The Balearic shearwater is the most threatened seabird in the Mediterranean and one of the rarest seabirds in the world (8) (10). Indeed, with an annual decline estimated at around eight percent, this highly threatened seabird could go extinct within the next 40 years (10). At present the greatest threat to the Balearic shearwater is thought to be unnatural adult mortality (11) (12). On land this may be caused by predation by introduced cats, rats and genets (Geneta geneta), and the species may have gone extinct on Cabrera Island as a result (6). At sea this species has a close association with trawlers from which it takes fish discards, and mass mortality has been observed where fish lines have come too close to large groups of birds (2).
The Balearic shearwater’s breeding habitat is under threat from encroaching urbanisation, as well as degradation from other human activities and introduced rabbits, which compete for nesting sites (2). The species’ foraging habitat is also under threat from pollution, particularly in the Ebro Delta, which has very high levels of mercury that have the potential to accumulate in the birds, causing poisoning and a reduction in reproductive success (2) (6).
The effects of the loss of its prey are, at present, poorly understood. However, in the 1990s the post-breeding population in the Bay of Biscay appeared to decline, while that in the western channel, several hundred kilometres to the north, become more numerous. The reasons behind this are unclear, but may be due to fisheries affecting the abundance of its prey, or the fish moving northwards in response to climate change, although both these scenarios require re-evaluating (9) (13). Changes in fisheries legislation also have the potential to threaten the Balearic shearwater, as this species has a strong dependency on discards for feeding its chicks during the breeding season (8).
Additional threats to the Balearic shearwater include attacks by yellow-legged gulls (Larus michahellis), which causes regurgitation and can inflict wounds, as well as hybridisation with the Yelkouan shearwater (Puffinus yelkouan) (2) (6) (12).