Seasonal species richness of birds on the world’s islands and its geographical correlates

Migratory birds occur on islands during different seasons of the year, which results in seasonal variation in species richness. These patterns and their geographical correlates have not been examined. We used 20 years of bird observations on 690 islands to examine how seasonal species richness estimates vary as a function of island area, isolation, and latitude. Species richness was highest on islands within the mid-latitudes of the Northern Hemisphere during migration and on islands within tropical latitudes during the non-breeding season. Area defined positive, non-linear relationships with species richness across seasons, with the steepest slopes occurring with islands >1,100 km2. Distance to mainland defined negative, non-linear relationships with species richness across seasons, with the strongest slopes occurring with islands located >150 km from the mainland. Species-area relationships were weakest for the most remote islands and strongest for islands at intermediate distan...