The biogeography of diet diversity of barn owls on Mediterranean islands

Aim Following the classical MacArthur-Wilson island equilibrium model of species richness, we hypothesize that island size and isolation affect prey composition and diet diversity of predators on islands. We used the barn owl as a model predatory organism that is known for feeding and habitat flexibility and explored the patterns of its diet on Mediterranean islands in relation to the island biogeography and human population density. Location Mediterranean Islands. Taxon Barn owl (Tyto alba) and its prey. Methods Data on taxonomic composition of the barn owl diet were obtained from pellets (Adriatic island of Korcula) and from published studies performed on 18 Mediterranean islands. For each island, diet diversity indices were calculated and correlated with island area, linear distance from the mainland, human population size and density, and geospatial trends. Results Murinae mice and rats were the dominant prey. The proportion of birds and reptiles consumed was 5 and 18 times higher, respectively, than in Europe in general. Diet diversity correlated with island size, but not island isolation. Island size was also a good predicator of the proportions of the vertebrate prey. Human population density and longitude had no effect on diet diversity. Main conclusions Diet diversity of the barn owl on Mediterranean islands only partly followed the island biogeography theory. Diet diversity was greater on the larger Mediterranean islands. However, a more diverse diet did not mean a higher number of taxa, but a wider range of abundant and evenly represented taxa. The smaller the islands, the more birds and reptiles were consumed, compared to higher proportions of mammals on the larger islands. These findings support the idea of barn owls' feeding flexibility and opportunistic predator behaviour. Despite the barn owl being a synanthropic bird, urbanization had no influence on its diet diversity. Also island isolation had no significant effect.