Use of unmanned aerial vehicles to quantify social affiliations in beef calves

We developed a novel method to quantify cow-calf social affiliations by using unmanned aerial vehicles (UAVs) to quantify the distance between pairs of individuals in a herd. Using two UAVs equipped with video-capable digital cameras, as well as photogrammetry and image analysis software, we measured the distance between cow-calf pairs. We hypothesized that this distance can be accurately measured by UAVs and has the potential to provide valuable information in studies of animal behaviour and welfare. Twelve cow-calf pairs were uniquely identified with the use of penning tags and coloured fabric squares glued to their backs. Aerial videos of the calves held with their dams in a pasture were made over four days using UAVs. Recordings were made during two periods on most days of study: Daytime (Day) and Evening (Eve). Calves were acclimated to the UAVs by flying one UAV intermittently at distance of 10 m away from the calf on the days before experimental recording began. We used two UAVs to video-capture the following: (1) the location of all individuals (UAV flown at 100 m) and; (2) the identity of cowcalf pairs (UAV flown at 15 - 30 m). Still-images extracted from the UAV-acquired video screenshots were used to produce orthomosaics using Agisoft Photoscan software to correct geometric distortions from the original images. The orthomosaics captured all of the cows and calves in a single image, from which we measured the distance between calves and their dams and the distance between calves and non-related cows in the herd. We constructed a mixed model to test the effect of time period and cow-calf relationship (related or non-related) on cow-calf distance. Time period and cow-calf relationship each had a statistically significant effect (F1,849 = 101.79 and F1,849 = 26.75, respectively, P < 0.001) on the distance between cows and calves, but there was no significant interaction between these variables (F1,849 = 1.35, P = 0.245). The mean distance between calves and their dams (X̅ = 43.8 m, SD = 24 m) was smaller than that between calves and non-related cows (X̅ = 55.6 m, SD = 28 m). The mean cow-calf distance was smaller in the daytime (X̅ = 39.8 m, SD = 15 m) than in the evening (X̅ = 84.3 m, SD = 24 m). Distance measurements were tested for accuracy using reference lengths that were measured in the field; the mean difference between our software-measured length and the true length was 0.9 ± 0.7 m (mean and standard deviation). Our novel and non-invasive method appears to be sufficiently accurate and precise to study social behaviour in beef cattle in a field setting.