The effects of detector height and season on bat activity have not been investigated in the UK and are poorly understood globally, yet are crucial to designing effective acoustic activity surveys. The aim of this study was to test predictions of species foraging preferences for the canopy or understorey and to determine whether this pattern shows seasonal variation. Bat activity was recorded using automated detectors in the canopy and understorey of an ancient broadleaved woodland in south-west England in spring and summer for a total of 40 nights. Activity levels were then compared between each of the two detector heights and two seasons. The canopy detector recorded significantly more bat activity overall and recorded significantly higher activity of Nyctalus, Pipistrellus pipistrellus and P. pygmaeus compared to the understorey detector. Nyctalus were more frequent in summer compared to spring. Pipistrellus pipistrellus consistently preferred the canopy, whilst P. pygmaeus showed this preference in summer only. Myotis were more frequent in spring compared to summer, favouring the understorey in spring but displaying a marginal preference for the canopy in summer. This suggests that it is advisable to record at multiple heights at different stages of the active season to assess bat activity in woodland. These effects should be tested in other woodland sites in order to determine the consequences of ground-based recording during a limited survey period.
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Vol. 14 • No. 2