Several factors were at play on Sept. 18 when thousands of broad-wings suddenly began funneling through the roughly seven-mile stretch between Quaker Ridge and Long Island Sound, O'Toole said.
Like most hawks, broad-wings migrate across the ridge's southern edge to take advantage of the warm air currents, known as "thermals," that bounce off the ground and propel them to high altitudes, he said.
Once atop the thermals, the hawks can soar for miles on the tailwinds without having to expend energy by constantly flapping their wings, as lower-flying birds do, to stay aloft.
However, this advantage fades when the hawks begin flying over water, where the cooler air currents make it much harder to sustain long periods of soaring. As such, O'Toole said, hawks typically hug the shoreline when the wind pushes them close to the ocean.
On Sept. 18, after being grounded by a cold front along their migratory route for several days, the hawks began taking off in large flocks, known as "kettles," as soon as the ground started to warm up.
At the same time, a moderate northwestern wind began pushing them closer and closer to the cool waters of the Sound, creating a bottleneck of birds trying to ride out the warm thermals between the ridge and the coastline, he said.
"It was just the right combination of factors, where you had this gradual build-up of hawks" in the region waiting for the cold front to lift," O'Toole said. When the air warmed and the wind began blowing, there was "this explosion of huge numbers."
While the one-day numbers are impressive, O'Toole added, they are not unprecedented: On Sept. 15, 1995, there were 31,988 sightings of broad-winged hawks at the ridge, likely due to similar wind and weather patterns.
Regular readers may remember that we have our own (so to speak) broad-wings here, nesting at the bottom of our driveway and attacking us and others whenever we walk past in May and June. Gory details here.