Abstract
Although seabirds that are trans-equatorial migrants show apparently broad overlap among populations in the non-breeding season, such large-scale pattern may conceal subtle but nevertheless 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.
Highlights
Trans-equatorial migrant seabirds travel huge distances across entire ocean basins, with apparent broad overlap between populations [1,2,3]
Birds from colonies close to each other may display differences in migratory behaviour and wintering ecology due to (1) population specializations arising from local adaptation to breeding environments [4,5], (2) carryover effects of conditions experienced during breeding [6], (3) asymmetries in competitive ability of birds of different origins, perhaps caused by 1 and 2, or by other factors, such as small differences in distance to stopover sites and wintering grounds
Recent developments in technology allow the year-round tracking of pelagic migrants, providing geographic position, and information on activity patterns derived from immersion sensors [7]
Summary
Trans-equatorial migrant seabirds travel huge distances across entire ocean basins, with apparent broad overlap between populations [1,2,3]. The large-scale overlap in nonbreeding distributions amongst populations, may, conceal important fine-scale differences in migratory behaviour. These aspects have been little investigated, given the paucity of ringing recoveries from pelagic seabirds and of tracking studies involving sufficient numbers of individual birds of the same species from different colonies. Studies using stable isotopes in feathers provide insights into their trophic ecology [8]. These methodological advances have not, yet been combined with the purpose of comparing the migratory and wintering behaviour of long-distance migrants from different colonies
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