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Introduction
The Bermuda Petrel or Cahow Pterodroma cahow is endemic to the islands of Bermuda (32°20'N, 64°45'W; 53 km2) in the western North Atlantic Ocean. Colonisation by Europeans early in the 16th Century severely depleted the bird's numbers (Butler 1619) and for more than 300 years the species was thought to be extinct. In 1951, a remnant population of 13-14 pairs was found nesting in rock cavities on three small rocky islets in north-east Bermuda (Murphy and Mowbray 1951). In 1961, a further survey of nearby islets found two more breeding sites and increased the known population to 18 pairs (Wurster and Wingate 1968, Madeiros 2005). A recovery programme began in 1962 (Wingate 1985), and by 2008, the breeding population had increased to 85 pairs (Madeiros et al. 2012). Despite this increase, the Bermuda Petrel remains one of the world's rarest seabirds and is classified as 'Endangered' (IUCN 2010).
In 2008, the Bermuda Petrel was restricted to four islets, having been extirpated from one islet by rats in 1967 (Madeiros et al. 2012). The four islets have a combined area of just 1.4 ha, are of low elevation (< 10 m) and prone to being overwashed during hurricanes and storms. Composed of soft limestone, the islets are subject to severe erosion during storm activity. Soil occurs only in isolated pockets and vegetation is sparse.
Historically, Bermuda Petrels nested in soil burrows (Zimmerman 1975) but the lack of soil at their current breeding sites has forced them to nest in deep rock cavities. Many of these cavities have needed to be reinforced or repaired and, over time, their limited number has been supplemented with artificial burrows constructed from concrete (Wingate 1978). Approximately 80% of the petrel population now breeds within these artificial structures. Difficulty of access to the islets makes the construction and maintenance of burrows a challenging and labour-intensive process. To date, this work has largely kept pace with the expansion of the population, but it is unlikely that this can be sustained. The increasing need for additional burrows as the population continues to grow exponentially (Madeiros et al. 2012) is exacerbated by the constant need to repair burrows damaged by...