DOI: 10.5061/dryad.h70rxwdhq
title: Detecting the presence of nesting nightjar in upland clear-fell using drone-mounted thermal cameras

description: 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.
description: 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 (&gt;100 hectare) forestry sites with continuous
 plantations of Sitka spruce (<i>Picea sitchensis</i>) and
 Norway spruce (<i>P. abies</i>) 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.
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