The systematic relationships of Puffinus mauretanicus, which breeds in the Balearic Islands in the western Mediterranean, have been disputed since its initial description as a subspecies of the Manx Shearwater P. puffinus. It is presently considered a species of its own, slightly differentiated from Yelkouan Shearwater P. yelkouan, a ‘sibling species’ which breeds elsewhere in the Mediterranean. However, birds seemingly intermediate between these two forms are breeding in Menorca, and further research is needed to confirm whether the two taxa really are different species. Bearing its limited breeding range and population size in mind, it is rather odd that the Balearic Shearwater has not been classified as threatened by BirdLife International in its Threatened Birds of the World, 2000. Since then, population studies have sounded the alarm, suggesting that the species might disappear within a few decades, and the Balearic Shearwater is now categorized as ‘Critically Endangered’. Published population estimates are not always reliable, however, and its population dynamics remains poorly understood. Threats are better known and include mammal predators at breeding sites, mortality induced by long-line fishing, and probably a greater difficulty to access food resources.