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Detecting the presence of nesting nightjar in upland clear-fell using drone-mounted thermal cameras

Citation

Shewring, Mike; Vafidis, Jim (2021), Detecting the presence of nesting nightjar in upland clear-fell using drone-mounted thermal cameras, Dryad, Dataset, https://doi.org/10.5061/dryad.h70rxwdhq

Abstract

1. Confirming the presence and location of European Nightjar Caprimulgus europaeus nests is a significant fieldwork challenge in ecological monitoring. Nest sites can be located through direct observation or capture and radio tracking of breeding individuals, however such work is time-consuming, disturbing and costly. 2. Unmanned Aerial Vehicles equipped with thermal sensors may enable rapid survey over large areas by detecting nest locations based on the contrast of relatively warm nests and the surrounding cooler ground. The application of this concept using UAV-mounted thermal sensors was trialled in two upland clear-fell forestry sites in South Wales, UK. 3. Detection trials were undertaken at five known nightjar nest sites to assess optimal timing and flight height for surveys. Nest heat signatures were clear during dusk and dawn, but not during the daytime. Nests were identifiable at flight heights up to 25m, but flight heights of 12-20m were optimal for the numbers of pixels per nest. 4. This approach was tested in a field trial of a 17ha forestry site where the presence and position of nesting nightjars was unknown. An automated transect at dusk and dawn at 15m flight elevation identified two active nightjar nests and four male nightjar roost sites. Without image analysis automation, the process of manual inspection of 2,607 images for ‘hot spots’ of the approximate size and shape of nightjar nests was laborious. 5. The UAV approach took around 18hrs including survey time, processing and ground verification, whilst a nightjar nest finding survey would take 35 hrs for the same area. The small size of nightjars and the low resolution of the thermal sensors requires low altitude flight in order to maximise detectability and pixel coverage. Low flight elevation requires more consideration of the risk of collision with trees or posts. Consequently, the approach would not be suitable for covering areas of highly variable terrain.

Methods

The study was undertaken at two field sites in South Wales, UK; Bryn Forest (‘Bryn’, SS 820 903) in Neath Port Talbot; and Cwmcarn Forest (‘Cwmcarn’, ST 230 928) in Caerphilly (Figure 1). Bryn is located between 200-350m above sea level (asl) whilst the Cwmcarn study area spanned 350-380m asl. Both field sites are owned by the Welsh Government and managed by Natural Resources Wales as part of large (>100 hectare) forestry sites with continuous plantations of Sitka spruce (Picea sitchensis) and Norway spruce (P. abies) and large areas of clear-fell.

The Falcon 8 (Ascending Technologies Ltd.) UAV with a gimbal-mounted Tau 640 thermal infrared (IR) camera with 19mm optics was used to collect 57 thermal images of the five European Nightjar nests at Bryn in 2018. These were taken at dawn (0430-0600), midday (1200-1400), and dusk (2030-2200). The Tau 640 has a 640 × 512 pixel resolution and 9hz framerate. 

Mean temperature values were extracted for all nest pixels (manually identified by species specialist - approximately 25 pixels per nest) and surrounding environments (500 randomly selected pixels) to achieve this, temperature values were extracted from each pixel identified as a nest pixels or background point pixels, these values were then summed and divided by the number of input pixels.  The difference between the two means was calculated for each image by subtracting the environment mean from the nest mean.

Usage Notes

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