THROUGHOUT Thord’s life, the island of Björkö, 12 miles west of Stockholm, served as a beacon, glowing in the lingering twilight of Swedish summer nights. There Thord and his brothers played among Viking graves and the remains of the once-majestic fortress of Birka. They harvested wild berries and hunted with bow and arrow in forests teeming with game. Björkö also provided the setting for eight-year-old Thord’s first solo adventure: his first test of survival skills outside the family fold.