Using temperature loggers to monitor den box use by American martens, fishers, and tree squirrels

Tree cavities are keystone habitat resources used by many animal species. Artificial den boxes can provide habitat for cavity-dependent wildlife where natural cavities are rare and have been used to address a variety of conservation, research, and management objectives. Accurate, cost-effective methods are needed to monitor den boxes to meet research and management objectives. Recent studies have used temperature loggers to monitor den box use, often using the warming effect of animal presence to detect presence of animals in den boxes. While effective, these studies have been unable to distinguish different species using den boxes from temperature data. Here, we describe a novel method for monitoring den boxes using internal temperature loggers that relies on temperature rate of change values. Our objectives were to describe the pattern of temperature rate of change corresponding to uses of den boxes by American martens, fishers, and tree squirrels; evaluate how effectively temperature rate of change could detect uses by these species; and evaluate whether temperature rate of change could be used to identify the species using den boxes. We built and installed 86 fisher den boxes in northern Minnesota, USA and monitored each box year-round for 3 years after installation using remote cameras and internal temperature loggers. We found that temperature rate of change showed a consistent pattern when animals entered, remained in, and then left the den box. Using temperature loggers, we were able to successfully identify all instances in which remote cameras documented fishers and martens using den boxes, but we rarely detected use by tree squirrels. The rate of change pattern when fishers used den boxes was statistically different from the pattern when martens used den boxes, demonstrating that species identification from temperature data is possible. Temperature loggers are an accurate, cost-effective method for monitoring den boxes to aid wildlife conservation, research, and management.