts of spotted bats. In these cases, 18 and 12 bats were seen and heard emerging from long vertical crevices in high walls of Echo Cliffs in Navajo Canyon and in Soda Canyon, respectively. Spotted bats began emerging just before dark (making observation difficult), and emergence of all individuals took about 25–35 minutes. Emergences were followed by calling sounds from spotted bats circling in the air near the roost entrance; during these observations, we heard what sounded like pairs of bats flying together, perhaps mother and young. Some bats were clearly observed circling at the roost and did not leave the area until joined by a second bat leaving the roost. The occult myotis was the only species at Mesa Verde that did not consistently roost in rock crevices during summer. In early summer, a pregnant female moved between a building in the Mancos Valley and a rock crevice and ponderosa pine snag in Morefield Canyon, whereas all other tagged female occult myotis roosted in buildings in the Mancos Valley.

TABLE 6.

Between-species comparison of reproductive rates of adult female bats captured at Mesa Verde National Park, Colorado, from 18 June to 26 July, in 2006 and 2007. Only species with sample sizes ≥5 adult females were included. Reproductive rates were based on total bats diagnosed as pregnant, lactating, or postlactating as a proportion of all adult females sampled and diagnosed during the 18 June to July 26 time period. To assure comparability between the 2 sampling years, only bats sampled at locations in common in both 2006 and 2007 were included.

TABLE 7.

Rankings by Akaike's information criterion adjusted for sample size (AIC_C; Burnham and Anderson 2002) of top logistic regression models (PROC GENMOD in SAS Institute, Inc. 2003) comparing proportions of reproductive female bats caught at Mesa Verde National Park, Colorado, in 2006 with proportions in 2007, based on sites sampled in common between the 2 years. Symbols: K = number of parameters in the model; Δ_i = difference in AIC_C value between the ith and top-ranked model; w_i = Akaike weight (probability that the ith model is the best approximating model among the candidate models). Under the general model, the probability of a female being reproductive was allowed to vary between the 2 years, whereas under the constrained model, the probability of a female being reproductive was assumed to be constant between the 2 years.

The distribution of sites used by roosting bats varied with species at Mesa Verde. Roosts of long-eared myotis and long-legged myotis were found throughout the study area. Roosts of other bat species were more limited in distribution, with occult myotis roosting only in Morefield Canyon and in the Mancos River Valley, fringed myotis only roosting in Soda Canyon near the southern end of Mesa Verde (perhaps biased by tagging location), and spotted bats only roosting in steep-walled canyons on the southern end of Mesa Verde. Reproductive female bats were found roosting in all parts of the study area within about 15 km of capture sites, but roosts tended to be at lower elevations in canyons and outer slopes of the cuesta (Table 8). Only 2 roosts used by a pregnant long-legged bat on sequential days were situated in what we considered higher elevations of Mesa Verde (≥2311 m).

The distance that bats traveled from tagging sites to subsequent roosts was variable (<1 km–14 km; Table 8). Spotted bats and occult myotis traveled the longest distances, and long-eared myotis and fringed myotis traveled the shortest distances. We also observed interspecific variation in the degree to which bats switched roosts from day to day. Long-eared myotis frequently switched roosts, with an average of 2.3 roosts (1–7 roosts per individual, SD = 2.1) discovered per bat followed. The occult myotis also switched roosts frequently, with an average of 2.0 roosts (1–3 roosts per bat, SD = 0.8). Our observations suggest that other species switched roosts less often, if at all, with <2 roosts found or suspected per individual tracked.

TABLE 8.

Summary of female bats marked with miniature radio-transmitters during the summers of 2006 and 2007 at Mesa Verde National Park, Colorado, grouped by species and reproductive status. Values in parentheses indicate the number of individuals successfully followed to roost sites (roost sites for others were not located). Also listed are average distances moved between capture sites and daytime roosts, as well as average elevations of roost sites.

Faunal Composition and Adult Sex Ratios

Sixteen species of bats occur at Mesa Verde. The bat fauna in 2006 and 2007 included every species of bat known west of the Great Plains in Colorado, except for the little brown bat (Armstrong et al. 1994, Fitzgerald et al. 1994). Our results illustrate the value of conducting thorough contemporary faunal surveys of management areas of interest. The first faunal list of bats at Mesa Verde documented 8 species based on 25 specimens (Anderson 1961): California myotis, long-eared myotis, western small-footed myotis, fringed myotis, long-legged myotis, big brown bats, Townsend's big-eared bats, and Brazilian free-tailed bats. The list increased by 2 species with the inclusion of canyon bats and hoary bats in 1963 and 1964 (based on 4 specimens—Douglas 1967). The Mesa Verde bat fauna was again surveyed in 1989–1994 (Chung-MacCoubrey and Bogan 2003). This effort resulted in capture of 177 bats of 11 species, adding 3 additional species: pallid bats, silver-haired bats, and occult myotis. In addition, 3 skulls of spotted bats were found in spotted owl (Strix occidentalis) pellets, and spotted bat calls were heard at night, although no bats were captured (Chung-MacCoubrey and Bogan 2003). The fauna we documented in 2006 and 2007 added captures of spotted bats, Yuma myotis, the first canyon bats since 1963, and the first Brazilian free-tailed bats since 1936 (Anderson 1961, Chung-MacCoubrey and Bogan 2003); we also documented big free-tailed bats by acoustic recordings.

The occurrence of spotted bats at Mesa Verde is noteworthy. This species was first found in Colorado in 1982, but its range in Colorado was thought to be restricted to the northwestern corner of the state. There were published records for just 3 specimens, all from north-western Colorado (Finley and Creasy 1982, Bogan et al. 1988). Past observational records of spotted bats in Colorado were acousticallybased foraging-habitat surveys in the northwestern part of the state (Navo et al. 1992, Storz 1995). We provide new distribution records and the first documentation of spotted bat maternity colonies in Colorado, as well as new information on locations and sizes of maternity colonies. To our knowledge, only one other study has observed maternity colonies of this species anywhere in western North America (Bogan et al. 1998), and our findings are consistent with that report. Records of other species are less noteworthy, although the higher number of big free-tailed bat calls in June differs from most past acoustic and capture records for this species in Colorado, which document more numerous calls in July and August (Navo and Gore 2001). The bat fauna at Mesa Verde contains species that are continental migrants, species with affinities for more-arid southwestern ecosystems (e.g., canyon bats, pallid bats, and Yuma myotis; Barbour and Davis 1969), species with more-montane affinities and distributions that tend to extend further northward (e.g., long-eared myotis, long-legged myotis, and fringed myotis; Barbour and Davis 1969), and species found throughout much of western North America (e.g., big brown bats, spotted bats, Townsend's big-eared bats, and western small-footed myotis; Barbour and Davis 1969). We suggest that the high species diversity of bats at Mesa Verde is a result of elevational zonation and topographical diversity. Although Mesa Verde is dominated by piñon-juniper woodland, the elevational zonation provides gradients in temperature and aridity, creating habitats that range from high-desert shrubland to pockets of montane forest; and the area has a complex topography with numerous cliffs and canyons (Floyd et al. 2003). Areas in North America with high topographical diversity have high bat diversity because such areas have abundant roosting opportunities for bats in crevices of cliff faces, canyon walls, and rock outcroppings (Humphrey 1975).

Based on captures in mist nets (Table 1), we also characterize the fauna as composed of 5 common species (each ≥ 10% of total captures), 5 uncommon species (each 2%–9% of total captures), and 5 rare species (each ≤ 1% of total captures). The 5 common species of bats at Mesa Verde (Table 1) are typical of mid-elevation forested sites in the Rocky Mountain and south-western states (Table 4; Jones 1965, Findley et al. 1975, Hoffmeister 1986). The 5 rare species can be categorized chiefly as inhabitants of zones that are warmer, more arid, or of lower elevation (pallid bats, Yuma myotis, and Brazilian free-tailed bats; Findley et al. 1975, Hoffmeister 1986) or species that may be more adept at maneuvering and avoiding nets (spotted bat and Townsend's big-eared bat)—habits that make them less likely to be captured. The 5 uncommon species are a mixed group and include species that can be categorized as (1) bats of more arid lowlands (California myotis and canyon bat; Armstrong et al. 1994, Fitzgerald et al. 1994), (2) bats typical of the zones in which we sampled but migratory (e.g., hoary bat) and not commonly captured in Colorado except during migration (Armstrong et al. 1994, Fitzgerald et al. 1994), or (3) bats typical but uncommon in Colorado and in some areas of other Four Corners states (fringed myotis; Table 4; Armstrong et al. 1994, Fitzgerald et al. 1994), except perhaps in areas near buildings used as roosts (e.g., occult myotis; Davis and Barbour 1970). Colonies of big free-tailed bats are generally limited to areas of the southwestern U.S. with deep canyons and cliff walls (Bogan et al. 1998, Corbett et al. 2008).

Comparisons with Other Bat Faunal Surveys

Piñon-juniper woodland is the dominant vegetation at Mesa Verde, and some stands are centuries old (Floyd et al. 2004). Fires dramatically reduced the extent of these woodlands between the 1989–1994 surveys by Chung-MacCoubrey and Bogan (2003) and ours. Fires in 1996, 2000, 2002, and 2003 burned 15,663 ha—about 73% of the park area and more than 3 times the total area burned in the preceding 100 years (Floyd et al. 2004, 2006). This large disturbance may have affected availability of prey or roosts (in other areas, some of the same species of bats found at Mesa Verde use crevices and cavities in piñon and juniper trees as sites for maternity roosts—Chung-MacCoubrey 2003a, 2003b), in turn causing shifts in species composition or in relative abundance in the bat fauna. We attempted to assess these effects by comparing our results with those from the 1989–1994 surveys and with similar bat studies in nearby regions of the other Four Corners states. Our 2006–2007 rankings of species at Morefield Canyon were similar with the 1989– 1994 rankings and with rankings in abundance at Soda Canyon, a site we sampled in 2007 with a similar configuration to sites sampled at Morefield Canyon in 1989–1994. Given the biases and unknown factors that can influence bat faunal surveys (e.g., Larsen et al. 2007, Kingston 2009), these data do not provide strong evidence for changes in bat fauna between the 1989–1994 survey and our study.

Similarly, we saw no evidence for major differences between the bat fauna at Mesa Verde and the bat faunas at other regional study areas with different and less extensive recent fire histories (Table 4). Compared to these other areas, Mesa Verde had identical or higher species richness and intermediate diversity and evenness of bat fauna. Species composition at Mesa Verde was most similar to the 2 geographically closest areas; and, excluding migratory tree bats, the 3 most abundant species were generally the same in most of these surveys (long-legged myotis, long-eared myotis, and big brown bats). These 3 species are typically considered to be “forest bats” (Barclay and Kurta 2007, Lacki et al. 2007a, 2007b). Forest bats remain among the most common species at Mesa Verde, and the relative abundances of uncommon and rare species of bats are similar to those at the other well-studied locations in the region (Table 4). Thus we find no evidence that the bat fauna at Mesa Verde is grossly dissimilar to the faunas in the other regional studies, despite the recent history of extensive fires.

Our findings of disproportionate sex ratios favoring males in several bat species suggest that for such species, the upper reaches of canyons and mesa tops that characterize much of Mesa Verde may be at elevations too high and too cool to be favorable for female reproduction. Results of tracking bats to roosts at Mesa Verde (see below) also indicate that females avoid roosting on mesa tops and within the upper elevations of Mesa Verde during summer. Lower elevations provide warmer roosts for female bats to rear young, whereas higher elevations can allow deeper daily torpor for males and nonreproductive females (see review in Weller et al. 2009). Sex ratios of silver-haired bats and hoary bats in their summer distributions are consistent with continental patterns, with males found primarily in mountainous areas of the Rocky Mountains and females found at lower elevations to the north and east (Cryan 2003). Reproductive female Brazilian free-tailed bats are not common in Colorado and usually favor lower, warmer, and moresouthern locations for formation of maternity colonies (Freeman and Wunder 1988, Fitzgerald et al. 1994). Big brown bats are widespread in Colorado; but there is increasing evidence that female big brown bats use the lower, warmer elevations to form maternity colonies (often in buildings), while males use higher, cooler elevations in summer—with both sexes hibernating in rock crevices at cooler elevations in winter (Neubaum et al. 2006).

Our netting results coupled with radio-tracking suggest a similar pattern for female occult myotis at Mesa Verde. We found little or no evidence for reproductive females at higher-elevation sites in the canyon bat, California myotis, western small-footed myotis, and occult myotis. Although reproductive female long-eared myotis were captured at higher-elevation sites at Mesa Verde, these sites generally had a greater abundance of adult males, and most roosts used by females were at lower elevations. The pattern for much of Mesa Verde to serve as habitat primarily for males and nonreproductive females of some species in summer is similar to patterns that have been noted at higher elevations in other areas of western North America (e.g., Grinnell 1918, Vaughan 1954, Fenton et al. 1980, Cryan et al. 2000).

At other study areas, researchers have also noted a preponderance of males at higher elevations in various species, including the California myotis, western small-footed myotis, and Yuma myotis (see review in Cryan et al. 2000 and citations below). Evidence for consistent use of netting sites at all elevations at Mesa Verde by reproductive females was strong only for the long-legged myotis. The long-legged myotis was also the only species that had adequate sample sizes with sex ratios skewed in favor of females. Long-legged myotis can be abundant in higher-elevation forests in Colorado, where sex ratios skewed toward females have been noted (all nonreproductive; Storz and Williams 1996). However, results of outstudy indicate that although female long-legged myotis frequent high-elevation habitats to forage and drink, based on radio-tracking, they did not show a greater tendency than other species to roost at higher elevations.

Bat Reproduction and Spring Precipitation

Findings regarding bat reproduction in relation to spring precipitation may have implications for populations of western bats in the future because climate change models project reduced spring moisture in large areas of western North America (Seager et al. 2007, Barnett et al. 2008, McAfee and Russell 2008). The effects of variable spring and summer weather on reproduction in western bats have not been well documented. Grindal et al. (1992) and Lewis (1993) reported an association between lengthy cool, rainy periods (precluding foraging) and decreased female reproductive success in 3 species of western bats. Spring drought has been suggested to lower reproduction of some species of western bats in only one other study (Bogan et al. 1998).

Between-year differences in female reproduction of some bat species at Mesa Verde paralleled differences in primary productivity of vegetation and a corresponding relative abundance of insects. Late spring and summer growth of herbaceous vegetation at Mesa Verde was greater in 2007 than in 2006 and was accompanied by a 1.6-times-greater abundance of nocturnal insects in 2007 (including coleopterans and lepidopterans), with marked differences in insect abundance by mid-June (Snider 2009). These differences occurred in areas that had recent fires (in 2002) as well as in unburned areas. Drought has been experimentally shown to reduce insect abundance (Frampton et al. 2000). Decreased abundance of nocturnal flying insects during drought has been reported in Arizona (Hovorka 1996); and reproduction is enhanced in insectivorous birds in areas with greater abundances of insects and other invertebrates (Burke and Nol 1998, Zanette et al. 2000). Curtailed reproduction in insectivorous bats during drought years has been noted in Australia (Rhodes 2007); and in some areas of the world, bat reproduction is thought to be timed with increases in food abundance associated with seasonally predictable rainfall (e.g., Fleming et al. 1972, McWilliam 1987, Bernard and Cumming 1997).

Insectivorous bats in temperate areas have physiological mechanisms that can facilitate curtailment of reproduction. They mate in autumn or winter, and sperm is stored in the female reproductive tract until ovulation in spring. Females under environmental stress can forego ovulation, fail to implant fertilized embryos, or resorb embryos (Racey and Entwistle 2000). Adult females in temperate areas thus act like “income” breeders in that they rely on food resources to support developing young during late gestation and lactation. Food resources are acquired more or less simultaneously to these phases of reproduction (Henry et al. 2002).

The 3 months with the lowest average precipitation at Mesa Verde National Park are April, May, and June—with June being the driest (Doesken and McKee 2003). These spring months may be critical for reproduction in bats because they are the period when bats leave hibernation and when fetuses develop in adult females. Births occur in mid- to late June, followed by the heavy nutritional demands of lactation over the subsequent 3–5 weeks. We conclude that increases in spring precipitation at Mesa Verde in 2007 resulted in conditions more favorable for reproduction in some species of bats than conditions in 2006. The literature for most species of bats that reproduce at Mesa Verde shows higher rates of reproduction elsewhere compared to our observations in 2006, also supporting our conclusion that reproduction in 2006 was lower than normal (e.g., Jones 1964, Easterla 1973, Barclay 1991, Morrell et al. 1999, Lacki and Baker 2007). Two of the species that did not show reduced reproduction in 2006 were the California myotis and the occult myotis. The California myotis may be more adapted to arid habitats than the other species of myotis we captured (Hoffmeister 1986), and radio-tracking showed that reproductive female occult myotis moved from Mesa Verde to lower elevations in the irrigated Mancos Valley, where precipitation effects may be ameliorated by human-subsidized water development and irrigation. The long-eared myotis also showed a shift in elevational distribution of the sexes in 2007 as well as increased reproduction.

Prolonged, multiyear droughts with reduced reproduction could have an additive impact on reducing growth rates in populations of bats and would likely lead to lower abundances of at least some species in some areas of western North America. Climate-change models suggest continued warming in western North America, with faster spring snowmelt and altered hydrology (Christensen and Lettenmaier 2007, Barnett et al. 2008). Current precipitation projections of global climate-change models for the broad southwestern region include a high likelihood for increased periods of aridity that will be more extensive than in the past (Seager et al. 2007). Decreased spring precipitation has occurred in the southwestern United States for the last 20 years (Holden et al. 2007) and will be a feature of future climate trends throughout the western United States (McAfee and Russell 2008). Given the differences in reproduction seen at Mesa Verde between the dry spring of 2006 and the moist spring of 2007, model projections for climate change have negative implications for reproduction, abundance, and perhaps even the continued presence of some species of bats. An increase in aridity also adds to the importance of managing for accessible drinking water to maintain high-diversity bat faunas in western North America (Adams and Hayes 2008).

Boosting Habits

Our radio-tracking results revealed very limited use of trees as roosts by the species of forest bats we followed. Many of these species tend to be roost generalists. In other parts of their ranges, long-eared myotis, fringed myotis, and long-legged myotis form summer colonies in trees, rock crevices, and buildings (e.g., Barbour and Davis 1969, Ormsbee and McComb 1998, Cryan et al. 2001, Chung-MacCoubrey 2003a, 2003b, 2005, Solick and Barclay 2006, Lacki and Baker 2007). We found all 3 species using mostly rock crevices at Mesa Verde, despite reports that they primarily roost in trees in some other forested regions, including piñon-juniper woodlands of west central New Mexico (Chung-MacCoubrey 2003a, 2003b, 2005). Piñon-juniper woodlands at Mesa Verde are among the oldest on the continent (Floyd et al. 2003), and structural characteristics of these ancient trees are likely as amenable to bats as the structure of younger forests of this type elsewhere. The older trees common at Mesa Verde may provide even better potential roosts for bats, with their complex, twisted trunks and limbs and deep cracks and pockets. However, our results suggest that females of the species we followed might generally prefer rock crevices over trees as summer roosts at Mesa Verde when both types of structures are available. The only other study that investigated use of roosts by bats in piñonjuniper woodlands took place in a landscape with much less exposed rock than exists at Mesa Verde (A. Chung-MacCoubrey personal communication). Differences observed between our study and those of Chung-MacCoubrey (2003a, 2003b, 2005) likely relate to the relative availability of rock crevices and trees suitable as roosts.

Many species of bats in western North America rely on rock crevices as roosting sites, but the extent of use and requisite characteristics of such sites are poorly understood (Bogan et al. 2003, Neubaum et al. 2006). Researchers working in other forested regions also noted the predominant use of rock crevices by long-eared myotis (Chruszcz and Barclay 2002, Rancourt et al. 2005, Solick and Barclay 2006) and fringed myotis (Cryan et al. 2001, Lacki and Baker 2007). Although long-legged myotis are known to form maternity colonies in rock crevices (Ormsbee and McComb 1998, Cryan et al. 2001, Baker and Lacki 2006), their exclusive use of rock crevices in forested habitats has not been previously reported. Our results from Mesa Verde indicate that use of trees as roosts by certain species of bats may not be extensive when suitable rock crevices are abundant. Bats generally tend to show greater fidelity to permanent roosts than temporary roosts (Lewis 1995) and thus may prefer more stable roost structures and microclimates in rock over those in trees. Preference for rock over tree roosts does not, however, indicate that Mesa Verde's piñon-juniper woodlands are not an important resource to bats. The woodlands host an abundant and diverse assemblage of insects on which bats forage (Snider 2009).

Bats spend the majority of their time sequestered in day roosts, and availability of suitable roosts is a major factor influencing bat populations (Kunz 1982). This work represents the first targeted research into use of daytime roosts by bats at Mesa Verde and emphasizes the importance of site-specific studies of roosting habits of bats. Spotted bats were the only species we tracked that exhibited consistent use of the same type of roost structure across their entire range. Only spotted bats are known to roost in crevices of high cliff faces; but because roosts are typically high above the ground (Leonard and Fenton 1983, Wai-Ping and Fenton 1989, Rabe et al. 1998b), observations at roosts are lacking (but see Bogan et al. 1998). Other species followed to roosts used a wider variety of structures but did not exploit trees at Mesa Verde to the degree they seem to in other regions. Thus, generalizations about the roosting habits of forest bats should be treated cautiously. Although radio-tracking studies of this kind are labor intensive, the knowledge gained is essential for the effective management of habitat for bats in specific areas.

The reliance of most bats at Mesa Verde on crevices in rocks and cliffs for roosts would suggest that a warming and more arid climate would not greatly impact bat roosting habitat in summer, although choice of specific roosts might shift upward in elevation as preferred temperature regimes also shift upward. The current pattern of sex-biased elevational distributions could also change with a warming climate; it is possible that bat species more typical of arid lowlands will increase in abundance. Additionally, almost nothing is known about overwintering habits of most bats that use Mesa Verde during the summer. It is likely that some may over-winter by hibernating in inconspicuous rock crevices at higher elevations, similar to big brown bats on the eastern slope of the Bocky Mountains (Neubaum et al. 2006). For example, we suspect that the female occult myotis we followed during early summer between natural roosts at Mesa Verde and a building in the Mancos Valley may have been in transition from winter to summer quarters. If hibernating sites at upper elevations warm at sufficient depth from the surface, then such sites may become unsuitable as hibernacula because bats would expend more metabolic energy at higher overwinter hibernacula temperatures.