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The Balearic Shearwater Puffinus mauretanicus is a Critically Endangered species endemic to the Balearic Islands, subject to a severe decline that could lead to the extinction of the species within three generations (Oro et al. 2004). Predation by introduced mammals is considered the main threat facing the species at its breeding grounds, and therefore conservation action is required along with subsequent monitoring in a species where such information is lacking. In order to assess the long-term impact of a rodent eradication project on the breeding success of the species on Sa Dragonera island, a series of study plots were established in April 2013. A survey was carried out to establish the minimum number of known Apparently Occupied Sites (AOS) within each plot. All potential nest sites within each plot were sampled for the presence of a bird(s) using a combination of methods: (i) tape playback, (ii) physical sighting (by eye or with an endoscope) and (iii) obvious signs of occupation. 33 AOS in 12 study plots were identified. This project was not a whole island estimate; rather it led to the establishment of a series of repeatable study plots, providing reference estimates of breeding pairs in defined areas on the island to enable monitoring of future changes in the population size following predator removal.