大象传媒

How ospreys find their way is intriguing. Adults can use their memory of landmarks from previous journeys to create a mental map of the route. But youngsters have never made the journey before. How they navigate is more of a mystery. Possibly they use prevailing winds or the sun and stars. They may sense the invisble lines of magnetism that circle the Earth, or even smell their way south. No matter how they do it they are extraordinary natural navigators. Steve Leonard is in Cape May, where thousands of ospreys gather at the end of the peninsula. They are waiting for the right wind conditions to fly over the bay. Southerly winds are critical to the ospreys. Rising thermals lift them a mile high, letting them glide for 5 or 6 miles to the next updraft. This lets them save huge amounts of energy. Young birds are inexperienced at finding thermals and have to resort to powered flight more often than the adults. It's energy they can ill afford. For youngsters this whole mission is balanced on a knife edge.