Increases in humidity will intensify lethal hyperthermia risk for birds occupying humid lowlands

Increasingly frequent and intense heatwaves are expected to elevate the risks of heat-related mortality among birds. Most studies have focused on arid-zone avifaunas and the extent to which risks will increase in other habitats, particularly humid lowlands, remains unclear. We tested the prediction that increasing air temperature and corresponding increases in humidity, and hence wet-bulb temperature (<i>T</i> _W; lowest temperature achievable via adiabatic evaporation), will increase exposure to conditions associated with lethal hyperthermia. We empirically determined maximum <i>T</i> _W (<i>T</i> _W-max) for an Afrotropical forest frugivore, the trumpeter hornbill (<i>Bycanistes bucinator</i>) as <i>T</i> _W-max = 31.7 ± 1.0°C. We then modelled current and future exposure to conditions associated with <i>T</i> _W > <i>T</i> _W-max across this species' range. Under a business-as-usual emissions scenario and assuming no vegetation buffering of air temperature (<i>T</i> _air), trumpeter hornbills will experience <i>T</i> _W > <i>T</i> _W-max for at least 1 day year^-1 over 46% of their current range, compared to 30% at present. However, the frequency of exposure will increase substantially and reach ~100 days year^-1 in parts of the southern Democratic Republic of Congo. When we incorporated the thermal buffering effect of vegetation, end-century exposure to <i>T</i> _W > <i>T</i> _W-max decreased by 0.3-66.7%, emphasizing the role of cool microsites provided by vegetation. Our analyses reveal the exposure of birds inhabiting humid environments at low latitudes to conditions associated with a risk of lethal hyperthermia under resting conditions will increase substantially in coming decades, putting a large fraction of global avian biodiversity at risk of population declines and local extinctions.