Tropical avenue of the raptors: twice a year, millions of migrating hawks, falcons, and vultures help link the biological heritages of continental North and South America.

• Author: Bildstein, Keith L.

We had just charged a little over a mile uphill through an organic cacao plantation on the Caribbean slope of Costa Rica. Our clothes were soaked in sweat, but we were happy. The destination for our trek, a small hillside opening, had come into view. A Kekoldi Indian communal house protruded from the forest on our left, and a small crowd had gathered atop a tiny wooden deck that jutted out over the precipice to our right.

Three individuals, a blonde Minnesotan and two dark-haired Costa Ricans, each gripping small metallic counters, were peering through binoculars into the distant coastal haze, while several small children clamored for attention at their feet. We approached the trio and were about to introduce ourselves when someone called out: "Incoming raptors!" It was the largest flock of the still-young day.

At once, priorities shifted, assignments were made, and before our eyes the trio, actually the staff of the world's newest million-bird hawkwatch, sprung into action. Half an hour later, more than five thousand birds of prey (about three per second!) had been identified and counted before s sailing southeast overhead into the Sixaola Valley on the border with Panama. The next day, the same birds would pass over the all-but-impenetrable Darien lowlands of easternmost Panama, en route to northwestern Colombia.

But this first day's count, on October 21, 2000, had only begun. By the time we left the site at five that afternoon, 51,442 raptors of at least eight species had been accounted for, bringing the season's total to more than half a million birds. Most hawkwatches need a few decades to reach this milestone. Here in Talamanca, Costa Rica, it had taken just seven bleary-eyed, sunburned weeks.

More than 97 percent of the day's total consisted of one-pound broad-winged hawks, two-pound Swainson's hawks, and four-to-five-pound turkey vultures; with the latter making up almost two-thirds of the overall flight. All but a few of these North American breeders overwinter in South America. Most of the broadwings melt away into the tropical enormity of forested Amazonia, while the Swainson's hawks continue into the temperate southern cone of South America, where they will spend the northern winter flocking after locusts in the pampas of Argentina. Although the geographic fates of the day's turkey vultures are less well understood, at least some of the birds--which are larger than tropical members of their own species--bully their local counterparts out of prime wetlands in the Venezuelan llanos.

Other species seen in lesser, but still impressive, numbers included twenty-three ospreys and fifty-seven peregrine falcons, avian predators whose populations only recently have rebounded from pesticide-era lows of the 1950s, 1960s, and 1970s.

To place the overall magnitude of the flight into a geographic perspective...