IN its most serious confrontation since losing more than one-quarter of its planes in the 1973 Arab-Israeli war, the Israeli Air Force has turned for assistance to a group of bird watchers. Monitoring the enormous migratory flocks passing over Israel - the second-densest bird migratory corridor in the world - to reduce the frequency of fatal plane-bird collisions, the Society for the Protection of Nature in Israel (SPNI) has been using a motorized glider to join the birds on their passage. By soaring with the flocks and mapping their migratory patterns, the society has helped the Air Force avoid clogged birdways and drastically curb its losses in collisions with birds.
The Air Force acknowledges losing several planes and pilots as a result of bird collisions before 1982. One pilot, who had safely survived the 1973 war in which more than 100 of his colleagues were downed by missiles, was killed a few weeks later when a pelican smashed into his cockpit.
A number of other pilots have been wounded in similar accidents. ``I was on a routine training flight at 3,000 feet when I suddenly heard a tremendous explosion and was knocked out,'' one such pilot said. ``After I came to myself a bit, I could hear the air rushing into the cockpit.... When I looked down, I saw feathers and a carcass. I realized I had been in a bird accident.''
Between 1972 and 1982, there were hundreds of these collisions. Although most were minor, total damage amounted to tens of millions of dollars.
The problem is as severe as it is because Israel lies at the narrow choke point of migration routes followed each fall by flocks of birds from the vast area between Germany and Afghanistan as they head south to wintering grounds in Africa.
Each spring, the flight is reversed. Only in Panama, according to ornithologists, is the migratory bird traffic heavier.
Unlike Panama, however, the Negev is also the site of intensive aircraft activity, a training area for one of the largest and busiest air forces in the world. The problem was severely aggravated in 1982 when, after the peace treaty with Egypt, Israel evacuated the broad Sinai Peninsula and transferred most of its air bases to the confined space of the Negev.
Although the phenomenon of bird migration over Israel has been known at least since biblical times (``The stork in the sky knows the time to migrate, the dove and the swift and wryneck know the season of return; but my people do not know the ordinances of the Lord'': New English Bible, Jeremiah 8:7), its extent and exact character were virtually unknown.
In 1980, the SPNI began establishing a string of ground observer stations. After the Sinai pullout, the Air Force enthusiastically agreed to finance the survey. Ornithologist Yossi Leshem attempted to supplement the ground observation by following the flocks in a light plane. It could not, however, fly slowly enough to keep pace. The solution was found in the motorized sports glider.
Lifting off in the morning with the flocks after their night's rest, Mr. Leshem and the glider pilot accompanied the eagles, storks, pelicans, and other flocks as they soared across the country.
``They just seem to regard us as a big bird,'' says Leshem. Data on altitude, direction, and times of these flights were channeled to the Air Force, which quickly established ``bird hit areas'' that pilots were forbidden to enter during migratory seasons.
``We discovered that there was a certain regularity in the timing of the migration, in the times when the large flocks appear and also in the route and height they flew,'' says Lesham. ``The results of these findings have been beyond anything we could have expected.''
In the five years since the program began, not a single pilot or plane has been lost to birds, despite the much-increased air activity over the Negev, nor has there been a single collision classified as serious. Rarely if ever before have bird watchers had such a direct impact on their country's military posture.
The survey has also brought to public attention for the first time a phe-omenon that has hitherto drawn hardly a passing glance in this busy part of the world. Hundreds of foreign bird watchers have begun streaming to Israel each migratory season.
The migrating birds, originally thought to number in the hundreds of thousands, are now reckoned in the tens of millions. Each fall, when their biological mechanism signals that the time come, birds of prey and other large birds begin draining out of the skies of Europe and western Asia into the airspace over the narrow Negev-Sinai land corridor to Africa.
Songbirds and other small birds take the direct route south, crossing he Mediterranean overnight.
Large birds, however, do not have the energy to flap their wings that far, given their weight. Instead, they ride rising currents of air, known as thermals, gliding for long distances between the top of one thermal to the bottom of another that will spiral them upward on their extended wings. In that way they migrate thousands of miles to their African wintering grounds, covering 200 to 300 miles a day. Thermals develop when the ground is warmed by the sun. There are no thermals over water or at night.
Some young migrants, such as eagles, are barely three months out of the egg. ``They have two months to grow to the size of their parents,'' says Leshem, ``and one month to pass their flight test.'' The birds store up fat in the weeks before the journey, which generally obviates the need for hunting en route.
Although tests in planetariums have shown that migrating birds can orient themselves according to the stars as well as the sun, their navigation on cloudy days is unimpeded. The birds are believed to be aided by unseen forces on their astonishing intercontinental journeys, perhaps by the earth's magnetic field.
The semiannual flypast of these heavenly pilgrims over the Holy Land was not the least of the miracles the prophets had witnessed.