We used stationary automatic ultrasound recording systems (Batcorder, EcoObs GmbH,
Marckmann, Schuster and Runkel, Nürnberg, Germany) to record bat occurrence and activity
above 50 grassland sites. A Batcorder was installed on top of a 1.80 m pole at the centre of each
grassland plot, directing the microphone towards open landscapes. Each grassland plot was
sampled five times (one night per month) between May and September 2010, except for 10
plots which could only be accessed four times due to logistic reasons (e.g., grazing bulls, hunting).
We surveyed five to six grassland plots simultaneously within the same night and sampling
plot combinations were randomized across the season. The recording system was
operating from sunset until 01:00 AM to limit recording time to the first peak of bat activity
during the night [39–40] and to allow the collection of data and replacement of batcorders on
other, and often quite distant, grassland plots for the next night. To control for the potential
confounding effect of moonlight, we visited grassland sites only in a two week period just
before and after new moon. Recording was aborted in case of rainfall and the survey was
repeated the following night.
Recordings were made in real time (sampling rate of 500 kHz, 16 bit) and triggered by the
sound pressure level at a threshold of -27 dB SPL. A frequency filter at 15 kHz was used to
avoid that environmental noise triggered additional recordings. A recording continued as long
as the sound pressure level remained above the threshold level within a post trigger time of 800
ms to assure that complete passes of bats with very long pulse intervals (e.g. Nyctalus noctula)
would be stored within one file.