Classifying avian drinking behaviour: ecological insights and implications in a changing world.

Water is a fundamental currency of life, and its availability significantly influences animal behaviour, physiology and distributions. However, our knowledge around the dependence on water for drinking and the direct and indirect mechanisms driving related behaviours remains partial in the context of changing climates. Here, we review patterns and classifications of avian reliance on water consumption and the role of abiotic and biotic factors in drinking habits. We bring to light the void of information regarding drinking habits in relation to thermal physiology, and then review existing literature related to the behaviour of drinking. We attempt to highlight misconceptions related to the typical classification of drinkers versus non-drinkers and direct future research efforts to improve our understanding of the dynamic relationship between water availability, acquisition, and maintenance under climate change. We conclude that despite the common dichotomous use of drinking versus non-drinking classifications in climate vulnerability assessments, this trait is highly flexible and occurs along a continuum from specialist non-drinking species to highly dependent drinking species driven by abiotic and biotic pressures. While some recent assessments continue to apply binary traits, our gradient-based approach advances ecological relevance by accounting for context-dependent flexibility. Consequently, previous approaches assessing drinking versus non-drinking species may not be as ecologically relevant as previously thought. Improving our understanding of the spatial and temporal trends in how species acquire or use water for drinking, along with sound empirical data, will continue to shape our theoretical understanding of this trait. We emphasise that understanding drinking as a trait is critically important for comprehending species vulnerabilities to anticipated future climates, especially given predicted changes and increased variability in water availability.