Later is Better: Optimal Timing for Walked Activity Surveys for a European Bat Guild

Bat activity surveys (walked surveys combining transect and point counts) are extremely important for collecting data throughout Europe in conservation and planning contexts. To ensure optimal data, it is vital to ensure synchronicity between survey time and peak bat activity. However, although protocols for two‐hour dusk activity surveys are well accepted, recommended start time in relation to sunset is a ‘best guess’ rather than based on empirical evidence. Accepted practice differs widely with recommended start times varying from 30 min pre‐sunset (finishing 90 min post‐sunset) to 30 min post‐sunset (finishing 2.5 h after sunset). We provide the first empirical test of optimal start times for dusk activity surveys by comparing bat activity at the same sites on the same nights. Four sites were surveyed, viz. two high‐quality woodland sites and two low‐quality agricultural sites. At each site, surveyors walked the same route and stopped at the same pre‐defined listening points for three repeat surveys per night: 1) starting 30 min pre‐sunset; 2) starting at sunset; and 3) starting 30 min post‐sunset. In total, 240 hours' of data were collected. Four species, all widespread and common throughout Europe, were recorded: common pipistrelle Pipistrellus pipistrellus, soprano pipistrelle P. pygmaeus , Natterer's Myotis natterai and noctule Nyctalus noctule. Recorded bat activity was highest on sunset and post‐sunset surveys both generally (overall bat activity) and for all specific species encountered. Findings were generally consistent for both low‐ and high‐quality habitats. The same species were generally represented in both point and transect data but point data yielded higher estimates of overall activity in low‐quality habitat and higher bat species richness in both high‐ and low‐quality habitat relative to transect data. We recommend that: 1) two‐hour dusk bat activity surveys start at/after sunset not before sunset and 2) both transect and point data are collected and analysed.