An Assessment of Nesting Seabirds within San Nicolas Bay, Aruba, 2009

Executive Summary:

The eleventh field season of the Aruba Tern Project was completed in 2009. The study continued its stated aim of producing a quantitative description of the status and distribution of nesting terns and laughing gulls within San Nicolas Bay and around the island of Aruba.

The 2009 field season began during the third week of June, which was later than in most previous years. Consequently, we were unable to gain accurate egg counts, egg measures and population counts for those species nesting early in the season, primarily Cayenne terns and sooty terns. We were also unable to confirm the nesting status of regionally threatened roseate terns and some common terns. Despite these challenges we were able to generate population estimates for each species, although the margin of statistical error and reliability of the estimates, for the earlier nesting species, was more variable.

We observed a continuation of the gradual increase in the number of nesting bridled terns. We also recorded average or greater than average numbers of sooty tern, common tern, brown noddy and laughing gull. The numbers of Cayenne terns were within the range of numbers experienced in previous years. The nesting island for Cayenne terns shifted from island 3 to island 2 and there were no Cayenne terns observed nesting on the islets opposite Oranjestad. The absence of Cayenne terns near Oranjestad was unusual but may be an observer bias as we were not on the island to see whether birds nested in that area and failed before we arrived.

The egg size, clutch size, chick growth and juvenile and adult survivorship was within the range recorded for all species in previous years. In general, and despite the challenges associated with relatively late data collection, we believe that 2009 represents a slightly better than average year for all tern species nesting on Aruba. The increase of nesting laughing gulls must be the result of immigration as the local population could not account for the scale of increase observed in this species. Laughing gulls continue to remain a serious threat to ground- nesting terns, most notably Cayenne terns. 

Aruba, and particularly the San Nicolas Bay islands are unique within the Caribbean and most probably the world. In 2009 San Nicolas Bay contained approximately 25% of the world’s population of Cayenne terns, over 90% of the Caribbean population of common terns, 25% of the Caribbean’s black noddy, 3% of the Caribbean’s brown noddy, >1% of the Caribbean’s bridled terns and 4% of the Caribbean’s laughing gull. We are unaware of any other location in the world that contains the diversity and abundance of terns observed in San Nicolas Bay. For this reason BirdLife International (2008) has declared the Bay islands to be a globally important bird area (IBA).

The project was conducted in collaboration with the Aruba Veterinary Service, the Department of Agriculture, Husbandry and Fisheries, and the Government of Aruba, whose support is gratefully appreciated. We thank Valero Aruba Refinery for funding the study and also providing logistical and lodging support. We thank Caribe Alaska for their continued support and encouragement with this project. 

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