WINTER MOVEMENT ANALYSIS

OBJECTIVES

King Eiders spend about 10 months of the year at sea, and their wintering locations range throughout the Bering Sea. From satellite telemetry analysis of wintering King Eiders we learnt that the birds do not remain stationary in winter, but that some individuals move substantial distances between different wintering sites. More than 60% of 95 birds we tracked during a full winter used more than one site, and the homerange used on average is orders of magnitude larger than what other sea ducks use during the wintering period. Why are King Eiders different from other sea ducks in their wintering behavior? Why do they depart from certain areas and fly up to 300 km to a new wintering site?

From other parts of the world we know that sea ducks can start evasive winter movements when sea ice covers up large areas of the sea. As the sea freezes up, the ducks have less area to forage in and eventually will take off and fly south to new foraging grounds. These movements are therefore characterized by changing sea ice conditions. King Eiders may be less tolerant to sea ice than other sea ducks (Phillips et al. 2006).

If King Eiders try to winter as far north as possible to reduce the costs of a longer migration, they might experience very cold weather conditions that lead to closure of the sea ice. If the movements we have observed are a response to deteriorating sea ice conditions we would expect to see two patterns: (1) ducks would depart from areas where the sea ice concentration increases strongly, and (2) ducks at the same site would depart from that site at the same time.

To examine whether the mid-winter movements of King Eiders tracked by satellite transmitters support these hypotheses we will use sea ice data to identify whether environmental conditions correlate with the timing of movements, and whether movements occur simultaneously among the birds we tracked.

Should I stay or should I go? King Eiders wintering in icy waters are often faced with the decision whether to endure conditions or leave a site.

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