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I just got an email and followup phone call from Bryan Murdock of Duluth. He was out on Connor's Point (in Superior, Wisconsin) beneath the Blatnik Bridge last night to photograph the sunset when he came upon this gruesome sight.
He said footprints went up to the carcass--they looked like fox or coyote. The bird could well have been knocked out of the air by the wintering Gyrfalcon or Peregrine.