Decades later: revisiting the physiological markers of avian dehydration

Understanding physiological responses is necessary to predict populations fate in a changing environment. Dehydration risk is expected to increase with climate change for several wild bird populations, especially in desert environments, and potentially leading to population collapses. Yet, very few field studies directly assess hydration state of wild birds. In this article, we provide ways to do so as we present a systematic review of the physiological markers of avian dehydration. We find that plasma osmolarity and plasma concentrations of sodium and chloride are the most reliable blood markers of dehydration, and seem to scale linearly with dehydration severity. Hematocrit appears to predict dehydration but multiple concerns impede its suitability as a marker. Alternatively, characteristics of the excreta -- such as urine osmolarity -- may represent less-invasive means to assess hydration state. There is currently no consensus on dehydration thresholds and tolerance, and further research is need to refine our dehydration diagnostic, representing crucial perspectives for future studies.