MIGRATORY TACTICS AND WINTERING AREAS OF NORTHERN GANNETS (MORUS BASSANUS) BREEDING IN NORTH AMERICA

Abstract

Migration has evolved to allow organisms to undertake life-history functions in the most appropriate place at the most appropriate time. Migration creates seasonal ecological linkages that have important implications for survival, population dynamics, response to climate change, and conservation. Although advances in bird-borne tracking technology have promoted knowledge of avian migratory ecology, major information gaps remain for most avian species, including seabirds. Ours is the first study to electronically track the migration and wintering of Northern Gannets (Morus bassanus; hereafter “gannets”) from almost all of their North American breeding range, in multiple years and with multiple tracks from individual birds. Gannets used distinct wintering areas and aggregated in several major hotspots. Most adults remained along the northeast North American coast, and breeding populations displayed only weak migratory connectivity. Unexpectedly, the Gulf of Mexico was revealed to be an important wintering area for adults. Individual gannets displayed remarkable winter-site fidelity with extensive range overlap across years. Timing, rates of movement, and use of stopovers during migration depended strongly on winter destination and also on sex, colony, and year. Females left the colony prior to males in fall, but, contrary to expectation, earlier spring arrival of males was not detected. Variation in seasonal constraints was emphasized by faster and shorter spring migrations compared with fall. Migratory duration, distance, and timing of arrival and departure from the winter grounds were all repeatable, suggesting strong individual conservation in these traits, while variability in the timing of colony departure and arrival, migratory speed, and the extent of stopovers en route imply greater environmental influences on these behaviors. Three of 46 gannets displayed a radically different round-trip migration and overwinter strategy by undertaking the first recorded (and repeated), round-trip trans-Atlantic migrations to the coast of Africa, where gannets breeding in Europe overwinter. Trans-Atlantic crossings were as rapid as 5 days. Gannets breeding at all of the North American colonies located in easternmost Canada in Newfoundland made trans-Atlantic migrations. This contrasts with no such crossings by a much larger sample of gannets breeding in the species' largest North American colony (Bonaventure Island) in the Gulf of St. Lawrence. Similar trans-Atlantic migrations have not been recorded in the well-studied European colonies. The discovery of this trans-Atlantic connection has implications for interaction, connectivity, and phylogeographic radiation between the eastern and western Atlantic populations and suggests that migratory animals have a surprising capacity for successful movement beyond their known migrations. We discuss our results in the broader context of seabird migration. The observed patterns of migratory timing and scale-dependent connectivity present a novel opportunity to assess the ecological and conservation implications during migration and on the wintering grounds. The overall weak connectivity suggests that gannets, as a population, may have the capacity to respond to modest environmental change. However, the repeatability of some characters implies that any response may be slow because the population will respond, but not necessarily the individuals. The lability of migratory tactics in the population as a whole combined with remarkable individual consistency in some, but not all, migration parameters offers rare insight into the relative contributions of genetic and environmental factors controlling migration.