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 can 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 of three (sub)tropical invasive passerine species [common waxbill (Estrilda astrild), red avadavat (Amandava amandava) and yellow-crowned bishop (Euplectes afer)] and three temperate native passerine species of comparable size and ecology co-occurring in the Mediterranean basin [goldfinch (Carduelis carduelis), serin (Serinus serinus) and great tit (Parus major)]. To assess heat tolerance, we exposed individuals to increasing air temperatures (Ta) during summer and measured their resting metabolic rate, evaporative water loss, the ratio of evaporative heat loss to metabolic heat production, and subcutaneous temperature. During winter, we measured cold tolerance as the minimum tolerated Ta (in a helium-oxygen atmosphere) and the associated changes in core temperature. As predicted, invasive species exhibited higher heat tolerance limits than native species, associated with a delayed onset of evaporative responses and higher evaporative scopes under extreme heat. Moreover, invasive species made greater use of facultative hyperthermia. However, cold tolerance limits did not differ significantly between invasive and native species. These results indicate that some invasive songbirds possess enhanced heat tolerance while maintaining comparable cold performance, which may allow them to occupy broader thermal environments than their temperate native counterparts. Our findings highlight that physiology plays a substantial role in the success of invasive endotherms.