Habitat destruction and introductions of invasive species have been primary causes of endemic species loss on oceanic islands. This paper addresses the question of whether a reintroduced population of a critically endangered island endemic, the Mauritius fody (Foudia rubra) is competing or co-existing with an exotic congener, the Madagascar fody (F. madagascariensis). We investigated habitat use and feeding ecology of the Mauritius fody during two early phases of its reintroduction onto a restored islet, and compared these parameters with the sympatric population of Madagascar fodies. The number of Mauritius fody territories increased from 4 to 20 in a seven month period following release, and overall mean territory area (c. 1 ha) was significantly larger than that of the more abundant Madagascar fody (c. 0.07 ha). The initial four territories were located in mature coastal forest, whilst those of the Madagascar fody were characterised by open canopy, smaller trees and grass-dominated ground cover. Mauritius fodies foraged mainly in dead leaves for insects, whilst the Madagascar fody fed primarily on the seeds of a native grass species. Mauritius fody territories later expanded to include a range of habitats. Differing ecological requirements indicate that divergence between these congeners may be sufficient for them to coexist and exploit contrasting resources on restored islets. We discuss the implications of these findings for both the long-term restoration of the Mauritius fody and other avian recovery programmes and highlight the critical importance of long-term post-release monitoring to assess the success of reintroduction to offshore islets as a long-term conservation strategy.