| Summary: | Southwest Greenland is an international important wintering area for Greenland and Canadian common eiders (Somateria mollissima borealis). They distribute both in the shallow habitats along the outer coastline and in the fjords which are generally very deep. In the fjord systems, eiders are typically dominated by adult birds gathered in large communal roosts on deep waters, which is in contrast to the coastal habitats that are dominated by younger birds with a more even spatial distribution on more shallow waters (< 50m). We hypothesized that fjord habitats served as a refuge during daytime when coastal areas are heavily disturbed by fishing and hunting activity. Surprisingly, satellite tracking of the eiders did not support this hypothesis, but revealed that there was little or no exchange between fjords and coastal habitats during the winter. Instead, observations of the feeding activity of the eiders indicated that predation by white-tailed sea eagles (Haliaeetus albicilla groenlandicus) was the key to understand the segregation between the fjord habitats and coastal habitats. We found that eiders in the fjords were feeding along the shoreline, but only during twilight and at night, whereas coastal birds were primarily diurnal feeders. Occasionally juvenile birds initiated feeding during daytime in the fjord, but were discontinued due to interactions with white-tailed eagles. Even in April when day length had increased by 5.9 hours (compared to February) the eiders appeared to rely on nocturnal feeding. The more extensive shallow waters in the coastal areas allow eiders to forage and feed at daytime at larger distances from land - presumably out of reach of eagle predation. We suggest that the nocturnal feeding strategy observed in the fjord is an effective anti-predator mechanism allowing eiders to utilize these habitats.
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