Often solitary or in small groups, these bats will roost in a variety of partially to fully concealed sites. Crevices in tree bark and exterior 'nooks and crannies' of buildings appear to be all that the bat requires for secondary (or temporary) roosts (Whitaker and Hamilton 1998). Caves or deeper recesses of buildings may be most used as primary or recurrent roost sites (Webster et al. 1985). From few maternity colony records, small groups of females locate in hollow trees, crevices in tree bark, bat houses and other manmade structures such as barns and little-used warehouses (Whitaker and Hamilton 1998).

NATURE SERVE GLOBAL HABITAT COMMENTS:

Generally associated with forested communities. Hibernates in caves, mines, and tunnels from late fall through early spring (Kurta 1982, Mumford and Cope 1964, Jackson 1961, Griffin 1940, and others). Hibernators frequently roost in crevices, drill holes, and similar sites (Caire et al. 1979, Pearson 1962, Layne 1958, Griffin 1940), but roosting in the open is not uncommon.

Use of different types of hibernacula can vary considerably among areas, depending upon quality and availability of sites. In a study of 71 potential hibernation sites, including large and small caves, overhangs, and mines, on the Shawnee National Forest in southern Illinois (Whitaker and Winter 1977), mines were the only occupied habitat. Mines also are the principal hibernation sites in northern Michigan where there are no caves (Stones 1981). In the northeastern U.S., hibernation sites include mines and caves (Griffin 1940, Hall et al. 1957) as well as large, cavelike water conduit tunnels (G. Hammerson, pers. obs.).

The principal requirements of a suitable hibernation site are winter-long, low temperatures above freezing, high humidity, and lack of disturbances, both natural (floods) and anthropogenic (visitation) (Barbour and Davis 1969, Hitchcock 1949). At least two studies have provided contradictory information on thermal habitat preferences, suggesting that warmer temperatures sometimes are selected or at least tolerated. In Illinois, Pearson (1962) found that the mean temperature at hibernation sites averaged 9.7 C. Stones (1981) studied the occurrence of bats in northern Michigan mines that were vertically thermally stratified. The mean ambient temperature was 5.9 C, with 43% of the population occurring in the range 7-8 C and 6.5% occurring in the range 9-11 C.

There appears to be a high degree of philopatry in hibernaculum use. In Missouri, over 90% of recaptured banded individuals, representing 5% of the original banded population of 945 (753 males and 192 females), were recaptured at their cave of origin (Caire et al. 1979). Mills (1971) recaptured 4.8% of 358 individuals at their cave of origin the year after banding. Griffin (1945) found that of over 13,000 banded bats of various species, of which about 8,500 were banded in their winter hibernacula, the ratio was 100:1 for bats that were observed to return to their cave of origin over subsequent winters vs. those that were recaptured elsewhere.

Night roosts used in summer between foraging bouts are in different habitats than day roosts. Caves, mines, and quarry tunnels are used as night roosts, typically by males, but also by nonreproductive females (Clark et al. 1987, Jones et al. 1967). They are joined later in the summer by juveniles and post-lactating females (Kunz, 1973). During the day, these same sites usually house no M. SEPTENTRIONALIS. Daytime observations typically are of individuals under loose bark on trees and in a variety of small spaces associated with buildings and other structures (Hoffmeister 1989, Caire et al. 1979, Hamilton and Whitaker 1979, Barbour and Davis 1969). At times M. SEPTENTRIONALIS has been found in or around caves on summer nights, but not actually roosting in them (Mills 1971). Early in the summer, these groups mostly comprise males, with females and young-of-the-year joining later in the season (Caire et al. 1979).

Copulation occurs in the fall during the swarming period when large numbers of bats congregate in and near certain caves (Baker 1983, Kurta 1980). Nothing has been published on the fidelity of individuals or colonies to particular swarming sites, nor the relationship of swarming site selection to hibernaculum and summer roost selection. Given the low numbers found in most hibernacula and summer night roosts relative to the higher numbers found at swarming sites, it appears that certain caves serve as congregation points for fall mating activity. However, short-term banding returns at swarming sites are very low, indicating movement among swarming sites (Kurta, pers. comm.).

Nursery colonies have rarely been located. Those that have been found were small and in a variety of sites, including a barn (Cope and Humphrey 1972) and a small cabin (Brandon 1961), though most likely the majority occur under the loose bark of trees, similar to the colonies reported from Indiana (Mumford and Cope 1964). Perhaps the single largest colony reported was found in a barn in Indiana (Cope and Humphrey 1972) on 22 June comprising 24 adult females, 12 immature females, and 18 immature males; 10 other adults escaped. Of the 24 females, 23 were lactating and 1 was pregnant. The ratio of number of adults to young confirmed the suspected litter size of one.

Maternity roosts are warm sites that maximize the growth rate of young while providing protection from predation and the weather. Cool summer temperatures can slow juvenile growth, thereby reducing the fat accumulation period prior to hibernation, and ultimately increasing the risk of overwintering mortality in young-of-the-year (McNab 1982).

Sex ratios from summer studies in different habitats demonstrate sexual dimorphism in habitat selection at this time of the year. In riparian areas in Iowa, Kunz (1971) found a 2:1 ratio of females to males, with most males taken in May and late August, apparently during migration. Caire et al. (1979), trapping at caves, found a preponderance of males.