Vocal activity of Nathusius' pipistrelle (Pipistrellus nathusii) was studied between April—October 1998 in Wrocław, SW Poland. This activity (nocturnal and seasonal), measured as the number of social and echolocation calls recorded along a 2 km convoluted transect, was lowest in April, May and June. In July, vocal activity increased to reach its highest level during August and September. Throughout the study, calls were emitted in the early part of the night from May through June, then switched to the latter part of the evening in August. Seasonally, the number of calls correlated positively with the air temperature; no significant relationship was found between the level of social and echolocation calls, and relative humidity or atmospheric pressure. It is considered that these changes in vocal activity in July were associated with the onset of the mating period, when the males emitted social calls during an advertisement display or from the roosts, performed to attract females for harem formation. The calls of P. nathusii emitted outside the mating period may also have played another social function.
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