Although the Eastern Tropical Pacific is well known for its diverse fauna, the seabirds occurring off Costa Rica’s Pacific coast have received little scientific attention. With seabirds now the fastest declining avian group, a better understanding of seabird diversity and abundance in this region is urgently needed. We report on observations of Costa Rica’s Pacific seabirds made during 19 days of observations on 11 offshore trips from 2006-2010. We provide, for the first time, spatially and seasonally explicit information on the distribution of 41 species of seabirds (nine families). Species diversity is higher during the dry-wet season (36 species) and wet-dry season transitions (36 species) than during the dry season (19 species). The fauna included three threatened species (Pterodroma phaeopygia, Procellaria parkinsoni, and Puffinus creatopus) and two near-threatened species (Psueudobulweria rostrata and Thalasseus elegans), highlighting the importance of Costa Rican waters for the conservation of seabirds.

Keywords: Costa Rica, diversity, Eastern Tropical Pacific, seabirds, seasonality, threatened species.