ABSTRACT. We observed walruses (Odobenus rosmarus) pursuing spectacled eiders (Somateria fischeri) within pack ice of the Bering Sea, 70 -90 km from the nearest land. We used both direct observations from a helicopter and a heligimbal camera system that can film animals from a helicopter at high altitudes. The eiders were in monospecific flocks of thousands of birds within large leads. The walruses apparently tried to catch the eiders from below; the eiders responded with a "flash expansion" (explosive radial movement), wing-flapping and running along the water surface to escape. Disturbance by individual walruses could restrict flocks of thousands of birds to small portions of the open water. In eight such events that we witnessed over 75 min of observations, we were unable to confirm that walruses captured any of these full-grown, flight-capable eiders. however, the high rate of attacks and the eiders' dramatic escape response suggest that walruses can at times be effective predators on them, and may affect the eiders' dispersion and energy balance.Key words: Bering Sea, heligimbal camera system, marine mammal predation on birds, pinniped predation on birds, predator avoidance, spectacled eider, walrus RÉSumÉ. nous avons observé des morses (Odobenus rosmarus) en train de pourchasser des eiders à lunettes (Somateria fischeri) sur la banquise de la mer de Béring, soit à une distance de 70 à 90 kilomètres de la terre ferme la plus près. nous nous sommes servis à la fois d'observations directes faites à partir d'un hélicoptère et d'un appareil héligimbal capable de filmer les animaux à partir d'un hélicoptère à haute altitude. Les eiders se tenaient en bandes monospécifiques constituées de milliers d'oiseaux faisant partie de gros groupements. Il semblait que les morses essayaient d'attraper les eiders par en-dessous; les eiders réagissaient en faisant une « expansion éclair » (un mouvement radial explosif), en battant des ailes et en courant le long de la surface de l'eau afin de s'échapper. Les perturbations exercées par les morses individuels pouvaient avoir pour effet de restreindre les bandes de milliers d'oiseaux à de petites nappes d'eau libre. Dans huit cas de telle nature dont nous avons été témoins pendant plus de 75 minutes d'observation, nous n'avons pas été en mesure de confirmer si les morses avaient réussi à capturer des eiders adultes en état de voler. Cependant, le taux élevé d'attaques de même que la réaction dramatique des eiders qui tentaient de s'échapper laissent entrevoir que les morses pourraient être des prédateurs efficaces en ce qui les concerne, ce qui pourrait exercer une influence sur l'expansion des eiders et sur leur bilan énergétique.Mots clés : mer de Béring, appareil héligimbal, prédation d'oiseaux par des mammifères marins, prédation d'oiseaux par des pinnipèdes, évitement des prédateurs, eider à lunettes, morse Traduit pour la revue Arctic par nicole Giguère. Some marine mammals regularly prey on other mammals. However, except for leopard seals (Hydrurga leptonyx) eating pengui...

Migratory birds that nest in the Arctic must be prepared to complete breeding within the brief polar summer, but must also have means of adjusting their physiological and behavioral readiness under widely variable environmental conditions. For Arctic breeding sea ducks, which may start laying within a week of accessing nest sites, hormonal status needed for courtship, pairbonding, and ovarian development must be achieved before arrival, often in the context of extreme and rapidly changing weather. Spectacled eiders (Somateria Wscheri) winter in pack ice, migrate to oVshore openings in ice to await snowmelt on nest sites, and then move rapidly inland to start nesting. Using a camera that can Wlm from a helicopter at high altitudes, we recorded behavior of spectacled eiders wintering in Xocks containing tens of thousands of individuals within openings in Bering Sea pack ice, 70-90 km oVshore. Rather than typical courting parties of single females pursued by a few males, the eiders formed crowded swarms of up to hundreds of birds in which males and females intermingled excitedly while performing courtship displays. In these "courtship scrums," which occurred in patches throughout very large Xocks, it did not appear possible for one individual to track other individuals to assess their characteristics for mate selection. Instead, this previously unreported behavior, which occurred near the seasonal onset of courtship, may provide visual and auditory stimuli that accelerate hormonal development for rapid transition from harsh winter conditions to breeding readiness. Such deviations from typical behavior may facilitate adaptation to advanced phenologies in a warming climate.

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