Invasive songbirds show greater heat, but not cold, tolerance than Mediterranean native counterparts.

Global climate change and biological invasions rank among the leading threats to biodiversity. Rising temperatures may promote invasions by favouring ectothermic species capable of maintaining physiological performance across broader thermal ranges, yet comparative evidence in endotherms remains limited. Here, we examined the heat and cold tolerance limits (HTL and CTL, respectively) of three invasive passerine species—Common waxbill (Estrilda astrild), Red avadavat (Amandava amandava), and Yellow-crowned bishop (Euplectes afer)—and three native species of comparable size and ecology co-ocurring in the Mediterranean basin—Goldfinch (Carduelis carduelis), Serin (Serinus serinus), and Great tit (Parus major). To assess HTL, we exposed individuals to increasing ambient temperatures (Ta) during summer and measured their resting metabolic rate, evaporative water loss, evaporative cooling efficiency (ratio between evaporative heat loss and metabolic heat production), and body temperature (Tb). During winter, we measured CTL as the minimum Ta reached (in a helium–oxygen atmosphere) and associated changes in Tb. As predicted, invasive species exhibited higher HTLs than native species, likely due to a delayed onset of metabolic and evaporative responses at high Ta. Moreover, invasive species made greater use of facultative hyperthermia, enabling them to store heat and postpone the activation of evaporative cooling. However, CTLs did not differ significantly between invasive and native species. These results indicate that invasive songbirds possess enhanced heat tolerance while retaining comparable cold performance, allowing them to occupy broader thermal environments. Our findings provide further evidence that physiology plays a substantial role in the success of invasive endotherms.