Eyes pressed to binoculars, they scan from the Middle Keys horizon to the highest clouds.
An alert goes out: "Kettle over Big Bob!"
The group of volunteers at the Florida Keys Hawkwatch shifts as one, looking to pick out details of distant hawks that follow the Florida Keys on the raptors' southern migration.
"No other site in the world has the documented numbers, for a single day and entire season, for peregrine falcons," said Rafael Galvez, coordinator of the Keys Hawkwatch.
"In the fall, this is the last place we can keep track of raptors and other bird species before they leave North America," Galvez said. "This is a bottleneck at the tapering end of the Atlantic migratory flyway."
Biologists Kerry Ross and Rachel Smith ran the spotting station at the Curry Hammock State Park's campground on Crawl Key earlier this week.
They helped set the Oct. 10, 2012, record for the highest one-day count of peregrine falcons at 651. For the fall migration season, generally running mid-September to mid-November, volunteers logged 3,836 peregrine falcons over the Keys.
This year's Florida Keys Hawkwatch shirts proclaim, "The Peregrine Falcon Migration Capital of the World." Volunteers also log broad-winged hawks (a record 7,236 counted last fall), sharp-shinned hawks, ospreys and more than a dozen other types of raptors.
Since making population estimates for wide-ranging bird species is virtually impossible, Ross said, the best system is to identify locations where species pass at specific times.
"You get trend lines that indicate whether a species is increasing or decreasing," Ross said. "You realize the peregrine falcon population seems to be increasing, but something has changed in the biology of the American kestrel that is causing its numbers to decrease."
If the main observation site at Curry Hammock was not running, Ross said, "you would miss huge numbers of birds."
Volunteers travel to serve as citizen scientists, enjoying the chance to spot a wide range of migrating hawks and other birds.
Watchers nickname landmarks, referring to specific trees with monikers like Big Bob and Little Bob for quick aerial reference. Utility poles get numbers.
A "kettle" refers to a group of raptors, usually gathering to ride the thermal of hot air arising from a land mass higher into the sky. The Keys are the last U.S. spot where the migrations can catch thermals before heading to breeding grounds in the Caribbean, Central America and South America.
Colleen and Charles Caudill of Tarpon Springs were casual birders before happening upon Hawkwatch three years ago on a Keys camping trip. Now they're back for their third migration. It's so fascinating and worthwhile," Colleen said. "We started at two weeks. Now we're here four weeks."
"It's addicting," Charles agreed.
Florida Keys Hawkwatch is coordinated by the Tropical Audubon Society, Florida Keys Audubon Society and Space Coast Audubon Society, with support from several conservation groups and businesses.
New volunteers, including beginning birders, are welcome. See the website: Floridakeyshawkwatch.wordpress.com; or email TASpublisher@gmail.com.