Autumn Activity of Myotis sodalis (Indiana Bat) in Bland County, Virginia

Northeastern Naturalist, 2006 by Brack, Virgil Jr

Diurnal roosts

During autumn swarming, M. sodalis congregates at caves that serve as winter hibernacula (Cope and Humphrey 1977, LaVal and LaVal 1980). Early in autumn, few bats roost in hibernacula, but as winter approaches, increasing numbers roost in the caves. About 50% of radio-tagged bats were tracked to roost trees near (mean = 0.8 km) the hibernaculum; roosts were not spread across the study area. However, 70% of bats left the project area one or more times and were logistically infeasible to locate. Gumbert (2001) located roosts for 20 of 26 bats radio-tagged in autumn and found roosts clumped near the hibernaculum (mean = 1.9 km).

Many bats used more than one roost, which is apparently characteristic of the species, regardless of time of year or type of colony. In Kentucky, M. sodalis in autumn often used more than one roost (Gumbert 2001), similar to summer maternity roosts in Michigan (Kurta et al. 2002) and Illinois (Carter and Feldhamer 2005), and males during summer (Brack et al. 2004) in Indiana. The number of suitable roosts may be limiting when a hibernaculum supports large numbers of bats, including both M. sodalis and congeneric species, which could lead to competition for roosts. It is possible that roosts closer to the cave provide an advantage during swarming, such as a savings in energy expenditures or a greater opportunity to mate.

Roost trees were often associated with gaps in the canopy, including forest edges and openings. Similarly, roosts of maternity colonies (Callahan et al. 1997, Carter and Feldhamer 2005, Kurta et al. 1993) and of individual males in summer (Brack and Whitaker 2004, Brack et al, 2004) are often located near canopy gaps to receive solar exposure. Roosts near openings and edges may also offer easy access to flyways (Brown and Brack 2003) or foraging habitat. The range in size of roost trees was large. Individuals or small numbers of bats sometimes roost in smaller trees than used by larger maternity colonies (Callahan et al. 1997, Gumbert 2001, Kurta et al. 1993). Larger trees may simply accommodate more bats. Alternatively, the thermal mass of a large maternity tree may be important for pups left alone by foraging females, while a smaller roost that warms quickly after sunrise may benefit single bats that leave the roost at night.

Reproductive activities

Most tagged bats left the project area or went into the cave one or more times for one or more days. Some bats were tracked as they left the study area. They were not found during nocturnal activity studies or in diurnal roosts. A similar phenomenon was observed at a hibernaculum in Kentucky (Gumbert 2001) and Indiana (Hawkins et al. 2005). Long-distance movements are in stark contrast to the clustering of day roosts in close proximity to hibernacula.

It is not known why bats left the area, but departure during swarming has implications for reproductive fitness. Abundance of females increases and then decreases over the season, but males are common throughout (Cope and Humphrey 1977, LaVal and LaVal 1980). Leaving the swarming area reduces or eliminates the opportunity to mate, especially for males. Bats that leave also spend energy traveling from and returning to the cave, so there should be a benefit. Possibly, bats visit and mate at other swarming locations, although distance to such sites and the relative abundance of bats (i.e., mating opportunities) should affect that choice. Alternatively, males seeking mating opportunities at the cave may need to intermittently leave to replenish energy supplies, especially as the season progresses and the time for both mating and foraging is compressed closer to sunset. Scarcity of a declining food supply and a contracting nightly and seasonal window to both forage and mate may be exacerbated by interspecific competition with large concentrations of swarming bats of several species. Over 16,000 bats, most of the genus Myotis, hibernate in the Skydusky Hollow Cave System (Brack et al. 2005).