The huge flight of 407 eagles smashed the old record of 245 set two years ago at Hawk Mountain Sanctuary, which has kept an annual tally of migrating hawks, eagles and falcons since its founding 76 years ago as the world's first refuge for birds of prey.
As the autumn raptor count at Hawk Mountain drew to a close Wednesday, sanctuary biologists and birding enthusiasts alike cheered what Keith Bildstein, the sanctuary's director of conservation science, recently called "possibly the greatest wildlife success story of our time."
The U.S. population of bald eagles suffered a steep decline between the 1870s and 1970s, first due to habitat destruction and hunting, and later because of the widespread use of DDT. The pesticide accumulated in fish, a major food source for eagles, and resulted in eagles laying eggs with weakened shells that broke during incubation.
By 1963, there were only 417 breeding pairs left in the lower 48 states.
They also were an extreme rarity at Hawk Mountain, which is situated along a major Appalachian flyway for migratory birds known as the Kittatinny Ridge. The low point came in 1975, when counters spotted only 13 bald eagles the entire fall.
"One of our members always used to bring champagne in hopes we would see one," said veteran bird-watcher Catherine Elwell, who has been visiting Hawk Mountain since the early 1970s. She said "great cheers would rise up" on North Lookout - an outcropping where the official count takes place - whenever an eagle was overhead.
Rachel Carson's 1962 conservation classic "Silent Spring" used data from Hawk Mountain to warn about the dangers of chemical pesticides. DDT was banned in the U.S. a decade later. The bald eagle began a gradual recovery that has seen its numbers reach more than 10,000 pairs, according to the U.S. Fish and Wildlife Service.
The eagle was removed from the federal Endangered Species list in 2007.
"This has been just the most remarkable recovery, and I don't think many of us thought it would happen," said Elwell, 68, of Alburtis.
Bald eagle numbers have risen steadily at Hawk Mountain, with a 10-year average of 235.
This year's count included one superlative day in late August, when research biologist David Barber tallied 36 eagles - 31 of them after 3 p.m., and 14 of those in a single hour. It was the second-highest single-day flight in Hawk Mountain's history.
"We just kept looking at each other, like, where are all these eagles coming from?" he recalled Wednesday.
Where indeed. While 2010 might turn out to be a statistical anomaly, senior monitoring biologist Laurie Goodrich, who coordinates the annual count at Hawk Mountain, speculates that a number of factors came together to make this year like no other.
The weather cooperated, with plenty of days of northwesterly winds that helped push migrants closer to the ridgetop. An increase in the number of breeding pairs in the Northeastern United States may also have contributed: Local eagles are territorial, pushing migrants from Canada farther south.
The record flight might simply reflect that there are more eagles than ever before. Three more were spotted Wednesday at Hawk Mountain.
Whatever the reason, Goodrich said, "It's something we can celebrate, as bird-watchers and as scientists."