Extra-pair copulation (EPC) is a widespread behaviour amongst birds, yet the rate of occurrence of extra-pair offspring is highly variable both within and across species. Two populations of Mediterranean blue tits, Parus caeruleus, one on the mainland and the other on the island of Corsica, are characterized by high levels of extra-pair paternity (EPP), with 14 and 25% of extra-pair offspring (in 46 and 68% of the nests), respectively. The rate of EPP is significantly higher in the island population. Such high rates on Corsica contradict the predictions of higher mate fidelity and lower levels of EPP on islands. Despite expressing several traits that characterize an insular syndrome, blue tits of Corsica do not exhibit lower genetic variation which could be associated with low levels of EPP on islands. Furthermore, our results do not support any of the hypotheses that have been proposed so far to explain variation in EPP rates at the local level. We tentatively suggest that proximate Mediterranean constraints, especially consistent food shortage, prompt females to seek better breeding opportunities through a two-step process including high rates of EPC and divorce.