Energetic metabolism and condition indices in native versus invasive common waxbills

Invasive alien species (IAS) pose a serious threat to global biodiversity, and unravelling the mechanisms driving their expansion across new ranges remains an important challenge for ecosystem conservation. Assessing similarities and differences in species' traits and phenotypic fitness proxies between native and invasive populations can show how species may have locally adjusted in the new areas and their capacity to expand further. Here, we compare basal metabolic rates (BMR), body and nutritional condition (based on feather proxies) and body size between native (South Africa) and invasive (northern and southern Portugal) common waxbills (Estrilda astrild). As expected, given prevailing ambient temperatures - similar between South Africa and northern Portugal but hotter in southern Portugal - our results showed that mass-independent BMR was similar between individuals from South Africa and northern Portugal, but lower in individuals from warmer southern Portugal. However, contrary to our expectations based on the enemy release hypothesis, feather density was higher in South African birds, challenging the assumption that invasive populations outperform their native counterparts. Additionally, feather growth rates trade-off with feather quality in the introduced populations, indicating that they may face energetically challenging environments in their invaded range. Common waxbills in South Africa were larger than those in Portugal, but further analyses showed that these size differences between native and invasive birds may be influenced by introduction history rather than ecological factors alone. This study highlights the importance of considering both native and invaded ranges when studying invasive species and provides insights into the strategies used by waxbills to thrive in different environments.