Angels on the Move

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 University's Dr. I.C.T. Nisbet and M.I.T.'s Dr. R. R. Richardson) team up with Ornithologist W. H. Drury Jr. of the Massachusetts Audubon Society to report the triumphs of electronic bird watching. Even the earliest radars, they say, picked up mysterious targets that operators call "angels." Most of the angels proved to be big birds--seagulls or wild geese--but when radars were improved, even small songbirds turned up as targets. They were such a nuisance on radarscopes that M.I.T. scientists worked out an electronic circuit to make radars blind to birds. But Nisbet, Richardson and Drury continued to study the nonelectronic aspects of radar bird watching.

Using a powerful experimental radar at South Truro near the tip of Cape Cod, the scientists soon finished off one well-established notion--that migrating birds follow coastlines. The all-seeing electronic eye showed that most birds cross the coast without changing direction. The few that are deflected have been so easy to spot as they fly along the beach that human bird watchers erroneously decided that coastline following was standard bird procedure. The Truro radar sometimes showed conspicuous angels moving out to sea at a 40-knot speed. These proved to be dense flocks of sandpipers, plovers, and other shore birds starting nonstop flights to the West Indies or South America.

Tracked by radar, the migrating birds did not seem so skilled at navigation as bird watchers like to believe. The Truro radar spotted many of them being blown out to sea at night by strong northwest winds. Apparently they did not know, in the darkness, that they were off course. The radar often picked them up heading back toward shore as soon as dawn came, as if in search of familiar landmarks, before they continued flying on their planned migration.

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