There’s a jaw-dropping bird migration spectacle that only happens in September, and I’m afraid you’ll have to cross the bridge to catch a glimpse of it. The problem is, Broad-winged Hawks hate to fly over water, and there are no winds strong enough to coax them across the bay to Cape Cod on their southbound flight each September.
The handful that breed in patches of more mature forest here on the Cape slip away quietly, only joining larger flocks somewhere south of Massachusetts.
During the breeding season Broad-winged Hawks are small, shy, and obscure when compared with their rampant highway-side cousins, Red-tailed Hawks. Whereas red-tails like to hunt open areas from farm fields to median strips, broad-wings are a retiring bird of woodland interiors, where they quietly hunt small animals from perches. They breed sparsely on Cape Cod from Orleans to Sandwich, but are rarely encountered.
When September comes around, Broad-wings throughout the eastern US and Canada leave their quiet forest homes behind and join together in sometimes massive migratory flocks, known to the hawk watching elites as “kettles”. Kettles, I suppose, have so many soaring birds swirling around that they resemble pepper flakes in a pot of boiling water – at least that’s how I’ve always explained the weird name. Because they all tend to migrate at the same time, usually following the passage of a cold front in mid-September, and because they all seek out the same flying conditions, it’s indeed possible to see hundreds or even thousands of broad-wings on a good day in Massachusetts around now.
Probably the best place in the Massachusetts to witness Broad-winged Hawk migration is Mt. Wachusett in Princeton, where the counters had a single-day tally of more than 11,000 on September 15, 2013. I guarantee that if you are lucky enough to witness a flight like that, it will blow your mind. So what concentrates so many hawks in such a small area? The answer is rising currents of air known as thermals. The warmer air over things like sun-baked parking lots and other warm ground rises faster than the surrounding air, and Broad-winged Hawks are brilliant at exploiting this phenomenon to power their long migration. By soaring to great heights in a thermal and then gliding several miles to the next thermal, these hawks can cover great distances without ever flapping, saving their precious energy reserve on their epic annual journey from Canada to Brazil and back.
As the birds continue south on their way to the South American wintering grounds, the kettles get even bigger. By the time they are passing through Texas and then the coastal plain of Veracruz, Mexico, they are, technically speaking, way, way bigger. Like, hundreds of thousands of birds passing in a single day bigger, swirling together in giant tornados of soaring birds. When the birds reach the tops of the kettles, they banner out across the sky in long sinuous strands of many thousands of gliding birds. I was lucky enough to see this first hand as part of the international hawk counting team in Veracruz some years ago, where the official count is conducted from a hotel roof in the small town of Cardel. Our job was to count the hawks and other migrant birds passing through, which included many thousands of Wood Storks, Pelicans, and other water birds in addition to the hawks.
The phenomenon there is known appropriately enough as the River of Raptors, and the season’s tally is typically over 4 million migrating raptors, mostly Broad-winged Hawks, Swainson’s Hawks, and Turkey Vultures, all passing in a couple of months. It’s the greatest global wildlife spectacle you’ve never heard of. And while Mexico is a bit further over the bridge than I had intended to take you today, I hope you’ll take a minute the next time you see a soaring hawk to think about what a thousand or even 100 thousand soaring hawks might look like, and know that somewhere out there, in the weeks ahead, that will be a real thing that people are seeing.
