Effective management of migratory animals requires an understanding of individual movement patterns throughout the annual cycle. We used satellite transmitters to track adult Barrow's Goldeneye (Bucephala islandica) captured at five wintering sites, one breeding site, and one molting site from across the species' geographic range in western North America. The data were analyzed to assess the strength of migratory connectivity and determine latitudinal and sex effects on migration phenology. At the range-wide scale, migratory connectivity was high; cluster analyses showed subpopulations aggregated at each stage of the annual cycle. Barrow's Goldeneye from all wintering latitudes traveled north and east to breed. Compared to females, males traveled farther north to molt and consequently traveled longer distances when returning to their wintering areas. Wintering latitude had little effect on migration distance but a large effect on the phenology of migration. Individuals that wintered at northern latitudes arrived on their wintering areas earlier and departed later than individuals that wintered farther south. Individuals that wintered at northern latitudes also arrived on and left their inland breeding areas later but left their molting sites earlier than individuals that wintered farther south. Sex also influenced the phenology of migration at all sites: males left their mates during incubation and consequently spent less time on breeding areas and more time on their molting and fall staging areas. The high level of migratory connectivity observed in this study suggests that the subpopulation of Barrow's Goldeneye in south-central Alaska is demographically independent from subpopulations in southeast Alaska and British Columbia, and could be managed separately. Connectivité migratoire et variation de la phénologie de migration au sein de la population du Pacifique du Garrot d'Islande (Bucephala islandica)RÉSUMÉ. Une gestion efficace de la faune migratrice passe par une compréhension de la tendance des déplacements individuels tout au long du cycle annuel. Au moyen d'émetteurs satellites, nous avons suivi des Garrots d'Islande (Bucephala islandica) adultes capturés à cinq sites d'hivernage, un site de reproduction et un site de mue dans l'ensemble de l'aire de répartition géographique de l'espèce dans l'Ouest de l'Amérique du Nord. Nous avons analysé les données de façon à évaluer la force de la connectivité migratoire et à déterminer les effets de la latitude et du sexe sur la phénologie de migration. À l'échelle de l'aire de répartition, la connectivité migratoire était élevée; les analyses par grappes ont montré que les sous-populations se regroupaient à chaque étape du cycle annuel. Les Garrots d'Islande de diverses latitudes d'hivernage ont voyagé vers le nord et l'est pour se reproduire. Comparativement aux femelles, les mâles se sont déplacés plus au nord pour muer et ont donc parcouru de plus grandes distances pour retourner dans leurs zones d'hivernage. La latitude d'hivernage a eu peu d'effets sur la dis...

Timing of seasonal bird migrations is broadly determined by internal biological clocks, which are synchronized by photoperiod, but individuals often refine their migratory timing decisions in response to external factors. Using 11 years of satellite telemetry data, we show that Pacific Barrow’s Goldeneye (Bucephala islandica), at higher latitudes, initiated spring and molt migrations later and fall migration earlier than individuals at lower latitudes. We further show that individuals refined migratory timing in response to interannual variation in environmental conditions. Individual Barrow’s Goldeneye initiated spring migration earlier in years with warmer springs at their overwintering locations and concluded spring migration earlier in years with earlier annual snow melt on their breeding grounds. Because individuals respond to conditions both where they initiate and where they conclude spring migration, our results suggest that Barrow’s Goldeneye update their migratory decisions en route. For all three migrations in their annual cycle, birds delayed initiating migration if they had been captured and tagged prior to that migration. Birds that initiated migration late for their latitude were less likely to include a stopover and completed that migration faster, partially compensating for delayed departures. Our results are consistent with the hypothesis that Barrow’s Goldeneye use a combination of endogenous cues and environmental cues in migratory decision making. Sensitivity to environmental cues suggests that Barrow’s Goldeneye may have behavioural plasticity that is adaptive when faced with ongoing climate change.