Thermal‐UAV assisted detection and relocation of roe deer fawns in managed grasslands

Advancements in modern fodder production have unintentionally increased wildlife mortality during grass mowing operations. Roe deer (Capreolus capreolus) fawns are particularly vulnerable because they remain motionless as a predator-avoidance strategy. Unmanned aerial vehicles (UAVs) equipped with thermal sensors offer a promising method for detecting fawns before mowing, but the post-relocation survival of relocated fawns remains uncertain. In this study, 40 roe deer fawns were marked, mainly with VHF collars, several days or weeks prior to mowing. An independent unmanned aerial vehicle (UAV) pilot, unaware of fawn locations, conducted aerial surveys in the early morning before mowing. The pilot detected 22 of 24 radio-marked fawns present in fields scheduled for mowing, corresponding to a >90% detection rate. All detected fawns were relocated less than 300 m away to safe areas and survived for at least 4 days post-relocation, consistent with successful mother - offspring reunion. These short-distance relocations were thus associated with high short-term survival. Overall, UAV-based thermal sensors proved to be an effective approach to reduce fawn mortality in mowed grasslands.