A practical comparison of manual and autonomous methods for acoustic monitoring
Free to read 1. Autonomous acoustic recorders are widely available and can provide a highly eﬃcient method of species monitoring, especially when coupled with software to automate data processing. However, the adoption of these techniques is restricted by a lack of direct comparisons with existing manual ﬁeld surveys. 2. We assessed the performance of autonomous methods by comparing manual and automated examination of acoustic recordings with a ﬁeld-listening survey, using commercially available autonomous recorders and custom call detection and classiﬁcation software. We compared the detection capability, time requirements, areal coverage and weather condition bias of these three methods using an established call monitoring programme for a nocturnal bird, the little spotted kiwi(Apteryx owenii). 3. The autonomous recorder methods had very high precision (>98%) and required <3% of the time needed for the ﬁeld survey. They were less sensitive, with visual spectrogram inspection recovering 80% of the total calls detected and automated call detection 40%, although this recall increased with signal strength. The areal coverage of the spectrogram inspection and automatic detection methods were 85% and 42% of the ﬁeld survey. The methods using autonomous recorders were more adversely aﬀected by wind and did not show a positive association between ground moisture and call rates that was apparent from the ﬁeld counts. However, all methods produced the same results for the most important conservation information from the survey: the annual change in calling activity. 4. Autonomous monitoring techniques incur diﬀerent biases to manual surveys and so can yield diﬀerent ecological conclusions if sampling is not adjusted accordingly. Nevertheless, the sensitivity, robustness and high accuracy of automated acoustic methods demonstrate that they oﬀer a suitable and extremely eﬃcient alternative to ﬁeld observer point counts for species monitoring.