Efficient non-invasive monitoring of an ecologically relevant physiological trait using thermal imaging of wild birds

Abstract

Physiology of free-living wild animals is often difficult to study in ecological/environmental monitoring contexts as it frequently requires invasive methods that involve capture and handling of individuals that can interfere in their physiology, stress, and behaviour. Infrared thermography (IRT) is a non-contact and non-invasive methodology that allows the remote measurement of body surface temperature. This individual physiological trait responds to ecological stressors and has been suggested to have potential as a non-invasive indicator of environmental stress in wild populations. In this study, we investigate the utility, efficiency, and reliability of IRT measurement in wild animals as a non-invasive tool for collecting physiological responses at scales and in data quantities needed to study ecological questions and to extend studies of thermal biology to explore population level differences. We used IRT to record eye region temperature in wild, free-ranging house sparrows while they visited feeders (n=130). We found the eye region temperature recorded during the first 3 seconds after the arrival to the feeder, which we define as baseline eye region temperature, consistently predicted the temperature measured across the whole of undisturbed feeding periods of different lengths (10, 15, 20, 25 s). Indicating that rapid and efficient data collection of representative measurements of a physiological response can be achieved under field conditions with high reliability (r > 0.9). By using GLMMs to explore a variety of factors that could potentially impact body surface temperature measurements in the field we found that they either had no significant impact or that significant effects (ambient temperature and feeding latency) can be readily controlled for. The results demonstrate that even in a species of highly active wild passerine bird IRT technology is suitable for non-invasive large-scale data collection of physiological responses to environmental stressors across many different sites. We suggest that thermal imaging will be an ideal tool for developing new environmental monitoring approaches based on changes in physiological state and for carrying out comparative thermal biology studies across different populations. We believe it can open new doors for monitoring diverse ecological and physiological questions on individuals, populations, and species of wild animals without interfering in their normal activities.