The flight of 404 eagles smashed the old record of 245 set two years ago at the ridgetop preserve, which has kept an annual tally of migrating hawks, eagles, and falcons since its founding 76 years ago.
This year's count included one day in late August when researchers tallied 36 eagles.
The bald eagle has made a comeback following a century of decline. It was removed from the endangered species list in 2007. Keith Bildstein, the sanctuary's director of conservation science, calls it "possibly the greatest wildlife success story of our time."