Resumo

Although seabirds that are trans-equatorial migrants show apparently broad overlap among populations in the nonbreeding
season, such large-scale pattern may conceal subtle but evertheless key differences in migratory behaviour.
These specializations could reflect adaptation to different environments during the breeding season, carry-over effects from
the breeding to the nonbreeding period, or asymmetries in competitive ability of birds of different origin. We compared the
migratory and wintering behaviour of Cory’s shearwaters Calonectris diomedea nesting in Berlengas and in the Selvagens, two colonies in contrasting oceanographic environments, separated by ca. 1200 km. Although no differences were found in winter distribution, there was a marked divergence in timing, route and the use of staging areas during the postbreeding (autumn) migration. Birds from Berlengas typically travelled to oceanic waters in the North Atlantic for an extended
stopover, whereas those from Selvagens rarely did so. In the South Atlantic, birds from Selvagens spent more time in flight,
perhaps because they had higher energy and nutrient requirements for feather replacement compared to birds from
Berlengas, which moult more flight feathers during breeding. Stable isotope analyses of feathers suggested that this
variation in activity patterns was unrelated to trophic ecology. Differences in migration routes and stopovers may expose
populations to distinct threats, and should be taken into consideration when defining units for conservation purposes and
developing appropriate management strategies.

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