Naturalized plants decrease diet similarity between an invasive bird and its most similar native species.
Although invasive animals can compete with native species for resources, detrimental competition for food is seldom reported in the avian invasions literature. In temperate climates, food limitation and energetic stress are higher during winter and, thus, winter diets might reveal competition that is not apparent during the rest of the year. We compared autumn and winter diets of the invasive common waxbill Estrilda astrild in northwest Iberia, and of the native bird most similar to it in foraging behaviour, the European serin Serinus serinus. Both bird species forage for seeds in open habitats, mostly in or around agricultural areas. We found that food preferences of the two species were substantially different when foraging on the same agricultural landscape. Contrary to the prediction that food scarcity in winter would make diets more similar, the diets of waxbills and serins diverged in winter, indicating low potential for detrimental interspecific competition. Differences in diet were in part due to foraging on non-native plants. Non-native seeds represented a large part of the waxbill diet in autumn (a non-native grass), and a small part of serin's (a non-native forb), substantially decreasing their diet similarity. Our results are consistent with the view that waxbills are using a vacant niche in human-modified habitats, in this case involving non-crop plants associated with agricultural practices (e.g. irrigation, tillage, harvest) that are underexploited by the native fauna. Understanding which invasive animals fill those vacant niches and, thus, may interfere minimally with native faunas, helps to prioritize conservation actions.