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söndag 4 april 2010

Daytime flight of emerging bats

In temperate regions bats survive the winter either by hibernation (the typical bat strategy) or by migration south combined with hibernation in the wintering area. The fuel for hibernation (and migration) is fat, which is consumed at a slow rate throughout the winter. About now (March/April) is the time to wake up for hibernators, when they are running low on fat loads and they have to find food to survive. It is not uncommon to see bats flying around during daytime at the time of emergence from hibernacula. On good Friday (2nd April) the Hedenström-Åkesson family had been bird watching around Voms ängar and Krankesjön, when they drove back a small bat flying back and forth along the road on the Revinge field was spotted at 14:20. They managed to get some photos, from which it is apparent we are dealing with a Pipistrellus species. The most common, and hence most likely, species in Sweden is P. pygmaeus (dvärgfladdermus), but P. pipistrellus (pipistrell) and P. nathusii (trollfladdermus) cannot easily be excluded on the basis of the photos. Why are bats emerging from hibernation sometimes flying in daylight? It could be that they are in such a desperate need for energy that they need to feed in daytime; perhaps diurnal insects are more abundant than nocturnal insects at this time of the year? It could also be that the circadian clock has drifted during the winter sleep, which makes them fly at the “wrong” time?

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