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

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.