Easter Island is located in the southeastern corner of Polynesia. This 163.6 km2 volcanic island is one of the most isolated places on Earth. During spring 2014 and 2015, we studied a breeding colony of Red-tailed Tropicbirds (Phaethon rubricauda) located on Rano Raraku Volcano. We specifically aimed to determine (1) main nest types used by tropicbirds, (2) nest distribution and abundance, (3) breeding success, and (4) threats faced by tropicbirds at this colony. Results showed presence of four nest types: Rock caves, moai nest, rockprotected, and vegetation nests. Nests were strongly associated with unfinished moai statues. Breeding success ranged from 37% in 2014 to 26% in 2015. Camera traps and direct observations showed interactions with five invasive alien species: The raptor Chimango Caracara (Milvago chimango) was the most severe predator of unattended eggs and chicks; cats were observed close to adult birds; dogs killed fledglings; ants preyed on newborn chicks; rats were recorded inspecting both occupied and unoccupied nests. We suggest that this is a relatively new and expanding colony. Urgent measures to control and/or eradicate invasive alien species on Rano Raraku are needed to improve breeding success and persistence of this colony in the future.
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