Factors underlying migratory bat aggregations in chestnut groves

Deciduous woodlands have undergone major structural changes in Europe in the course of the last century. Dense woodlands have, for instance, replaced traditionally managed, open sweet chestnut groves, and have led to biodiversity losses. Surveys carried out in the southern Alps have shown that the frequency of occurrence of the rare migratory Leisler’s bat Nyctalus leisleri in bat boxes is much higher in managed than in unmanaged chestnut groves, pointing to the importance of traditional groves as stopover sites. To identify the reasons for this habitat preference we investigated whether managed chestnut groves constitute a more appropriate foraging ground than closed stands, and whether bat boxes in managed chestnut groves offer more suitable temperature conditions for roosting. Foraging habitat selection and roost usage were investigated by means of radiotracking and temperature loggers, respectively. Foraging Leisler’s bats predominantly selected deciduous woodlands over other habitat types, but showed no distinct preference for either type of chestnut grove over other forest types. In managed chestnut groves, bats used cooler boxes than others available in the same habitat, but there was no difference in temperature between the boxes used in managed versus unmanaged groves. Our results provide no evidence that foraging opportunities or roost temperature dictate the forest-specific pattern of bat box occupancy. As an alternative explanation, we suggest that managed chestnut groves may represent optimal lekking arenas due to their open structure and high roost density. Whatever the reason for this habitat preference, partially maintaining traditional management, resulting in a semi-open, mosaic landscape, is likely key to maintaining chestnut groves attractive for this migratory bat.