Heat tolerance limits of Mediterranean songbirds and their current and future vulnerabilities to temperature extremes

Songbirds are one of the groups most vulnerable to extreme heat events. Although several recent studies have assessed their physiological responses to heat, most of them have focused on arid-zone species solely. We investigated thermoregulatory responses to heat in eight small-sized songbirds occurring in the Mediterranean Basin, where heatwaves are becoming more frequent and intense. Specifically, we determined their heat tolerance limits (HTL) and evaporative cooling efficiency and evaluated their current and future vulnerabilities to heat in southwestern Iberia, a Mediterranean climate warming hotspot. To do this, we exposed birds to an increasing profile of air temperatures (Ta) and measured resting metabolic rate (RMR), evaporative water loss (EWL), evaporative cooling efficiency (the ratio between evaporative heat loss and metabolic heat production) and body temperature (Tb). HTL ranged between 40 and 46°C across species, and all species showed rapid increases in RMR, EWL and Tb i..., We captured birds and measured physiological key traits (RMR, EWL, EHL/MHP, Tb) in response to heat by using open-flow-trough respirometry. As specify in Material and Methods we only considered for the analyses those data from calm animals (activity levels from 0-3). 
We extracted phsyiological data in response to heat from previews studies to compare those results with that from our species. To to that, we limit the comparisons to songbirds species from previous studies among a body mass range between 5-60 grames. 
Besides, we modelled their current and future vulnerability to heat by employing daily maximum temperature data provining from 16 regional predictive models from AdapteCca platform, which use data from EUROCORDEX. Then we determined the number of days in which maximum temperature surpass the upper critical temperature (risk of sublethal effects) or their heat tolerance limits (risk of lethal hyperthermia) thorugh Extremadura (which was divided in grids, referred to them in t..., We employed R studio from R Core team to perform all the analyses, and all the graphs were made with ggplot2 package.