Citation: Larsen, H.L.; Møller-Lassesen, K.; Enevoldsen, E.M.E.; Madsen, S.B.; Obsen, M.T.; Povlsen, P.; Bruhn, D.; Pertoldi, C.; Pagh, S. Drone with Mounted Thermal Infrared Cameras for Monitoring Terrestrial Mammals. Drones 2023, 7, 680. https:// doi.org/10.3390/drones7110680 Academic Editors: Humberto L. Perotto-Baldivieso and Aaron M. Foley Received: 11 September 2023 Revised: 12 November 2023 Accepted: 16 November 2023 Published: 18 November 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). drones Article Drone with Mounted Thermal Infrared Cameras for Monitoring Terrestrial Mammals Hanne Lyngholm Larsen 1, * , Katrine Møller-Lassesen 1 , Esther Magdalene Ellersgaard Enevoldsen 1 , Sarah Bøgh Madsen 1 , Maria Trier Obsen 1 , Peter Povlsen 1 , Dan Bruhn 1 , Cino Pertoldi 1,2 and Sussie Pagh 1 1 Department of Chemistry and Bioscience, Aalborg University, 9220 Aalborg, Denmark; cp@bio.aau.dk (C.P.); sup@bio.aau.dk (S.P.) 2 Aalborg Zoo, 9000 Aalborg, Denmark * Correspondence: hannell@bio.aau.dk Simple Summary: This study investigates the use of a drone equipped with a thermal camera for recognizing wild mammal species and to determine population sizes of red deer (Cervus elaphus) and roe deer (Capreolus capreoulus) in a 13 km 2 moor in Denmark. The thermal images of wild animal species in the area could be distinguished by their body measures when the drone filmed with the camera perpendicular to the ground in an altitude range of 60–120 m. The thermal drone images of the mammal species’ body measures showed significant distinctiveness (F = 6.8, p < 0.001) with MANOVA and population studies of larger mammals, and at the same time was more time-efficient and less invasive for species recognition and population studies than traditional methods. Abstract: This study investigates the use of a drone equipped with a thermal camera for recognizing wild mammal species in open areas and to determine the sex and age of red deer (Cervus elaphus) and roe deer (Capreolus capreoulus) in a 13 km 2 moor in Denmark. Two separate surveys were conducted: (1) To achieve drone images for the identification of mammals, the drone was tested around a bait place with a live wildlife camera that was often visited by European badger (Meles meles), stone marten (Martes foina), European hare (Lepus europaeus), roe deer and cattle (Bos taurus). The thermal images of wild animal species could be distinguished by their body measures when the drone filmed with the camera pointed perpendicular to the ground in an altitude range of 50–120 m. A PCA ordination showed nonoverlapping body characteristics and MANOVA showed that the combined body measures used were significantly distinctive F = 6.8, p < 0.001. The reactions and behavioral responses of the different species to the altitude and noise of the drone were also tested in this place. (2) On a 13 km 2 moor, a drone was used for a population study of deer. Red deer and roe deer were counted and separated by body measures. Red deer individuals could, at the right altitude, be separated into adults and calves, and males and females. Body length was the most conclusive body measure, and therefore a reference measurement in the field is recommended. The frame thermal images were effective in species recognition and for use in population studies of deer, and are thought to be more time-efficient and less invasive than traditional methods. In autumn, the number of stags and the life stage of red deer, as well as the distribution of deer in different types of vegetation, could be determined. Keywords: UAV; red deer; roe deer; badger; fox; marten; European brown hare; ungulates; game species 1. Introduction Population and species monitoring are important elements in the management and conservation of species [1]. Hunters’ harvest statistics are often used as an indirect mea- surement of population size. However, this only concerns species that are not protected, and game bags may reflect hunters’ tradition rather than changes in population size [2–4]. Drones 2023, 7, 680. https://doi.org/10.3390/drones7110680 https://www.mdpi.com/journal/drones