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100 to more than 1,000 kilometres from the place of birth. It is remarkable that during thesemigrations they use those woodlands and tree holes that are occupied annually in theseseasons by migrating bats. Such tree holes are often used also for hibernation in winter.4.1.5 Hibernation roostsHollow trees are important for some bat species as hibernacula. They lie in climaticallyfavourable lowlands predominantly in woodland areas but also in parks. Those hollow treesare approached by the bats with the first frost, which, in Central Europe, is usually in mid-November. Bats sometimes form large groups in hibernation roosts, e.g. the noctule where acolony may contain more than 900 individuals (Frank & Dietz 1999). Several hibernationroosts are in trees standing close to each other (Frank 1997). They are consistently used overmany winters (Bock 2001). The hibernation colonies start to disperse from the end ofFebruary until the middle of March.Table 3 Function of tree holes for bats in the course of the yearFunctionMonth1 2 3 4 5 6 7 8 9 10 11 12NurseryMalesMatingMigrationWinter4.2 Which roost types are important?Due to their insectivorous dentition, bats are not able to make suitable roosts themselves.Rather, they depend on natural tree holes or those made by other animals. Different typesinclude woodpecker holes, crevices, rotted knotholes and loose bark (cf. Meschede & Heller2000).Species have different requirements from a tree hole and these might change throughout theyear depending on the function of the roost (Tables 3 & 4). In general bats seek protectionfrom the weather and predators in their roost. Furthermore a tree hole should enable bats tostay away from their droppings. An ideal tree hole has a hanging place above the entranceand a hollow below into which droppings can fall. Woodpecker holes with the former nestsite below the entrance hole fulfil this criterion if they have been enlarged upwards by theinfluence of fungi and rotting over time (van Heerdt & Sluiter 1965, Stratmann 1978).Similar development processes are observed in other roost types that have arisen by barkdamage or branch loss.The volume of a tree hole is another criterion of its quality. Maternity roost sites must offerat least enough room that a group of females with their young can be resident. This is evenmore important for hibernation roosts, e.g. for the noctule or other tree hibernating species.A roost of noctules in the Philosophenwald in Giessen (Hesse, Germany) with up to 900hibernating animals had a volume of 53 litres. In an examination of twelve hibernation roost38