Tracking wildlife energy dynamics with unoccupied aircraft systems and 3-dimensional photogrammetry

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’.