Characteristics of Summer and Winter Roost Trees Used by Evening Bats (Nycticeius Humeralis) in Southwestern Missouri
Boyles, Justin G., Robbins, Lynn W., The American Midland Naturalist
We radio-tracked 13 evening bats (Nycticeius humeralis) to 34 trees during the summer of 2003 and 10 evening bats to 29 trees during the winters of 2003 and 2004. We captured males in every month of the year and provide evidence suggesting that females are also year-round residents of southwestern Missouri. These captures extend the known winter range of N. humeralis at least 110 km northeast. Evening bats selected trees in late stages of decay during the summer, but during the winter they selected a higher proportion of live trees. Comparisons of summer and winter trees suggest that habitat characteristics are more important than tree characteristics in explaining variation between roosts used in the two seasons. Winter roost trees were located in areas with lower average tree height and higher densities of trees. Those attributes would likely cause problems with clutter when leaves are on the trees during summer.
To devise effective conservation and management plans for a bat species, a thorough understanding of its summer and winter roosting habits is vital (Fenton, 1997). Despite being one of the most locally abundant bat species throughout the southeastern United States, relatively little is known about the ecology of the evening bat (Nycticeius humeralis). Nycticeius humeralis is believed to be migratory, but the portion of the population discussed herein appears to remain in the area throughout the year (Boyles et al, 2003). Evening bats roost communally at various times of the year and commonly are found roosting in large numbers in trees and buildings. Many of the studies on roosting sites of evening bats have been done with colonies in man-made structures (e.g., Hooper, 1939; Watkins, 1969; Watkins and Shump, 1981; Bain and Humphrey, 1986; Cope et al, 1991; Wilkinson, 1992a, b), but recently studies have reported the tree roosting (Wilkinson, 1992a, b; Bowles et al, 1996; Menzel et al, 1999, 2001; Hutchinson, 2001; Boyles et al, 2003) and ground roosting habits of evening bats (Boyles et al, in press).
Unlike endangered bats in the region such as the Indiana bat (Myotis sodalis) and the gray bat (Ai. grisescens), the level of interest directed toward the evening bat has been relatively low. In Missouri, N. humeralis currently is not listed as endangered or threatened; however, there is reason to believe local populations could be declining in other states (Whitaker and Hamilton, 1998). For example, in Indiana many previously known maternity colonies no longer exist (Cope et al, 1991; Whitaker and Gummer, 2003), and evening bats are not as abundant in Indiana as they have been in the past (Whitaker et al, 2002). The evening bat is listed as endangered in Indiana (Whitaker and Gunimer, 1988). They also occur in Tennessee and Kentucky, but they are no where abundant (Harvey et al, 1991). If it is true that evening bat populations are declining in some locations, research in areas where they are common could be vital to preserving the species in the future. Research on common bat species also provides pertinent information for less common or endangered species (Agosta, 2002).
There are limited data about how tree-dwelling bats overwinter in the eastern United States. Most forest-dwelling bats in Missouri are assumed to either migrate and or hibernate in caves; however, there is a paucity of data on winter roosting ecology and whereabouts of Nycticeius humeralis. It is commonly assumed that evening bats migrate south during the winter, but winter captures in southern regions are too rare to make assumptions about the entire winter range. There are scattered published records of evening bats taken or seen flying during the winter in Florida (Genoud, 1993; Hutchison, 2001), Arkansas (Sealander, 1960; Baker and Ward, 1967), Oklahoma and Texas (French and Bunyard, 2002). One of the winter records in Arkansas was in Washington County, approximately 110 km southwest of our study site (Sealander, 1960). …