Over the last 30 years, HawkWatch has counted an estimated 20 million hawks and banded 125,000 of them, an essential task in understanding and protecting the birds. They were first to identify three flyways in the American West that follow a pattern similar to flyways in the East where migrating hawks navigate along coastlines and mountain ridges featuring strong thermal updrafts.  Hawks today face many threats, said Allen Fish, director of the Golden Gate Raptor Observatory on the Marin headlands north of San Francisco, which makes the need to count and band them more important than ever. For example, American kestrels are in sharp decline, while there is a potential decrease in golden eagles.  As more giant wind farms are erected, an increasing number of hawks are slashed and killed by turbine blades. Oil and gas exploration is fragmenting many hawk habitats. Urban-suburban growth, pesticides, herbicides, electricity lines and climate change are other stressors, he said.  The only way to understand what is happening to hawks is to collect data over many decades, banding as many birds as can be captured, said James Dawson, a hawk biologist with 30 years of field experience and curator of wildlife at the High Desert Museum in Bend, Ore.  Hawks, eagles and other birds of prey are tricky to catch, however. The term “eagle-eyed” did not come out of nowhere. Mechanical lures that appear overwhelmingly realistic to human eyes are unconvincing to hawk eyes, Mr. Fish said. His organization spent 17 years trying to perfect such lures but suspended the effort when researchers realized how easily hawks see the deception.  Hawks see 8 to 10 times better than we humans, Mr. Fish said. They have four foveas (the part of the eye that sees detail) instead of our two, the ability to detect reflected ultraviolet light and greatly accelerated speed of visual processing. When a hawk dives at a mechanical lure, he said, it pulls up 30 to 50 feet before the target as if to say, “there’s something wrong here.”  The only way to catch hawks is to use live birds as lures, Mr. Fish said. It’s the only method trappers use. So when the juvenile harrier approached the Manzano mountains last week, two trappers concealed in a blind 50 yards below the lookout had their lures and nets ready.  The live pigeons, doves and starlings that are used to attract hawks are outfitted in full body suits made of leather or Kevlar for protection. With only their heads, wings and feet exposed, the birds stand near the traps or are tossed up to 20 feet in the air, their feet securely attached to fishing lines manipulated from the blind.  At the Manzano site, three kinds of nets were arrayed on a spacious apron of ground in front of the blind. A mist net, made of nylon with a large mesh and multiple compartments to catch birds that fly into it, was suspended, like a volleyball net, between two poles.  A dho-gaza net, with a smaller nylon mesh, was also affixed to two poles, but set to collapse when a hawk flew into it. A remotely triggered bow net, a hooplike device with a spring mechanism, lay on the ground.  Ms. Weston described what happened next (HawkWatch would not allow a reporter to observe the trappers working): When a hawk that her colleague Tim Hanks had identified flew overhead and spotted a pigeon waddling on the ground, the trappers pulled on the line attached to the pigeon to jiggle it, hoping it would pass for a small mammal, the favored prey of harriers.  When the hawk struck, Ms. Weston said, it entangled itself in the mist net before it could reach the pigeon. In the standard routine for all trapped birds, it was measured, weighed, checked for fat deposits under its wing and inspected for parasites. The harrier was healthy, Ms. Weston said, displaying the bird.  The trappers had fixed a metal band to the harrier’s leg, assigning it an identity should it be recaptured elsewhere or found dead. Ms. Weston let it go amid a flurry of powerful wing beats.  During this fall season, Aug. 27 to Nov. 5, the hawk team has been observing between 30 and 200 hawks a day, capturing and banding about 10 percent of the migrants. The most common species have been sharp-shinned hawks, Cooper’s hawks, red-tailed hawks, American kestrels, golden eagles, northern harriers and Swainson’s hawks.  The pigeon, unharmed, was soon able to take the rest of the day off. Like other lure birds, it has an enviable schedule, to limit job stress: 60 to 90 minutes every other day.  An article on Tuesday about a program to monitor migrating birds of prey misstated the elevation of the Capilla Peak Observatory in the Manzano Mountains of New Mexico. It is about 9,000 feet above sea level — not 9,000 feet above the Rio Grande Valley.  A version of this article appears in print on October 26, 2010, on Page D2 of the New York edition with the headline: Serving Up Feathered Bait to Attract Ecosystem Data. Order Reprints| Today's Paper|Subscribe
The trapping of the birds is only one aspect of hawk watching done each autumn at 10 sites to focus on birds of prey as indicators of ecosystem health.