Abstract :
The Galapagos Petrel Pterodroma phaeopygia is endemic to the Galapagos Archipelago, where it
nests on only five islands. The species is considered ‘Critically Endangered’, mostly due to the
effects of alien invasive species, which impair its reproductive success. During 2002–2003 we
studied the breeding biology of the petrels nesting on San Cristo´ bal island. The study revealed
particular characteristics of the San Cristo´ bal petrel population and differences compared to
those of other islands, mostly related to nesting habitat, phenology, reproductive success and
causes of mortality. On San Cristo´bal, petrel nests were primarily located along ravines, in areas of
dense vegetation cover formed by the endemic shrub Miconia robinsoniana and a wide variety of
native ferns. Over 90% of the nests on the island were located on private agricultural land. The
petrel population has a prolonged reproductive period covering 10 months. Laying dates occurred
mostly from May to October, with a peak during August, although eggs may be occasionally laid
between November and March. The incubation period averaged 50.8 days (range: 46–53), and
parental care 103.7 days (range: 98–108). Overall reproductive success was 23.6%; 63.8% for
eggs and 37.1% for chicks. Predation by rats was the primary cause (72.2%) of nest failure. Rat
control campaigns and clearing of exotic plant species in areas of high density of petrel nests, as
well as promoting cooperation agreements between conservation authorities and landowners of
the properties where nests are located, are suggested among other critical management measures
intended to reduce nest mortality and ensure the survival of the San Cristo´bal petrel population
