The consequences of ant invasions on ecosystems may only become apparent after long periods. In addition, predicting how sensitive native fauna will respond is only possible if the underlying proximate mechanisms of their impact are identified. We studied the attraction of the native and invasive ant community to artificial bird nests. Further, we studied reproduction of a wild native songbird over five consecutive breeding seasons in relation to the presence of an invasive ant species. We analysed biometric, reproductive and individual blood parameters of great tits Parus major breeding in invaded as compared to uninvaded sites by Argentine ants Linepithema humile. Great tits bred preferably in uninvaded territories by the Argentine ant. Moreover, Argentine ants were more abundant at nests in invaded sites, than any native ant species were at uninvaded sites. Further, Argentine ants recruited at the artificial nests more intensively and responded to a larger variety of nest contents than native species. Although breeding success and adult condition did not vary in relation to invasion status, offspring quality was negatively affected by the presence of Argentine ants. Nestlings reared in invaded sites were lighter, with lower wing/tarsus length ratio and had a reduced nutritional condition and altered oxidative stress balance as measured from several blood parameters. The interspersed distribution and small distance between invaded versus uninvaded territories suggest that ant presence affects nestling condition through direct interference at the nest. Our results highlight the importance of evaluating the proximate effects like physiological parameters of the native fauna, when studying invasive ant-native bird interactions.