Recogniser construction
To construct the recognisers, we used the technique ‘binary template matching’ – a technique that first converts a spectrogram into a binary template and then matches ‘on’ and ‘off’ points of the template to the file the recogniser is applied to (Katz et al., 2016b; Towsey et al., 2012). This was done using the monitoR package in R, which provides flexibility in its construction parametres (Katz et al., 2016b). To build the initial recognisers, templates were constructed from a minimum of 10 reference calls (from the pool of 100 candidate calls) that were both clear and representative of the variation in calls and environmental conditions. Binary templates comprise ‘on’ and ‘off’ regions (call and non-call), which are based on a user-defined amplitude cut-off (Figure 1). Each template’s amplitude cut-off was determined through manual inspection of templates using the makeBinTemplate function in monitoR (Katz et al., 2016b). Amplitude cut-offs were set arbitrarily and progressively altered and reviewed. A cut-off that clearly showed call structure and was not masked by background noise, was deemed appropriate, with some background noise deemed acceptable.