25 February 2010

To stay or to go?

Especially during a winter like this those birds that migrated south last autumn probably did a good choice in a Darwinian sense. In mild winters, however, those that decide to stay near their breeding areas may be at an advantage because they can select territories first and they don't have to pay the cost of migrating. Many bats have evolved the alternative strategy of hibernating, which means that they lower their body temperature almost to the ambient, and therefore they spend very little energy. But there is an energy cost and the fat they accumulate in the autumn must last until spring, when insects emerge again. In that sense they can be compared with bird migrants, but instead of converting fuel into distance, they migrate in time. In bats, there are species that do migrate, often in combination with hibernation at the winter destination, and we may think about how this strategy came to be? Here, I want to share some images from a recent winter bat count in the mine at Taberg, Småland. Six species were encountered, and here are pictures (lower) of Whiskered bat (Myotis mystacinus; the two small ones) and Brandt's bat (M. brandtii; the two larger ones on the right), and the picture on top shows a Natterer's bat (M. nattereri), which is quite a rare species in Sweden.