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Acoustic monitoring : listening to what Nature has to tell us

Recorders : "Testing 1,2."

Last spring, we launched a field experiment to monitor biodiversity with passive acoustics in the "PRAXIS lab", our open-nature laboratory in the Luberon region. The site consists of a Mediterranean south-alpine forest with some semi-open areas (agricultural experimentation area, low shrubs). There is little anthropogenic noise in the area thanks to its distance to main roads and urban zones. This makes it an ideal place to monitor wildlife acoustic emissions without having to filter external noise. Available habitats, vegetation, climate and environmental variables can host a certain set of potential animal species. The presence of these species was confirmed by data found in open-source citizen wildlife surveys.

Vegetation at the PRAXIS lab
Stereo recorder with 4 microphones

Let's introduce the singers

In January, female and male tawny owls (Strix aluco) mark their territory and strengthen their bond by hooting at dusk - sometimes for several hours. This nocturnal raptor is the most abundant in Europe. Heterogenous semi-open lands harboring old trees with nesting cavities are their favorite spots.

Spring coincides with the comeback of migratory birds such as the unmistakable Common Cuckoo (Cuculus canorus). This opportunistic species can be found in a wide variety of habitats as long as there are some trees from which it can watch over the area. It is one of the rare birds to feed on pine processionary caterpillars in early spring [1].

Pine processionary caterpillar

At this time of the year, this species of caterpillars has stinging hairs for protection during migration from a tree to another. In the last decades, its population has dramatically increased in pine forests due to milder winter climates [2].

Cicadas rythme hot summer days with a typical sound, produced by an organ called tymbal vibrating up to 100 times per second [3]. This vibration can be heard at over 2 km and is used by males to attract females. There are around 15 species of cicadas in the South of France.

In August, migratory birds such as the colorful European Bee-eater (Merops apiaster) gather before crossing the Mediterranean sea to spend winter months in Africa. This species can be locally threatened due to heavy declines in insects populations and loss of sandy banks nesting-sites [4].

At night, bats use ultra-high frequency sounds (above 20 kHz, the human auditory threshold) to hunt insects with echolocation, which allows them to obtain information about their surroundings even in the dark. They are very beneficial to crops due to an insectivorous diet [5]. Post-recording treatment is required to be able to hear their acoustic emissions. The following recording is an example of what Common Pipistrelle (Pipistrellus pipistrellus) sound like when the recording is slowed down by a factor 10.

In September and October, it is sometimes possible to hear the roar of the Red Deer (Cervus elaphus) in large forested areas. The powerful roar is used to intimidate other male deers and attract a female.

Why study wildlife sounds ?

All above-mentioned species are difficult to spot by sight - camera traps can catch mammals but hardly birds or bats, let alone insects. Furthermore, sending experts on the field is expensive, time-consuming and invasive: it may change the behaviour of acoustically-active wildlife.

Roe deer caught by a camera trap in the PRAXIS lab

Sound recorders offer an attractive solution to monitor acoustically-active species when they are not easy to spot. Surveys based on acoustic activity are often the only way to obtain tangible proof of their presence.

Soundscapes change throughout the year, and they are likely to change with human perturbation and environmental changes.

What if we could :

  • detect changes in populations (e.g. tawny owls in the PRAXIS lab), communities (various species found in the PRAXIS lab), landscapes (species found in the mountain range containing the PRAXIS lab) at different time-scales ?

  • tackle some ecological questions (e.g. habitat use and connectivity, biodiversity assessment, land-use effect on biodiversity, efficiency of actions such as reforestation and vegetation management practices) ?

Ecoacoustics is the science dedicated to these questions [6].

What's next ?

GreenPraxis is currently acquiring recording units that can be used independently or as part of a network of recorders to provide spatialised data. The R&D team will come up with solutions to provide as much automation as possible in the data collection, treatment and analysis phases. We are talking about big data (around 0,5 TB for one week of continuous monophonic recording), which cannot possibly be analysed by humans. Here is where artificial intelligence and deep learning algorithms come in.

How are we trying to answer specific research questions with ecoacoustics and what results do we obtain ? This will be the topic of future articles. Stay tuned!



[1] Barbaro, L., & Battisti, A. (2011). Birds as predators of the pine processionary moth (Lepidoptera: Notodontidae). Biological Control, 56(2), 107–114.

[2] Battisti, A., Stastny, M., Netherer, S., Robinet, C., Schopf, A., Roques, A., & Larsson, S. (2005). Expansion of geographic range in the pine processionary moth caused by increased winter temperatures. Ecological Applications, 15(6), 2084–2096.

[5] Charbonnier, Y., Papura, D., Touzot, O., Rhouy, N., Sentenac, G., & Rusch, A. (2021). Pest control services provided by bats in vineyard landscapes. Agriculture, Ecosystems and Environment, 306, 107207.

[6] Sueur, J., & Farina, A. (2015). Ecoacoustics: the Ecological Investigation and Interpretation of Environmental Sound. In A. Sharov, M. Tonnessen, & T. Maran (Eds.), Biosemiotics. Springer.



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