Aim  We measured the changes in an island avifauna over more than 100 years (1898–2006), using community indices accounting for difference in expected species sensitivity to land‐use and climate changes.

Location  Ouessant Island, France, Great Britain.

Methods  We assessed the temporal trend of the relative proportion of generalist species breeding on Ouessant island and whether high‐temperature tolerant species have replaced less tolerant species over this time period. We further tested the relationship between the observed change in the avifauna composition, and long‐term population species’ trends measured independently in potential source regions of colonist species (France and Great Britain).

Results  During the whole study period, Ouessant island has experienced a strong increase in species richness (+41%), but a severe decline in specialist species. In contrast, we found no change in species composition in terms of their temperature‐tolerance. The observed trend was highly correlated with species trends measured in the Great Britain.

Main conclusions  Our results revealed an ongoing biotic homogenization process towards more generalist species, coupled with a strong local increase in species richness. The observed trend was most likely driven by a strong habitat change in the island occurring during the period considered, favouring the colonization of generalist species. Our results show that an increase in species richness can be misinterpreted as a sign of conservation improvement and that assessing change in community composition using species‐specific ecological traits provides more accurate insights for conservation planning purposes.