Academic journal article The American Midland Naturalist

Temporal Patterns in Capture Rate and Sex Ratio of Forest Bats in Arkansas

Academic journal article The American Midland Naturalist

Temporal Patterns in Capture Rate and Sex Ratio of Forest Bats in Arkansas

Article excerpt


We quantified changes in capture rates and sex ratios from May to Sept. for eight species of bats, derived from 8 y of extensive mist netting in forests of the Ouachita Mountains, Arkansas. Our primary goal was to determine patterns of relative abundance for each species of bat captured over forest streams and to determine if these patterns were similar to patterns of abundance found in other types of studies, including studies of bat mortality at wind turbines. We also wanted to discern regional patterns in sex ratios that have implications for seasonal distributions and migration. Capture rates for eastern red bats (Lasiurus borealis) were up to 25 times greater in Aug. and Sept. than in spring or early summer. Although not significant (P = 0.063), capture rates of hoary bats (L. cinereus) peaked in both late spring and late summer. Silver-haired bats (Lasionycteris noctivagans) were abundant in late spring and late summer but were absent during mid summer, suggesting they migrated from the area. Sex ratios of red bats were predominately male in late spring and late summer but were dominated by females in mid summer, possibly because of increased activity of lactating females during mid summer. Female Seminole bats (L. seminolus) were only captured after Aug. 1, suggesting a seasonal geographic separation of sexes. Our results suggest that patterns of bat abundance derived from mist netting over forest streams may be similar to patterns of bat fatalities at wind turbines, communication towers, aircraft strikes, roads and patterns derived from trapping at cave entrances for many species, but it is unclear why this pattern appears ubiquitous.


Conservation efforts for forest bats are complicated by a lack of information on migration, seasonal changes in distribution and abundance and sex-based geographic separation. For example, migratory tree bats, including hoary bats (Lasiurus änereus) and eastern red bats (L. borealis) are the species of bats most often killed at wind turbines during late summer and early autumn (Cryan and Brown, 2007), and information on regional and temporal changes in relative abundance and migration eventually may help managers reduce these mortalities. Consequently, information on abundance and sex ratios in different regions throughout the year is important for understanding ecology, critical periods and critical areas of habitat needed for bats.

Although broad-scale geographic distributions generally are known for most species of bats in the United States and Canada, seasonal changes in bat abundance within their geographic range are poorly understood and temporal differences in abundance and sex ratios may occur throughout the range of many species due to sexual segregation and migration. Information compiled from museum records often notes temporal and spatial differences in sex ratios (Findley and Jones, 1964; Barbour and Davis, 1969; Cryan et al., 2000) and studies frequently suggest regional differences in abundance or sex ratios. However, only a few studies (e.g., Cryan, 2003) have demonstrated geographic changes in abundance and sex ratios throughout a season.

Many bats are migratory and sex-biased migration may occur in some species whereby the sexes differ in their tendency to migrate, distances traveled or geographic location (Fleming and Eby, 2003) . Among the bat species that occur in the Ouachita Mountains of Arkansas and Oklahoma, tri-colored bats (formally eastern pipistrelle, Perimyotis subßawis), evening bats (Nycticeius humeralis) and possibly Seminóle bats (Lasiurus seminolus) are considered regional migrants (typical movements of 100-500 km; Barkalow, 1948; Humphrey and Cope, 1968; Barbour and Davis, 1969; Watkins, 1969), whereas red bats, hoary bats and silver-haired bats (Lasionycteris noctivagans) are considered long range migrants (movements frequently >1000 km; Fleming and Eby, 2003).

Although bat captures from mist netting can be affected by factors such as weather, bat activity and avoidance of nets by experienced bats, data from mist netting may be used as an index of relative abundance for comparing yearly or seasonal changes in bat abundance if appropriate measures are taken to ensure methods are standardized. …

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