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Bats of the temperate region of North America avoid winter by some combination of migration and hibernation at a location that provides the right conditions for minimizing energy expenditure over winter. Such optimal conditions are commonly found underground, and most of the best known hibernacula occur in caves or mines. Where winters are milder, some bat species hibernate in hollows in trees. Haida Gwaii, British Columbia, has the dual distinction of being relatively far north in terms of bat distribution, but with a relatively moderate oceanic climate. We hypothesized that because of the moderate winter temperatures, hibernating in trees could be an option for bats on Haida Gwaii. We used data loggers to monitor temperatures inside potential roost trees during 9 winters between 2002-2003 and 2012-2013. We found that mean winter temperature inside the trees ranged from 2.3-6.5°C, and in most years temperatures either did not drop below freezing or else did so only for short periods of time. We calculated that a 6 g bat would have required between 2.49-2.92 g of fat to hibernate in the tree roosts that we monitored, which is well within the limits that Little Brown Myotis (Myotis lucifugus) are known to accumulate in autumn. Our acoustic data demonstrated that California Myotis (Myotis californicus) were periodically active during all winter months except December, which we view as evidence that this bat hibernates locally either in trees or buildings. The absence of Little Brown Myotis, Keen's Myotis (Myotis keenii) and Silver-haired Bat (Lasionycteris noctivagans) observations during winter suggests that they may either use more common hibernacula, such as caves, or migrate off the islands. [ABSTRACT FROM AUTHOR]