Detecting Animals in Repeated UAV Image Acquisitions by Matching CNN Activations with Optimal Transport

Repeated animal censuses are crucial for wildlife parks to ensure ecological equilibriums. They are increasingly conducted using images generated by Unmanned Aerial Vehicles (UAVs), often coupled to semi-automatic object detection methods. Such methods have shown great progress also thanks to the employment of Convolutional Neural Networks (CNNs), but even the best models trained on the data acquired in one year struggle predicting animal abundances in subsequent campaigns due to the inherent shift between the datasets. In this paper we adapt a CNN-based animal detector to a follow-up UAV dataset by employing an unsupervised domain adaptation method based on Optimal Transport. We show how to infer updated labels from the source dataset by means of an ensemble of bootstraps. Our method increases the precision compared to the unmodified CNN, while not requiring additional labels from the target set.