Improving daytime detection of deer for surveillance and management

Maximising the detection of a target species reduces the uncertainty of survey results and can improve management outcomes. Deer (Cervidae) populations are managed worldwide due to their impacts on anthropocentric interests. In the UK, deer can only lawfully be shot during the daytime, from 1 h before sunrise to 1 h after sunset, when deer activity is at its lowest. We evaluated performance of a thermal imager relative to binoculars for their ability to detect deer during the daytime and at twilight (1 h either side of dawn and dusk). Transect surveys on Thorne Moors, UK, revealed that more roe and red deer were observed using a thermal imager than when using binoculars. More deer in much larger groups were observed at twilight than during the other daylight hours. Variation in animal detectability at different times of the day must be considered during wildlife surveys if their outputs are to be as accurate and precise as possible. The results support the continued focus of deer culling efforts during the hours of twilight. They also highlight the potential utility of thermal imagers for maximising detection probability at twilight.