Tracking wildlife energy dynamics with unoccupied aircraft systems and three‐dimensional photogrammetry

Abstract

Abstract We present a novel application using unoccupied aircraft systems (UAS; drones) for structure‐from‐motion three‐dimensional (3‐D) photogrammetry of multiple, free‐living animals simultaneously. Pinnipeds reliably haul out on shore for pupping and breeding each year, accompanied by dramatic female‐to‐pup mass transfer over a short lactation period and males lose mass while defending mating territories. This provides a tractable study system for validating the use of UAS as a non‐invasive tool for tracking energy dynamics in wild populations. UAS imagery of grey seals Halichoerus grypus was collected at Saddle Island, Nova Scotia. A multirotor UAS was piloted in 360‐degree orbits around relatively dense animal aggregations and georeferenced images were used for construction of a 3‐D point cloud, orthomosaic and Digital Surface Model for animal volumetric measurements. Directly following UAS survey, a subset of adult females were hand‐measured (morphometrics, blubber depth, n = 21 handlings [15 were unique animals]) and female–pup pairs were weighed (adult females: n = 32 [24]; pups: n = 33 [23]) to validate that UAS 3‐D photogrammetric models provided accurate animal volume and mass estimates. UAS two‐dimensional body length measurements were sensitive to animal recumbency and posture. The new UAS 3‐D photogrammetric method overcame these constraints, and aerial‐derived body volume measurements were equivalent to those collected from the ground. UAS body volume measurements precisely predicted ‘true’ body mass (mean absolute error, adult female: 3.8 kg, 2.1% body mass; pup: 4.1 kg, 9.8%), and exhibited a stronger relationship with total body mass than with blubber volume. The method was applied to 673 free‐living animals to characterize volume and mass dynamics across lactation and breeding for a much larger sample size than would be possible using traditional ground methods. Indeed, 1–46 animals (M ± SE: 9.2 ± 1.2) were modelled concurrently within the focal area of a UAS flight. Application of the method also captured significant inter‐annual variation in body volume/mass dynamics, and female‐to‐pup energy transfer efficiencies were lower when there was low sea ice extent. The UAS 3‐D photogrammetric method presented in this study is likely to be broadly applicable to other species, and the ability to measure whole groups of free‐living animals at once makes strides towards ‘weighing populations’.