Despite the peering binoculars of generations of earnest bird watchers, no one yet really understands the migrations of birds. This is partly because birds are small, fast and hard to see, partly because many species do most of their flying at night. But a new, giant bird watcher has recently taken the field. Radar, used since World War II to track man-made flying machines, can spot small birds at night or behind clouds, as high as 10,000 ft. or as far off as 50 miles. The secrets of migration are rapidly being unraveled by electronics.
In Natural History magazine, two physicists (Cambridge...
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