Florida’s raptor highway

There are other, better-known places than the Florida Keys to watch hawks migrate.

There’s Hawk Mountain in Pennsylvania, where it all started, and where the rocky outcroppings folks perch on to catch sight of passing raptors seem like hallowed ground.

There’s Cape May, New Jersey, with throngs of onlookers so numerous that the official counter often has to sit in an elevated lifeguard’s chair to see the horizon.

There’s the Goshute Mountains in Nevada, a lonely, high-altitude forest surrounded by desert, and, of course, there’s Veracruz, Mexico, where some days hawk counters are forced to estimate numbers of birds by the tens of thousands, the raptor freak’s equivalent of drinking from a fire hose.

I’m a devotee of the Keys. I may be biased because I live here, but compared with other, more venerable sites, it’s undiscovered country — the edge of the United States and the end of the road. Also, if you give credence to the old conventional wisdom that hawks don’t migrate across large bodies of water, a slightly perverse pleasure accompanies knowing that most of the raptors that dot the sky every fall shouldn’t be here.

For my first five years in the Keys, I was clueless about the 26,000 raptors that moved overhead every fall. Being so long unawares does not bother me, though, as pretty much everyone else was in the same boat.

Now that my eyes are open, in the fall I see migrant hawks everywhere: Peregrine Falcons standing like sentinels atop radio towers, kettles of Broad-winged Hawks swirling above the cemetery, Sharp-shinned Hawks diving into bushes, Merlins strafing shorebirds or picking fights with one another.

One of my favorite recollections (to date) is of standing on top of the La Concha Hotel in the heart of Key West, binoculars up, watching a tiny speck of a Peregrine track across the island, slipping in and out of the wispy fringes of clouds, and then, without hesitation, lighting out over the ocean for Cuba and points unknown. It’s something I’ve managed to see only once, but from mid-September to late October, it happens almost every day.

The Keys are a narrow chain of islands that fall away from the Florida mainland like coins from a hand, dropping into the Atlantic Ocean and the Gulf of Mexico and for a time forming the only clear demarcation between the two. The drive from the Everglades, at the extreme south of the peninsula, to Key West, the end of the island chain, is about 120 miles. It will take you three hours and involves crossing 42 bridges and an almost equal number of less obvious causeways. When you get out of your car at the end of US 1, you are 90 miles from Havana — 40 miles closer than you are to Miami.

According to secondhand accounts, John James Audubon noticed a plethora of raptors when he visited the Keys, but it seems unlikely he had much of an idea about the migration. He was here in April and May of 1832, after all; the major raptor migration takes place in autumn.

Along with Roger Tory Peterson, Robert Porter Allen published a seminal paper in The Auk in 1936 about observations of the hawk migration at Cape May, and Allen worked for several years as a field biologist in the Keys — he studied Roseate Spoonbills — yet he gives no mention of the phenomenon in his published work.

Southbound snapshot

Enough people must have looked up into the sky and noticed that something was afoot, or at least awing, by the late 1980s, when Harry Darrow and Wayne Hoffman, biologists who worked at the Audubon Science Center in Tavernier, on Key Largo, organized an annual series of fall hawk counts. Their method was to disburse teams up and down the Keys and do one-day censuses. From 1989 to 1992, they recorded consistent sightings of significant numbers of predominantly southbound birds.

People began to have a snapshot of an idea, but there was no story. If raptors were coming into the Keys, the next questions were obvious: When, specifically, were they coming? How many were coming? What species were coming? And what were they doing once they got here?

Since raptors are large birds, in order to use as little energy as possible when migrating, they eschew powered flight, preferring soaring and gliding to flapping. Some raptors, particularly accipiters and buteos, are thought to be capable of only a very limited amount of powered flight before needing to either soar or land. To make their way along their routes, they seek out thermals, ride the rising air until the currents falter, and then glide off, seeking more upwellings as they travel. (They also seek out ridges and ride the updrafts created by wind, although this is a phenomenon that doesn’t occur significantly on the eastern seaboard south of North Carolina.)

Although it’s debatable how reluctant raptors are to cross large, open bodies of water, they are reluctant. Strong thermals don’t form over them, and crossing requires either sufficient altitude to glide (something that is not always attainable), powered flight, or a combination of the two. Since raptors can’t swim, a bird that doesn’t make it from shore to shore in one go will most likely die.

As a flyway, the Keys function as a funnel. Hundreds of small mangrove islands dot Florida Bay, the body of water between the Florida peninsula and the Upper Keys, offering a multitude of safe routes for raptors to travel. Approaching the Middle Keys, though, the out islands disappear and the land narrows, sometimes to a width not much greater than the two lanes of the Overseas Highway. It creates a bottleneck.

In 1996 and 1997, Cindy Brashear, a graduate student at Florida International University, under the guidance of her professor, Phillip Stoddard, took a more expansive and tactical approach to documenting the migration here. She hired experienced hawk counters to do all-day censuses, seven days a week, from September 1 to November 15, stationing them poolside at the Gulf View Resort on Grassy Key. (One of the observers I spoke to could not recall whether poolside drink service was available at the time but indicated that it did not matter, as the counters weren’t paid enough to cover such extravagances.)

Impressive tallies

The initial results were impressive. More than 20,000 raptors were counted moving southbound each year, and approximately 6,500 were counted going back north, producing annual nets of about 13,500 birds that went south and did not return. (It should be noted that the Keys lie not in a straight line but in an arc, moving both south and west, and that northbound and southbound are not compass-correct directions. The terms mean either toward the mainland or away from the mainland.)

In 1998 Casey Lott, who’d worked as a counter on the Brashear and Stoddard survey, started the Florida Keys Raptor Migration Project with the support of HawkWatch International. His mission was to refine the censusing techniques and to commit to long-term monitoring of raptor population trends. The project has since been renamed the Florida Keys Hawkwatch.

Lott moved the count a few miles down the road to a more permanent home at the publicly owned Curry Hammock State Park, specifically the elevated back deck of the building that housed the facility’s large array of composting though surprisingly odor-free toilets, where the hawk watch is still based. He also set up an educational program to let Keys residents and visitors know about the migration, as well as a trapping and banding operation a quarter mile across US 1 on Long Point Key.

The project’s overall numbers, an average of 15,000 southbound birds a year, were lower than Brashear and Stoddard’s, but for the most part that was due to Lott’s decision to stop counting Turkey Vultures, which milled about in large kettles near the count site, disinclined to move clearly southbound or northbound. (The counters became sure that they were seeing the same vultures over and over again after an albino vulture joined the flock.)

The data collected have shown a number of things. One is that the significant pulse of migratory raptors usually begins in mid-September and trails off by the first week of November. Another is that, at most, only 21 percent of the birds reversed course when they found themselves confronted with a significant water crossing at the end of the Keys. Christmas Bird Count data and birder observations make it clear that the vast bulk of those non-retreating birds are not overwintering in the Lower Keys. Most in fact are undertaking the water crossing, heading either toward Cuba, which is 90 miles away if they fly in a straight line, or possibly the Yucatan, a distance of at least 400 miles.

Eight species make up 98 percent of the roughly 12,000 annual net of birds, meaning birds that go south and stay south — approximately 2,800 Sharp-shinned Hawks, 2,700 American Kestrels, 1,800 Peregrine Falcons, 1,700 Broad-winged Hawks, 1,100 Ospreys, 540 Northern Harriers, 475 Merlins, and 460 Cooper’s Hawks. The other two percent consists of a smattering of Swallow-tailed Kites, Short-tailed Hawks, Mississippi Kites, Red-tailed Hawks, Red-shouldered Hawks, Bald Eagles, Swainson’s Hawks, one Snail Kite (in 2007), and one Zone-tailed Hawk (the day after Hurricane Wilma passed in 2005).

The numbers of Swallow-tailed Kites are not representative. Most quit Florida for South America half a season earlier than other raptor migrants. The Swainson’s Hawk numbers might be slightly low also, as small kettles tend to show up in late November, after the counters have given up their posts.

The percentage of hawks that decide not to cross varies, of course, from species to species. Broad-winged Hawks are most likely to reverse course; fewer than half the birds counted are known to continue south. American Kestrels and Northern Harriers tie for the most undaunted birds, percentage-wise: 96 percent continue on. (On average, more than five times as many American Kestrels pass through as Northern Harriers, though.)

Migration mysteries

Much of the variation in a species’ willingness to cross can be traced to physiological characteristics, such as wing loading and musculature, but the numbers indicate that more birds, and more types of birds, undertake the venture than previously thought.

The Peregrine Falcon numbers are especially significant because they are the highest totals of any North American count site and are only intermittently surpassed by the count in Kekoldi, Costa Rica. They represent an estimated 10 to 12 percent of the combined total population of the species in North America, including Greenland, and provide an important way to monitor the health of a population of birds nearly extirpated from the continent in the 1970s.

The Broad-winged Hawk numbers are interesting, also. While close to two million Broad-wings are counted in Veracruz annually, their passage dates are earlier than those of the 3,900 birds counted entering the Keys every year. The independence of these birds may indicate a sub-population of the species.

The real mysteries, though, are the accipiters. The common belief is that Cooper’s and Sharp-shinned Hawks, because of their short wings and sprinter’s musculature, will not make long water crossings. Observations at places like Cape May, where the birds confront a 13-mile bay crossing, and Whitefish Point, Ontario, where they face a 11- to 16-mile lake crossing, indicate that Sharp-shins will redirect their course until they find a less risky route if conditions are not completely favorable. They are known to completely bypass the 50- to 75-mile crossing from Fire Island to the New Jersey coast that Peregrine Falcons regularly make.

Cooper’s Hawks are thought to avoid any crossing over 15 miles. Yet the data show that approximately 460 Cooper’s Hawks and 2,800 Sharp-shinned Hawks leave the mainline Keys and head out over the Florida Straits every fall.

How many birds complete the crossing is unknown. There are those who believe that, like the apocryphal lemmings, almost all of the accipiters perish midway. In Field Guide to the Birds of Cuba (Comstock, 2000), the most recent guide to the island nation’s avifauna, authors Orlando Garrido and Arturo Kirkconnell list Sharp-shins as “very rare” and make no mention at all of the Cooper’s Hawk.

Casey Lott believes that a large number of the birds are probably making it across and then moving on to other areas in the Caribbean or Central and South America. The lack of sightings, he points out, could be due to the large number of potential landing points along hundreds of miles of open coastline, the lack of high-quality optics, the dearth of birders, and the absence of migratory-bird censuses. The country’s long-term political estrangement from the U.S. could also be a factor.

It’s nice to think the birds could be moving unnoticed through Cuba, the same way they moved unnoticed through the Florida Keys for so long.

Mark Hedden is a writer and birding guide in Key West. He blogs occasionally at www.boneisland.com. 

  Originally Published October 1, 2008