Shearwaters take 'females only' summer holiday

Male and female Balearic Shearwaters may head for different migration hotspots over the summer period a new study suggests.

The Balearic Shearwater is a frequent visitor to southern UK coastal waters, yet with just 3,200 estimated breeding pairs left in existence it is the only European seabird to be officially classified as Critically Endangered. Accidental by-catch in fishing gear is thought to be its greatest threat away from land, but surprisingly little is known about this species’ elusive life at sea.

In a study published this week in PLoS ONE, scientists from Oxford University’s Department of Zoology led by Professor Tim Guilford, together with colleagues at the National Oceanography Centre in Southampton, the British Geological Survey, and local scientists from Mallorca, have, for the first time, succeeded in tracking the bird’s at-sea movements year-round using miniature geolocation devices. 

For most of the year shearwaters feed over the Iberian shelf in the western Mediterranean, close to their breeding colonies in caves on remote coasts of the Spanish Balearic Islands. Then, at the end of June, the shearwaters make a three-month migration out of the Mediterranean to feed in productive waters along the Atlantic coasts of Iberia and France.

In 2010, 26 individuals were successfully tracked from a colony on Mallorca and, although all made the post-breeding Atlantic migration, none reached the UK. This contrasted with visual observations indicating that up to a quarter of the World population was feeding in the English Channel at that time, which suggests that there may be strong age-related partitioning in migratory destinations. 

The tagged breeding birds were instead concentrated in two relatively small areas off west Portugal and southwest Brittany, highlighting the need for marine protection in these offshore areas, as well as the Spanish breeding grounds, if the species is to survive extinction. Intriguingly, only females migrated to France (south-western Brittany), hinting that there may be sex- as well as age-related segregation in migratory strategy. 

Furthermore, using data from the logging devices alone, the scientists unravelled a complex pattern of synchronised visits to the cave colony by paired males and females long before breeding takes place, suggesting that breeders may be vulnerable to predation and disturbance on the islands even outside the breeding season.

In addition to ongoing studies by the UK and Mallorcan team, parallel studies by French and Spanish scientists at other island colonies are being conducted under the EU-funded FAME initiative. The results of these initial studies are already being used to inform conservation measures for the species, and it is hoped that a fuller picture of the bird's life at sea will emerge in time to help avert its extinction.

The research was funded by NERC, Total Foundation, Merton College Oxford, and Microsoft Research Cambridge.