Addressing the patterns of variation in thermal traits is crucial to better predict the potential effects of climate change on organisms. Here, we assessed seasonal (winter vs summer) adjustments in key thermoregulatory traits in eight Mediterranean-resident songbirds. Overall, songbirds increased whole-animal (by 8%) and mass-adjusted (by 9%) basal metabolic rate and decreased (by 56%) thermal conductance below the thermoneutral zone during winter. The magnitude of these changes was within the lower values found in songbirds from northern temperate areas. Moreover, songbirds increased (by 11%) evaporative water loss within the thermoneutral zone during summer, while its rate of increase above the inflection point of evaporative water loss (i.e., the slope of evaporative water loss versus temperature) decreased by 35% during summer — a value well above that reported for other temperate and tropical songbirds. Finally, body mass increased by 5% during winter, a pattern similar to that found in many northern temperate species. Our findings support the idea that physiological adjustments might enhance the resilience of Mediterranean songbirds to environmental changes, with short-term benefits by saving energy and water under thermally stressful conditions. Nevertheless, not all species showed the same patterns, suggesting different strategies in their thermoregulatory adaptations to seasonal environments.

Acknowledgments We thank Vito M. R. Muggeo for helping in modifying the R- segmented package, Juan Varela Simó for allowing us to use the bird drawings for Figure 3, Andrea Soriano for suggestions in data analysis, and the DG Environment (Junta of Extremadura) for permissions. We also thank the State Meteorological Agency (AEMET) for giving us access to climate data, and two anonymous reviewers that improved an earlier version of the manuscript. Funding This research was funded by the project IB16183 (Junta de Extremadura and European Regional Development Fund ERDF), and the grant GR21081 (Junta de Extremadura and ERDF). EG-M was supported by a postdoctoral fellowship by The National Council of Science and Technology (CONACYT) grant 473980, and Junta de Extremadura (Consejería de Economía, Ciencia y Agenda Digital) and Fondo Europeo de Desarrollo Regional (FEDER) grant GR18169. NP-M was supported by a predoctoral fellowship from Junta de Extremadura and European Social Fund (PD16099), and JC-V was supported by Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación and European Social Fund grant PEJ2018- 003697-P. JSG was supported by the Government of Extremadura grant TA18001.