Breeding biology and population increase of the Endangered Bermuda Petrel Pterodroma cahow

Summary The Bermuda Petrel Pterodroma cahow was thought to have become extinct early in the 17 th century due to a combination of hunting by human colonists and predation by introduced rats,cats,dogs and pigs. However, single individuals were found on four occasions during the first half ofthe 20 th century, and in 1951 a small population was discovered breeding on several rocky islets innorth-east Bermuda. Recovery actions began in 1962 when the population numbered just 18 pairs,dispersed among five small islets. Although rats extirpated one of these five colonies in 1967,thepopulation has grown steadily to 56 breeding pairs in 2000. We investigated the breeding phenol-ogy, productivity and population size of the Bermuda Petrel between 2000/2001 and 2007/2008.Each year, the birds began arriving in Bermuda around mid-October. They departed on a pre-breeding exodus between 19 November and 14 December, returning after 32–56 days to lay a singleegg between 31 December and 31 January. Eggs hatched from 16 February to 26 March after a mean(6 SD) incubation period of 53 6 2 days, and young fledged from 15 May to 25 June after a meanfledging period of 91 6 5 days. Between 2000/2001 and 2007/2008, reproductive output rangedfrom 29 to 40 fledglings per annum. Mean annual breeding success ( 62%) was reasonably highrelative to other Procellariiformes, largely due to the provision of arti ficial (concrete) nestingburrows. In 2008, the population numbered 85 breeding pairs. Monitoring since 1961 indicates thepopulation has been increasing exponentially, doubling approximately every 22 years. This rate ofincrease, together with the increased incidence of storm damage, is making it progressively moreimpracticable to construct sufficient concrete burrows on the current nesting islets to accommodateall breeding pairs. The vulnerability of these sites to accelerating storm damage and erosion asa result of anthropomorphic climate change is now the greatest threat to the Bermuda Petrel.