(from the Fall 1998 Eastern Mass Hawk Watch Newsletter)

Paul Roberts and Tom McCullough have both devoted many hours to watching hawks and the weather, and thinking about how various factors affect the path that Broad-Winged Hawks take as they move through our region in the fall. Recently, these two gentleman put their heads together over beer and chicken wings on this most interesting subject. The following “debriefing” sets down some of their observations and theories developed over the years.

We hope it will encourage EMHW members to cover new locations in an effort to test these hypotheses and help us better understand the dynamics of our Broad-Wing flight.

How the Flight Develops

It is likely that most of the Broad-Wings we see in EMHW territory nest in Maine and the Canadian Maritimes. When the urge to migrate strikes, large numbers of birds begin moving in a southwesterly direction, with the curvature of the Atlantic coast, funneling birds towards Massachusetts. Given our regional topography, they probably begin forming sizable kettles for the first time somewhere east of the White Mountains.

Experts disagree on the extent to which wind speed and direction affect the flight path of migrating Broad-Wings. But Tom and Paul essentially share the belief that “our” Broad-Wings fly on a heading of 230 to 240 degrees regardless of daily conditions. If they encounter strong northwesterly winds (that is, a cold front), Paul and Tom both believe that the birds modify their flight path only slightly, and work harder to stay on course. This could explain why places like the Cape and Rhode Island never have big Broad-Wing days, no matter what the weather. It could also help explain why large Broad-Wing flights along our coast in the fall are comparatively rare and presumably the direct result of exceptionally strong wind drift.

Paul and Tom are convinced that once they get the itch to start, Broad-Wings will move every day in any wind, under almost any conditions short of heavy rain. If weather conditions create good thermal lift, the birds congregate in the massive numbers we’re all so eager to see. But their urge to migrate is strong and insistent, driving them to move whenever and however they can. In less than favorable conditions, that may often be at lower altitudes and in ones and twos, even if they don’t get very far.

Data from telemetry studies carried out at the Golden Gate Raptor Observatory suggests that at other species of hawks migrate at a fairly leisurely pace, stopping off here and there for a few days to rest and hunt, or wandering off to the east or west or even back north for a while before continuing on their southward migration. Broad-Wings appear to be more single-minded. Our experience of the 10-day peak Broad-Wing “window” is mirrored at other watch sites, including Corpus Christi, where radar tracking over several years has shown huge concentrations of Broad-Wings moving through every spring within the same three- or four-day period. The birds couldn’t accomplish this if they sat around and waited for ideal conditions before moving.

How the Birds Use Lift

In our region, Broad-Winged Hawks are the birds most dependent on thermal lift to migrate.

Contrary to popular belief, Broad-Wings can and do fly from 8 AM to 6 PM (EDT) under reasonable conditions. It is likely that Broad-Wings also feed where and when they can, at least on our northern end of their route. Hawkwatchers regularly see migrating Broad-Wings with full crops, sometimes even packing lunch, defecating, and snatching at dragonflies on the wing. (The occasional unwary immature makes an ill-advised grab for a Monarch butterfly, as well.) Tom McCullough has particularly noted an abundance of full crops on days after the birds have been grounded by rain.

Studies suggest that Broad-Wings can see one another at a distance of roughly 6 miles under average conditions. Thus, the formation of large kettles would tend to “drain” birds from a corridor about 12 miles wide. No doubt they benefit (as do human observers) from the presence of moisture in the air, which provides visible evidence of thermals in the form of clouds, as well as a visual backdrop.

By 11 AM to 12 PM “bird time” (noon to 1 PM EDT) on a good day, thermals may begin to reach their peak, lifting the birds high and allowing them to glide for long distances between kettles. The famous “noon lull,” in which the birds are flying so high that they’re literally out of sight, does not seem to apply to mountain sites like Wachusett and Watatic. But on those classic crisp, sunny fall days, the high-flying birds may be miserably hard to see against a dazzling blue, cloudless sky.

Thermal intensity does not reach its peak until well into the afternoon, after which it begins to drop off with the lengthening angle of the sun. (Obviously, significant weather changes during the course of the day can delay the build-up of thermal intensity or hasten its drop-off.)

As evening falls, the earth releases its radiant heat, forming gentle thermals all over the landscape. Observers still out there near dusk are frequently rewarded with a late push of birds gliding low and slow on this soft, twilight lift. It’s worth noting that some very large flights have been observed from Wachusett between 3:30 and 6:00 PM (EDT) in recent years.

Another curious phenomenon that hawkwatchers encountered repeatedly last fall is a situation in which strong, gusty northerly winds produce numerous small, ill-organized kettles that repeatedly break up and reform at relatively low altitudes, while simultaneously other streams of birds glide by high overhead.

What’s going on here is that those gusty winds break up the thermal columns shortly after they form, and many of the birds that try to ride them up lose their concentrated lift before it really gets going. In these conditions, most kettles are small, short-lived and disorderly — forming, breaking up, and re-forming a short distance away — as the hawks search for a consistent updraft.

Yet even in these conditions, some proportion of the birds, either more knowledgeable or simply luckier, manage to get on an elevator that lasts long enough to take them up where they want to be and sends them streaking high overhead, past their friends and relations struggling along beneath them.

Perhaps the ideal conditions from the hawks’ point of view are the combination of a steady, dry Northeast tailwind and a clear, cool, sunny day creating strong thermals all over the landscape. Here the wind literally pushes the tops of the thermals together, smearing them into a kind of “street” of rising air. Once on the “street,” the hawks can glide for many miles on the near-continuous lift, pushed along by a helpful tailwind going just their way.

There’s also lift available inside cumulus clouds, created as water vapor condenses into droplets. This lift can push birds right up through a cloud, and explains why soaring birds frequently don’t peel off into a glide as they enter a cloud. Watch for this.

Where Are They?

Although count data from the longest-established watch sites suggests the Broad-Wing population may be declining slightly, the astounding numbers that have come out of more recently established sites at Corpus Christi and Veracruz should give us a clearer sense of the total numbers in time. But Northeastern hawkwatch sites, including Wachusett and Watatic, have not had a 20,000-bird year in quite a while. Why?

The pessimists among us wonder whether changes to weather patterns caused by global warming or the effect of development on thermal production and lift factors may be shifting the migration corridors to send the birds wide of our traditional observation points. Certainly, last year’s anxiety throughout the Northeast about the dearth of immature birds observed was allayed by the huge numbers seen in Texas and Mexico, so they got there somehow.

But assuming the population is relatively stable and that the weather pessimists are mostly wrong, we know that at some point in mid-September, tens of thousands of Broad-Winged Hawks will move through our area on a known heading. So why is it that some years 20,000 birds are observed from Mt. Wachusett, and in other years only 2,000?

Undoubtedly, a big part of the answer is that the birds are simply moving on a path that does not bring them within viewing range of our most frequently manned watch sites. Consider this: The distance from Mt. Wachusett to Mt. Watatic is 14.5 miles, and the folks at the two sites can see each other through scopes. Yet studies have shown that observers cannot see (much less count) kettles of hawks more than 2 miles away with 8x optics. Even with scopes and under ideal conditions, it’s unlikely that birds more than 3 miles away are detectable. That leaves a corridor roughly 9 miles wide between Wachusett and Watatic where, theoretically, thousands of birds could motor on by out of range of even the scopes on the two peaks.

In reality, the updrafts created by the mountains themselves probably draw the majority of passing hawks close enough to one or the other of the two peaks to be seen and counted, but the example demonstrates why it’s important to have observers at as many sites as possible around the region in order to get a fix on the size and path of the fall flight.

When conditions all along the route contrive to put many of the birds on a flight path that’s wide of Wachusett and Watatic (either East or West), watchers at those two sites are going to end up with smaller numbers. However, observers at locations like the Fruitlands Overlook, the new Bolton Flats site, or High Ridge Wildlife Management Area in Westminster would likely be able to see good numbers of these “missing” birds. And the value of reports from these locations in helping to understand our regional hawk flight would be tremendous.

The point is that tens of thousands of Broad-Wings move through Massachusetts each year, but many may be flying over locations where nobody’s watching out for them. Tom and Paul strongly encourage all EMHW members to check out new sites, like any high terrain in or near Groton, MA; the Fruitlands Overlook; or the parking lot of the Wal-Mart in Lunenberg.

Even if you can’t make it out to the central part of the state, make every effort to hawkwatch wherever you are, especially during that peak “Broad-Wing window” between September 10th and 20th. A lot of birds will move through during that time, and if you’re not looking, you’ll miss them for sure. Dunback Meadows and Whipple Hill in Lexington, Round Hill in Sudbury, Prospect Hill in Waltham, Pinnacle Rock in Melrose, or the Page School in West Newbury might produce a good flight. The ability to compare count data from new locations in relation to data from Mt. Wachusett and other well-covered locations will be invaluable in developing our regional coverage picture

— Scott Cronenweth, Jane Stein

Most Broad-Wings from Main and the Maritime Provinces probably migrate through southern Maine and coastal NH. into the eastern half of Massachusetts. The westward curvature of the coastline coincides with their general direction, funneling birds into a relatively narrow corridor (c. 40 miles?) as they enter Massachusetts. The hawks move through this corridor on a “broad front,” but excellent weather conditions can further channel them into ephemeral “rivers in the sky.” On a number of occasions, 75 percent of the Broad-Winged Hawks seen at a site in one season have passed within 3-4 hours, including flights of as many as 5,000 hawks at West Newbury, 10,000 at Mt. Watatic and almost 20,000 at Wachusett. Such counts are by no means an annual event, but reflect the magnitude of the migration through our area.