ONLY GIRL SHEARWATERS FLY TO FRANCE

Puffinus shearwater. Credit: Jofre Ferrer via Flickr. 

  
The Balearic shearwaters (Puffinus mauretanicus) who breed on the Spanish Balearic Islands don't go far when they migrate... out the Strait of Gibraltar then north to summering grounds off the Atlantic coasts of Portugal and France.
  
Yet, curiously, only the females go to France. 
  
Perhaps because of these longer migrations, they also spend longer away from their breeding grounds than the males: 

• Median duration of time away for females: 91 days
• Median duration of time away for males: 83 days

Credit: Tim Guilford et al. PLoS ONE. DOI:10.1371/journal.pone.0033753.

These are the findings of a team of researchers who tagged 26 shearwaters with miniature geolocation trackers and followed their annual movements from the Balearic Islands. The results are published in a new paper in PLoS ONE.
  
You can see in the maps above the routes of the individual birds, each with its own color track:
  

• Inset map shows where all the birds occurred statistically half the time throughout the year.
• Larger maps shows where all the birds occurred statistically half the time on migration to and from their breeding islands.
• The colored circles mark where four birds made trips back into eh Atlantic after their migrations.
• The red symbol is the position of the breeding colony at Sa Cella cave on Mallorca.

  

Longline hooks. Credit: Isaac Wedin via Flickr.

The gender-specific migrations are more than a curiosity. They're vitally important knowledge since Balearic shearwaters are Europe's only critically endangered seabird. From the IUCN Red List:
  

This species has a tiny breeding range and a small population [known breeding population: ~3,200 pairs] which is undergoing an extremely rapid population decline owing to a number of threats, in particular predation at breeding colonies by introduced mammals [cats, genets, rats, rabbits] and at-sea mortality as a result of interactions with commercial fisheries [hooked and drowned on longlines]. Population models predict an extremely rapid decline over three generations (54 years), qualifying the species as Critically Endangered.

  

Balearic shearwater. Credit: Roger Montserrat via Flickr.

Obviously if you have a large proportion of the females flocking and feeding in one location then the entire species becomes susceptible to mass mortality fishing events like the one that killed ~50 birds off Spain in 1999-2000. As the authors note:
  

[F]or approximately ¼ of the year, a large percentage of the world's population of breeding birds will be vulnerable to [fisheries] by-catch in these two core areas within the territorial waters of Portugal and France.    

      
The paper:

• Guilford T , Wynn R , McMinn M , Rodríguez A , Fayet A , et al. (2012) Geolocators Reveal Migration and Pre-Breeding Behaviour of the Critically Endangered Balearic Shearwater Puffinus mauretanicus. PLoS ONE 7(3): e33753. doi:10.1371/journal.pone.0033753