Body surface temperature as an indicator of physiological state in wild birds

Understanding physiological processes is key to answering the questions of why organisms behave in the way they do, and how they interact with each other, and their environment. Despite technological innovations in recent decades, assessment of physiological state in free-living animals still generally requires subjects to be trapped and handled, so tissues or blood can be sampled, or so measurement devices can be attached or implanted. Such methods limit research to species and individuals that can be caught, potentially restricting the generalisability of findings, and introducing bias. Additionally, natural behaviours are interrupted, and subsequent physiology, behaviour or performance may be altered as a result of the stress of capture, the burden of attached apparatus, or the effects of surgery. Consequently, alternative techniques such as inferring physiological state from traits that do not require invasive sampling would be a valuable development. Body temperature is a particularly promising candidate trait, linked with an array of physiological functions, and having previously been used as a proxy for metabolic activity, stress state and immune challenge. With the advent of low cost, highly portable thermal imaging cameras, physiological ecologists are now presented with unprecedented opportunities to measure body surface temperature non-invasively, and at high frequencies from free-living animals. In this thesis, I investigated relationships between body surface temperatures, measured using thermal imaging from free-living blue tits or captive zebra finches, with physiological measures or situations relevant to the assessment of physiological state. I developed reliable thermal imaging techniques to take non-invasive measurements of body surface temperatures in a variety of contexts, allowing characterisation of physiological responses in real time. My studies of captive birds revealed that activity levels influence body surface temperatures measured from free moving animals, and so should be accounted for in experimental designs. I also successfully acquired body surface temperatures from overwintering blue tits visiting food-baited traps, and from breeding blue tits entering and leaving their nest. Using this data, I showed that body surface temperature exhibits a characteristic response to acute stress, which differs with stressor type. While the mechanisms require explanation, much potentially useful information appears to be stored within body surface temperature dynamics during acute stress. Additionally, I established links between body surface temperature and longer term physiological processes in free-living blue tits. I observed near identical correlations between body surface temperature and body condition across differing seasons and life history stages. Also, I found evidence suggesting both that repeated acute stressors (predation risk and human disturbance) had a chronic effect on body condition breeding blue tits, and that surface temperature in those birds was linked to body condition. If confirmed, these results would be particularly interesting in a conservation physiology context, as it may prove possible to detect a signal of persistent physiological effect(s) relating to human disturbance, non-invasively. Furthermore, my discovery of a further correlation between baseline plasma glucocorticoids and body surface temperature in overwintering birds implies links with the hypothalamic-pituitary-adrenal axis. All of these results combined suggest that body surface temperatures measured using thermal imaging are highly likely to prove useful in determining aspects of physiological state non-invasively from free-living animals. While further investigation and validations are necessary, this work has laid the foundations for an exciting new methodology that could help solve many questions that remain unanswerable using current techniques.