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 in response to increasing Ta. However, only the crested lark Galerida cristata achieved an evaporative cooling efficiency greater than 1. The studied songbirds currently experience summer Ta maxima that surpass their upper critical temperatures of their thermoneutral zone and even their HTL. Our estimates indicated that five of the eight species will experience moderate risk of lethal dehydration by the end of the century. We argue that the limited heat tolerance and evaporative cooling efficiency of small-sized Mediterranean songbirds make them particularly vulnerable to heatwaves, which will be exacerbated under future climate change scenarios.

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 the database as point-x). 

Finally, we employed evaporative water loss slopes to estimate lethal risk of dehydration following the methodology employed by Albright et al. (2017) (see Material and Methods for deeper description). Hourly temperature data were obtained by averaging the hourly profiles from the 10 hottest days in the last 20 years in the study site. To this profile we added 1.7 a 2.8 ºC to estimate future risk of dehydration under a mitigated climate change scenario and an unmitigated scenario, respectively. See Material and Methods for complete description of the methodology. 

We employed R studio from R Core team to perform all the analyses, and all the graphs were made with ggplot2 package.