We present a novel application using unoccupied aircraft systems (UAS; drones) for structure-from-motion three-dimensional (3-D) photogrammetry of multiple, free-ranging 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: 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-ranging 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 (mean±SE: 9.2±1.2) were modeled 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-ranging animals at once makes strides towards ‘weighing populations’.

README_SheroMethodsEcolEvol2021.xlsx contains metadata for each of the following datasheets:

Shero_MEE2021_masses_volumes.csv file contains ground acquired mass and animal body volume measurements, as well as the UAS-derived volumetric measurements for each animal. Animal posture, terrain, and flight characteristics are also provided.

Shero_MEE2021_larger_area_surveys.csv file contains UAS body volume measurements collected from larger area surveys. Note that under Age Cagetory, Adult female = "AF", Pup = "P", Unaccompanied pup = "UP", and Male = "M".

Shero_MEE2021_Alta6_flight_specs.csv file contains information on flight attributes for the FreeFly Alta 6 multirotor UAS projects used in the manuscript.

Shero_MEE2021_Morphs_datasheet.csv file contains original ground morphometrics measurements that were used to calculate animal total body volume using elliptical truncated cones.

Shero_MEE2021_supplemental_DJIPhantom_flightspecs.csv contains flight attributes of the DJI Phantom 4 Pro V2 flights used in the supplemental materials.

Shero_MEE2021_supplemental_DJIPhantom_animal_volumes.csv contains UAS volumetric measurements of animals imaged with the DJI Phantom Pro V2 UAS.