The Fall Migration of Raptors

By Emily Brodsky

Just about when the leaf peepers begin flocking to the roadways to observe Vermont’s spectacular autumn foliage, an equally-enthusiastic set of nature lovers is trekking up the peaks to watch a different seasonal event: the fall migration of raptors.  Also known as “birds of prey,” this majestic group includes the eagles, falcons, hawks, vultures, ospreys, and the less-familiar but no-less-impressive group called the harriers, of which North America has only one (the beautiful Northern Harrier).   Perched on a mountain outcropping, one can predictably see large numbers of these birds as they make their way to southern climes.

Whether you’re a veteran bird-watcher or a novice, raptor-watching (usually referred to as “hawk-watching,” even though other types of raptors are included) is a great way to spend an autumn afternoon.  One of its draws is that the birds are highly visible.  Unlike the diminutive songbirds, which hop around incessantly and hide in dense shrubs, raptors are large, steady, and during migration, exposed.  Also, because each group of raptors flies differently and has a distinctive shape, these birds are easy to tell apart.  The peregrine falcon, for example, has long, pointed wings, which it flaps continuously for its fast, powered flight.  In contrast, the bald eagle rarely flaps and its broad, sturdy wings make it look like a flying plank.  At the popular hawk-watching sites, you’re likely to find fellow observers on the summit to help you with identification; learn the shapes and flight patterns of the major groups and you’ll be a hawk-watching maven in no time.

 

So when and where is a Vermonter to begin?  The peak of fall raptor migration is from mid-September to early November; try going at different times of the season to see different species.   The most popular hawk-watching sites in Vermont are Mount Philo, 15 miles south of Burlington, and Putney Mountain in the southeast corner of the state.  Snake Mountain in Addison and Mount Ascutney in Windsor are also decent spots, as are Coon Mountain, just beyond the ferry terminal in Essex, New York, and Mount Tom in Massachusetts, straight down the Connecticut River from Brattleboro.

In addition to being a popular place for recreational hawk-watching, Putney Mountain is also an official migration monitoring site.  Because raptor migration is predictable and easy to watch, people have been counting migrating raptors and recording their numbers since 1934, when the first official count site was established at Hawk Mountain Sanctuary in Pennsylvania.  Since then, numerous similar counts have been established all over the globe, from the Panama Canal to the Strait of Gibraltar.  The long-term migration data collected at these sites allow scientists to monitor raptor populations; numbers vary greatly from year to year, but over long periods of time, scientists can identify trends.  The decline in juvenile Bald Eagles migrating past Hawk Mountain Sanctuary in the 1970s alerted Rachel Carson to the threat of DDT to these important predators, and she wrote about this trend in Silent Spring, the influential book which led to the ban of that harmful pesticide.  Visit the Putney Mountain Hawk Watch just for fun, or participate in the count to play a role in history.

You may be wondering why people hike up mountains to watch raptors migrate, instead of just observing from their driveways.  Do mountains simply afford better views of the sky?  The answer is that raptors concentrate along specific routes during the fall migration, and just as you’re more likely to find lots of cars on I-89 than on a dirt road in the sticks, you’re much more likely to see large numbers of raptors along these migration flyways.  Flyways tend to stick to mountain chains, because these topographic features allow for easy flight.

As you can probably imagine, migration is exhausting.  When we humans are exhausted, we can take a nap and recharge; to a raptor, exhaustion usually means death.  Some raptors, such as Broad-winged Hawks, fly as many as 4,500 miles in about nine weeks to reach their wintering grounds.  To make it that far, they must do whatever they can to save energy along the way.  Lucky for raptors, there are some great energy-saving tricks.

When winds blow against a barrier such as a mountain, they’re forced upwards.  During migration, raptors fly along the sides of mountain ridges to take advantage of this upward push of air, called an updraft.  Instead of flapping their wings to generate lift, raptors can simply spread their wings wide and ride the updrafts like a surfer rides a wave.  Updrafts can carry raptors hundreds of miles along a continuous mountain chain like the Appalachians, which conveniently runs from north to south.  Not only does this strategy save migrating raptors an enormous amount of energy; it also makes for a great show, since updrafts carry the birds right past the slopes.

Updrafts are helpful when the wind blows.  Early in the fall, however, when the sun is still high and the air is calm, raptors rely more heavily on another phenomenon of physics. You’ve probably seen hawks or vultures flying in circles, high in the sky with their wings outstretched.  These birds are using a trick called soaring flight.  As you know, the surface of the Earth is quite variable; some spots are covered with rocks, some with woodlands, and some with houses and streets.  When solar radiation hits these surfaces, they each heat up at a different rate, and thus, the air just above the ground heats up unevenly.  In spots where the ground is warm, the air rises, forming columns called thermal air currents (or thermals, for short).  Raptors find these thermals, and spiral upward without having to flap their wings.  When they get nice and high in one thermal, they exit and glide toward another (losing altitude but gaining distance), and they rise up again.  In this way, they can travel long distances without expending much energy.  Mountain slopes heat up faster than the valleys below them, which means they’re good places for thermals; thus, raptors stick to the mountains even on calm days.

Mountains aren’t the only places in which to spot large numbers of migrating raptors; these birds tend to follow shorelines as well.  Thermals don’t form above water bodies like they do over land, because water releases heat slowly and evenly.  Without thermals or updrafts, raptors must use flapping flight – the most costly kind of flight.  For migrating raptors, flapping across a large expanse of water is risky business: if they run out of energy, they drown.  Consequently, most raptors avoid flying over large water bodies, and when they reach one along a flyway, they hug the coast – or, if they must cross, they find the shortest crossing.  Short crossings and narrow strips of land between water bodies act as concentration points, or bottlenecks, funneling thousands of raptors over the land as they avoid the surrounding water.  Examples are the south-facing peninsula of Cape May, New Jersey, the narrow crossing from Europe to Africa across the Strait of Gibraltar, and the thin strip of coastal plain at Veracruz, Mexico.

Migration behavior varies among species.  Broad-winged hawks, for example, depart for their approximately 4,500 mile trek to northern South America in early September when the thermals are strong.  Aptly named, Broad-winged Hawks are built for soaring flight.  Although Broad-winged Hawks are solitary for most of the year, they flock during migration.  Scientists believe flocking helps the birds to find the best thermals, although it could serve other purposes as well, such as protection; even most raptors have to worry about predators.  Broad-winged Hawks are one of the main attractions at raptor watch sites, since it’s possible to see hundreds or even thousands of them soaring together.

Unlike Broad-winged Hawks, Cooper’s Hawks are mediocre long-distance flyers.  These birds have stubby wings and long, rudder-like tails; they’re built for maneuvering among the branches in their forested habitats.  Cooper’s Hawks don’t generally migrate very far, and some don’t migrate at all.  Those individuals that do migrate tend to do so later in the season than Broad-winged Hawks, departing in October and November, and dropping off along the way as they find suitable wintering grounds.  They rely heavily on updrafts to save energy during the trip, and are easy to spot on north-facing ridges.

You may ask: why do the birds go to all this trouble, anyway?  Or, better yet: if they don’t like the cold, why don’t they just stay in the south, where the weather is toasty-warm year-round?  A common misconception about migration is that it’s prompted by temperature change.  Since we like to follow the warmth of the sun in the wintertime and many of us head south to Florida beaches, we assume birds and other migratory animals share our preferences.  In most cases, however, migration relates to temperature only indirectly.  In actuality, migration is mostly about food.

As the northern days grow shorter and the temperatures drop, plants cease to produce fruits.  Annual plants reach the ends of their lives, while perennials drop their leaves and transfer their sugars into stems and roots for winter storage.  Many of the insects and mammals that feed upon these plants turn in for a months-long slumber, or stock their larders with seeds, nuts, and other high-energy morsels and settle into their winter dwellings.  Ice creeps over the surfaces of lakes and ponds, sealing in their inhabitants until the spring thaw.  Carnivorous birds suddenly find themselves with little to eat.  So, they follow the food.  And, because it coincides with warmer weather, the food just so happens to be in the south.

When migratory raptors reach their wintering grounds, they must compete with resident birds for food and roosting sites.  This works out okay in the winter, when the birds need only worry about themselves; once spring comes along, however, the birds must compete for nest sites, and food for their offspring as well as for themselves.  Making the grueling return journey is worthwhile, since the raptors will have their choice of nesting spots when they reach their mostly vacant northern homes.  They’ll also get there just in time for dinner; after the snow and ice melt, there will be fish, rodents, songbirds, and juicy insects around just about every corner.