In two study plots on Mayotte Island, the territorial, neotenic-endemic race brutus of the Frances s Sparrowhawk occurred at world-record breeding densities for a small raptor of 2.0- 2.6 ha/pair, with average inter-nest distances of 140-159 m in the breeding season of 1997. Measured in nine transects, the abundance of birds of potential prey-size correlated negatively with sparrowhawk abundance on Mayotte, while numbers of birds larger than sparrowhawks varied in parallel with sparrowhawk abundance, both corresponding to rainfall and lushness of the vegetation. Throughout the Comoro archipelago, there is a negative relationship at island level between sparrowhawk abundance and the proportion of small birds in the community. Several species of small birds are lacking from islands with sparrowhawks compared to islands without sparrowhawks; anomalies that cannot be explained by island size, distance to source or habitat. More species are lacking from islands with high sparrowhawk abundance. We suspect that the chances of survival for colonizing birds may have been affected historically by the abundance of Frances s Sparrowhawks on some of the Comoro Islands.