A Balearic shearwater breeds exclusively on the Balearic Islands in the Mediterranean Sea. Photo: Kirk Zufelt.
The critically endangered Balearic shearwater managed to avoid extinction through frequent hybridisation.
The Balearic shearwater (Puffinus mauretanicus) breeds on the Balearic Islands and is regarded as a flagship species for biodiversity. Listed as ‘Critically Endangered’ on the Red List, it is Europe’s most endangered seabird. Geneticists at the University of Barcelona have now shown that it was precisely hybridisation with the Yelkouan shearwater (Puffinus yelkouan) that saved the species from extinction (PNAS, 20 August). The gene flow, which has occurred on multiple occasions, has even been decisive in preserving genetic diversity and enabling adaptation to the natural habitat. This demonstrates that hybridisation ‘can be crucial for maintaining genetic diversity in threatened taxa’.
Both species were long regarded as subspecies of the widespread Manx shearwater (Puffinus puffinus), which breeds on islands in the Atlantic Ocean. The Balearic shearwater breeds exclusively on the Spanish Balearic Islands in the western Mediterranean. The population there is estimated at just 1,800 to 2,500 breeding pairs, roughly half of which breed on Mallorca. The influence of hybridisation with P. yelkouan is greatest on the more easterly Menorca, with a decreasing gradient towards Ibiza and Formentera. Outside the breeding season, the Balearic shearwater can be found in many more locations: in the Bay of Biscay, southern England, and (occasionally) in the North Sea.
The genus name Puffinus dates from 1760, derives from the French zoologist Mathurin Jacques Brisson, and can probably be traced back to the French word for shearwater: puffin. Remarkably, this was also the term for a former delicacy: a cured, fat nestling of a shearwater. The French name for the colourful Atlantic puffin (Fratercula arctica) is, incidentally, macareux. The British ornithologist Percy Lowe devised the (sub)species epithet mauretanicus in 1921, referring to the North African region of Mauritania from which his type specimen originated. He was equally unaware that this species breeds and engages in extra-pair liaisons on the – then still demure – Balearic Islands.