This study assesses the extent of midwinter activity (June-July, 1996, 1998) in an island population of lesser short-tailed bat (Mystacina tuberculata) located at the southern limit (= highest latitude) of the species' distribution. Activity of radio-tagged bats (n = 22 bats) was monitored over 38 nights (years combined). A mean ± SE of 64.1 ± 6.5% (range = 0-100%) of radio-tagged bats flew on each night monitored when mean minimum temperature was 3.4 ± 0.5°C. Temperatures ranged from -1.0 to 8.9°C, and 33% of tagged bats flew on the coldest night. Video-monitoring revealed high levels of activity at 11 large communally occupied tree roosts on 100% of nights monitored (n = 31 nights, years combined). Mean minimum temperatures on these nights was 3.0°C ± 0.5 (range = -1.4-7.1). Periods of activity were associated with feeding, social displays and changing roost sites. Movements of bats were dynamic with large numbers emerging from and entering roosts, often simultaneously, throughout most of the night (x = 81.9 ± 3.3% of night). Maximum numbers of active bats in tree cavities at one time numbered > 100 individuals on 75% of nights monitored. These included nine nights when individuals numbered > 500, and five nights when individuals numbered >1,000 (maximum = 1,443). Radio-tagged bats spent 57.1 ± 9.7% of time monitored roosting alone and the remainder roosting communally. They changed roost site on average every 4.2 ± 0.8 days moving between a total of 35 different roosts. Roosts were often re-used either by the same individual or by several different bats. Most radio-tagged bats visited communal night roosts that were different from those they had used during the day, with up to eight radio-tagged individuals visiting simultaneously. Lower levels of activity were recorded at six roosts that were occupied on the same nights as large active communal roosts. Video-monitoring over a total of 31 nights revealed external activity at these roosts on only 11 nights (35.4%). One radio-tagged bat did not emerge for 13 days. 1 suggest that winter activity may not be as energetically expensive for M. tuberculata as for many other cold-temperate bat species. Their ability to forage on terrestrial invertebrates, which are not commonly available to other species, and to select different roost sites where they could either remain active or relatively inactive, may allow them to be active more frequently and for longer durations.
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