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 distances to the mainland. Intermediate proximity to other islands was a poor predictor of species richness. Our findings emphasize the presence of seasonally dynamic relationships, the enhanced role of evolutionary processes on larger islands, the unique ecology of the world’s most remote islands, and the importance of islands as stopover sites and wintering grounds for migratory bird species.