Falco rusticolus; the Gyrfalcon is a typical polar-circumferencing species of the northern hemisphere. It is about the same size as common European goshawks but has characteristically more pointed wing tips and the general falcon shape. The color is variable and depends also on their habitat. Greenland gyrfalcons are very light. The Icelandic individuals are darker but on average still lighter than their mainland Scandinavian relatives. Ptarmigan is their main prey on Iceland and elsewhere as well. However they will also hunt waterfowl and small rodent-like mammals. Daniel Bergmann, the nature photographer who has offered me amongst others photo's of the gyrfalcon, has an interesting story about his personal experiences in photographing these birds as well as more general information (history of hunting, export for falconry, ecology, threats) about the gyrfalcon on Iceland. You can find his site here.