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Changing climatic conditions are affecting ecosystems worldwide, but polar ecosystems are experiencing the most rapid changes as the extent of summer sea ice has diminished over the last several decades. In the Canadian Arctic, thick-billed murres Uria lomvia can be used as samplers of the marine environment, and we examined changes in the diet of the birds between recent and historical studies from the 1970s and 1980s. In the low Arctic, the amount of Arctic cod, an ice-dependent species, declined in the murre diet; and capelin, a subarctic species, has now become the primary prey item. In the mid-Arctic, where summer sea ice is still present during the summer, Arctic cod persists as an important prey item, but capelin is now regularly observed, marking an apparent northward expansion of this species over the last 30 yr. In the high Arctic, summer sea ice continues to dominate the seascape in most years …