The importance of wetland margin microhabitat mosaics; the case of shorebirds and thermoregulation.
Wetlands, and the species that rely upon them, are under significant threat world-wide, with wetlands often being completely removed or drastically altered. Successful wetland management requires an understanding of the interactions between wetland species and the microhabitats they use. The use of microhabitats for thermoregulation in wetland species is poorly studied, though anthropogenic influence on wetlands can reduce the diversity of microhabitats and thus the thermoregulatory options for animals. At high ambient temperatures birds may use the water-logged wetland margins to help with thermoregulation, and are often observed roosting in the sitting position within this microhabitat. However, whether sitting on the wet substrate helps in thermoregulation is unknown. In this study, we tested whether birds selectively use microhabitats across temperatures by conducting field observations of nine species of shorebirds. We use comparative analysis to determine whether birds roost more on wet substrate in the sitting posture, that is, 'wet-sitting', at high ambient temperatures. We found substrate type across the wetland margins to be important in shorebird thermoregulation, with the time spent sitting being significantly mediated by the substrate on which the bird roosted. Individuals tended to sit on bare, wet ground much more under high ambient temperatures compared with low ambient temperatures. Vegetation on the other hand was used similarly across temperatures, and likely does not provide the same thermoregulatory benefits. By roosting on wet substrate at high ambient temperatures, birds may increase the potential for heat dissipation across the uninsulated legs, as water-logged wetland margins are known to remain cooler than the ambient temperature or vegetated microhabitats under hot climatic conditions. Synthesis and applications. Wetland creation and management requires an understanding of the functional significance of such microhabitats, not only for foraging and breeding, but also for roosting. We demonstrate that managing wetland margins is likely important in minimising heat stress in birds, with our findings emphasising the importance of maintaining open spaces in habitat mosaics for birds to use for thermoregulation. The ability of wetland species to manage heat stress is becoming exceedingly important as they are threatened by both decreased wetland availability and increasing ambient temperatures under climate change.