The impact of environmental factors in birdsong acquisition using automated recorders

Abstract

The use of automatic acoustic recorders is becoming a principal method to survey birds in their natural habitats, as it is relatively noninvasive while still being informative. As with any other sound, birdsong degrades in amplitude, frequency, and temporal structure as it propagates to the recorder through the environment. Knowing how different birdsongs attenuate under different conditions is useful to, for example, develop protocols for deploying acoustic recorders and improve automated detection methods, an essential part of the research field that is becoming known as ecoacoustics. This article presents playback and recapture (record) experiments carried out under different environmental conditions using twenty bird calls from eleven New Zealand bird species in a native forest and an open area, answering five research questions: (1) How does birdsong attenuation differ between forest and open space? (2) What is the relationship between transmission height and birdsong attenuation? (3) How does frequency of birdsong impact the degradation of sound with distance? (4) Is birdsong attenuation different during the night compared to the day? and (5) what is the impact of wind on attenuation? Bird calls are complex sounds; therefore, we have chosen to use them rather than simple tones to ensure that this complexity is not missed in the analysis. The results demonstrate that birdsong transmission was significantly better in the forest than in the open site. During the night, the attenuation was at a minimum in both experimental sites. Transmission height affected the propagation of the songs of many species, particularly the flightless ones. The effect of wind was severe in the open site and attenuated lower frequencies. The reverberations due to reflective surfaces masked higher frequencies (8 kHz) in the forest even at moderate distances. The findings presented here can be applied to develop protocols for passive acoustic monitoring. Even though the attenuation can be generalized to frequency bands, the structure of the birdsong is also important. Selecting a reasonable sampling frequency avoids unnecessary data accumulation because higher frequencies attenuate more in the forest. Even at moderate distances, recorders capture significantly attenuated birdsong, and hence, automated analysis methods for field recordings need to be able to detect and recognize faint birdsong.

Comparison between lek counts and bioacoustic recording for monitoring Western Capercaillie (Tetrao urogallus L.)

“…In addition, there are a number of temporal and environmental conditions that can affect detectability and occupancy at leks (Walsh et al 2004;Raynor et al 2017;Fremgen et al 2018). An understanding of these factors is critical to improving population estimates for the species being monitored (Drummer et al 2011;Sadoti et al 2016;Priyadarshani et al 2018). Within this study, the levels of vocal activity were affected by date, time and altitude, but (perhaps surprisingly) no relationship with weather variables was found.…”

“…the song of a particular bird species), and groups these into a number of clusters based on their similarity. This study represents one of the first published tests of this software, filling a recently identified gap in the literature (Knight et al 2017;Priyadarshani et al 2018).…”

“…Recorders can be left in the field for long periods to acquire significant quantities of data, but manual processing by human observers can then be labour intensive. To counteract this, automated methods are being developed, either to recognize species vocalizations outright or to classify and group recordings, to make manual checks more efficient (Priyadarshani et al 2018). Due to the recent release of the Kaleidoscope Pro software employed in this study, tests of its use and validity are rare in the literature.…”

Experimental test of birdcall detection by autonomous recorder units and by human observers using broadcast

“…Previous work indicated differences in transmission of bird sounds between day and night (Priyadarshani, Castro, Marsland, ) and so the experiment was conducted between 21:00 and 23:30, which is in the time range where the selected species naturally call. Calls were broadcast at natural volume (Section 2.4 below).…”

Acoustic localization of terrestrial wildlife: Current practices and future opportunities

“…In air, the amplitude of a sound decreases farther from the source of the sound, a process called attenuation . Higher frequencies attenuate faster than lower frequencies, meaning that for two sounds produced at the same amplitude but different frequencies, the lower‐frequency sound can be heard from a greater distance than the higher‐frequency sound, although these effects are mediated by factors such as habitat and weather conditions (Goerlitz, 2018; Priyadarshani, Castro, & Marsland, 2018; Spiesberger & Fristrup, 1990).…”