Recommended method: Roost counts. Note that roost counts can only give an absolute abundance estimate within a localized area, and only if one can be sure that all roosts sites in the area are known and accounted for. These conditions are generally not realistic. Roost counts, however, will provide a fairly accurate estimate of colony size for a given roost.
Suitable roost sites are of paramount importance to bats (Kunz 1982). Roost sites may be night roosts (sites used temporarily between feeding bouts during the evening), day roosts (sites that bats use during the day), or hibernacula (sites where hibernation occurs during the winter months). Roosts may also serve as maternity colonies where adult females aggregate to raise young. Bats tend to be more faithful to day roost sites and hibernacula, and have more stringent requirements for characteristics such as the thermal properties of roosts (Brigham et al. in press).
Recent research suggests some species of bat in B.C. regularly change day roosts (trees; Vonhof and Barclay 1996, Rasheed and Holroyd 1995). Many species have very specific requirements for roosts in terms of shelter, temperature, humidity, and environmental stability (Kunz 1982). These requirements must be met if a bat is going to be able to use torpor in an energetically efficient way. This is especially true for hibernacula. The limited number of suitable roost sites may set an upper limit on bat population sizes. In contrast, the solitary species that roost in foliage, such as Lasiurus cinereus and L. blossevilli, do not seem to have special requirements for roost sites. The advantage of roost monitoring is that roosts tend to be permanent and can be logistically easy to study (Thomas and Laval 1988).
Locating roosts is largely a matter of identifying potential roost sites (such as cliffs, buildings, caves and mines; see also Table 3). These sites should then be observed (visually and with detectors) around dusk to see if bats emerge. If they do, then visual counts or counts with bat detectors can be made to assess the size of the colony. Some species of bat use trees as roosts, particularly older trees and snags, with cavities or crevices in the bark (Kunz, 1982). It may be possible to identify roost trees by signs such as urine stains on trunks, feces accumulations on the ground or by hearing audible bat sounds (Mayle, 1990).
During the winter, bats use a wide variety of sites for hibernation in British Columbia, including caves, mines, buildings, and trees (Nagorsen et al. 1993). Identification of these sites is important in order to improve our understanding of bat ecology, and to improve the protection of important hibernation sites. Hibernacula should generally not be entered as this causes disturbance of the roosting bats and may lead to abandonment. However, in some circumstances, entering hibernacula may be warranted. These may include annual checks on the status of a site, long-term studies of changes in bat numbers, or scientific studies of hibernation and arousal mechanisms. If winter hibernacula are identified, they should be periodically monitored from the inside (we suggest every five years) by experienced individuals, to assess if any changes in population size are occurring. However, disturbance should be kept to a minimum and hibernacula should be afforded protection. At present very few hibernacula are known in B.C. (Nagorsen and Brigham, 1993), although this presumably means they are relatively free from disturbance.
If it is necessary to enter a hibernacula, safety of the surveyors must be a prime consideration, as well as minimizing disturbance to bats. Repeated disturbance to bats in hibernation sites can reduce their survival by forcing them to use valuable stored food reserves (Thomas et al., 1990, Speakman et al. 1992). The acceptable frequency of surveys to hibernacula will vary with the configuration of the site, the number of bats, and the purpose of the survey. Surveys should last for only short periods, in order to minimize disturbance to bats. Maximum party size should be related to the size of the hibernacula, and the density of bats. However, generally only two (for safety purposes), or a few trained and experienced individuals should be involved in the survey. Hibernacula should only be entered once in the winter (between November and March), preferably when the maximum number of bats is present. For most sites, the peak in occupancy probably occurs during January or February, although there is likely variation between sites and species in British Columbia. If extensive mapping of the hibernacula is required, this should take place during the summer when bats are absent (Kunz et al. 1996).
Although not a method for assessing absolute abundance of bats, radio-telemetry is a technique for locating and identifying roost sites. By clipping the fur on the backs of bats and attaching radio-transmitters, it is possible to track them to a roost site. However, certain species (e.g., P. townsendii, E. maculalum) are particularly sensitive, and should not have their hair clipped for tagging purposes. In addition, smaller species (e.g., M. californicus, M. ciliolabrum) may be inappropriate for tagging because of their mass relative to that of a radio tag (see below). Adult male bats of appropriate size can be tagged without great concern. However, only adult females that are in early pregnancy, non-parous, lactating, or post-lactating should be tagged. Adult females during late pregnancy and juveniles should not be tagged.
For flying animals, the recommended maximum mass that should be attached is 5 percent of body mass. Note that for bats, the calculation of body mass does not include a stomach full of insects. Additional mass will have significant effects on an animal's behaviour (Aldridge and Brigham, 1988). If this '5% rule' is followed then only 6 of the 16 species of bat in B.C. are suitable for radio-telemetry using a 0.44 g transmitter (see masses of bats in Table 1). However, there is some controversy as to the effects of putting a transmitter on smaller bats (D. Thomas, Personal communication). Although there is no doubt additional mass may change a bat's behaviour, especially when foraging, it can be argued that carrying around a transmitter is not that much different from carrying around a fetus (which typically weighs more than the smallest transmitters) which adult females obviously do each year. However, it is unclear what effects changing the center of gravity may have on a bats flying ability, especially on males that are unaccustomed to such weight gains (see also Kalcounis and Brigham 1995 for effects of weight on bats).
Arguably, radio-telemetry may be appropriate for locating the roosts of most species of bat. However, this assumes that the roosting habitat of tagged bats are the same as those for untagged individuals. Radio-telemetry is a useful, though expensive and time consuming (Wilkinson and Bradbury, 1988) method for locating roost sites, but it is not useful for obtaining realistic information on foraging habitats or behaviour of smaller species.
The smallest transmitters currently available weigh about 0.44 g, have a battery life of 8 to 10 days and a maximum detection distance of 2 to 3 km which varies depending on topography. Larger transmitters (0.7 g) may last 3 to 4 weeks, with slightly greater detection distances. Transmitters usually remain attached for 1 to 14 days. However, some species are more adept at removing (via chewing or grooming) the transmitter, thereby reducing its effective life.
Non-random (for roost counts). Observations should be conducted at known roost sites (Table 3). See above for protocols for mist netting (to acquire bats for radio-telemetry).
For emergence data, usually only one roost exit per night per observer can be monitored. Data should be collected on more than one night to account for emergence number variation.
Counts of emerging bats provide an estimate of population size for that specific roost (Thomas and LaVal 1988). However, to extrapolate the information to larger geographic areas or populations, a researcher must:
Due to these constraints, population estimates should reasonably be limited to individual roosts. If large colonies or winter hibernacula are identified, they should be periodically monitored from the inside (we suggest every five years) by experienced individuals, to assess if any changes in population size are occurring. However, disturbance should be kept to a minimum and large colonies or hibernacula should be afforded protection.
When conducting roost counts:
When conducting radio-telemetry: