It was a cool March day, on the backside of a high pressure system moving over the area from the west. I was standing on a high hill that rose just above the surrounding trees, a few miles east of the Illinois River. At the limit of vision, aided by binoculars, I picked out the mere speck of a bird against the broken cloud cover of the southwestern sky. By its quick wing beats alternating with gliding, I knew this bird, even at a great distance, to be a sharp-shinned hawk. When the bird was directly overhead, my neck and arms ached from the strain of holding the field glasses. But I kept the hawk in sight as long as possible, until it blinked out of view in the northern sky, again at the limit of vision.
Over the last few decades, hawk watching has gained in popularity, and this has helped provide insight into the complexities of bird migration and behavior. Yet as we observe the many ways that these raptors follow age-old routes and also adapt to unpredictable environmental factors such as weather, we gain an appreciation for how our world came to be as it is, and how it may inevitably change.
The Strategies of Flight
Weather conditions would seem to be the most pressing and immediate issue confronting any migrating bird. In midwestern North America, high barometric pressure systems as large as several states may move from west to east with winds blowing clockwise around a central area. As a high pressure system moves through an area, winds blow from a northerly direction, and then eventually switch to southerly with lower pressure as the system moves off to the east. Northerly winds, in general, favor fall migrant hawks moving toward the south. During spring, migrants move mostly on southerly winds. This I saw quite easily one April day along the Illinois River, when a strong southerly wind brought in hundreds of broad-winged hawks – as well as ominous-looking storm clouds. Usually, though, few hawks will move during storms, a fact I once verified by standing, perhaps foolishly, for several hours in the rain... observing no hawks.
A phenomenon extremely important to the soaring flight of migrating hawks is the formation of vertical air movements called "thermals." Thermals are formed when solar energy is absorbed by the land, which warms the adjacent surface layer of air. The warm air rises in a column and is replaced as fast as it rises by nearby cooler air. A soaring hawk can glide over the rising air, and gain altitude without expending much energy from wing flapping. Though rising air columns will eventually dissipate, at heights sometimes exceeding one half mile, a hawk may then glide down to the next thermal on its migrational pathway.
A hawk can soar because its body weight in relation to the area of its wings – a term referred to as "wing loading" – is low. A bird with heavy wing loading, such as a mallard duck, must constantly flap its wings or it will rapidly lose altitude. A turkey vulture, in contrast, has a very light wing loading, even for a soaring bird, and may seem unsteady while soaring, actually appearing to be buffeted about by the winds. During migration, hawks take advantage of their ability to soar, combined with weather conditions and local landscapes, to minimize energy use.
Migrating hawks probably proceed as a broad front of birds until they encounter an obstacle or guiding line such as a river or shoreline. At such places they may concentrate in large numbers as each bird responds to the landscape in a similar way, pre-conditioned toward such behavior by inherited traits. For example, as fall migrants in Wisconsin are moving toward the southeast on northwest winds, they encounter the western shore of Lake Michigan. Thermals do not form over water because of the high capacity of water to absorb heat. For this reason, many raptor species are reluctant to cross large bodies of open water. Instead of crossing Lake Michigan, they follow the "leading line" of the lake’s shore southward, and then most likely move again as a broad front when south of the lake. At such times, an observer on a hill along the western shore of the lake would have a good chance of counting thousands of hawks. Similar phenomenon probably occur all around the Great Lakes. At northeastern Minnesota in the fall, for instance, hawks soaring on thermals moving from the northwest encounter Lake Superior, and then travel toward the southwest, rounding the lake at Duluth.
The influence of the Great Lakes on hawk flight paths is similar during spring migration. Raptors moving north through Wisconsin and Michigan on a southwest wind may be concentrated toward the tip of the Keweenaw Peninsula, which extends nearly to the middle of Lake Superior. From the top of Brockway Mountain, near the tip of the peninsula, one may see the hawks passing overhead apparently on their way toward rounding the peninsula in order to head back southward to the mainland. They most likely then pass around the lake at its eastern or western tip. Although of longer duration, this strategy is evidently safer than attempting to cross the large expanse of the frigid lake, with its unpredictable and sometimes violent weather.
Hypothetical Hawk Migration in the Midwest: 1 = Hawk Ridge, Duluth Minnesota; 2 = Brockway Mountain, Michigan; 3 = Whitefish Point, Michigan; 4 = Cedar Grove, Wisconsin; 5 = Mt. Hoy, Blackwell Forest Preserve, Illinois; 6 = H.A. Gleason Nature Preserve, Illinois; 7 = Pere Marquette State Park, Illinois; Nelson Dewey State Park, Wisconsin.
On the other hand, although crossing large bodies of water may not be a preferred strategy for raptors, it certainly occurs to some degree for many species. Those that rely less on soaring flight, such as the falcons, are more likely to cross a large lake than a turkey vulture or rough-legged hawk. In any case, it is likely that many raptors of all kinds die every year from being blown out over the Great Lakes or from attempting a crossing under poor conditions.
Updrafts, or horizontally moving air deflected upward, are used by hawks in much the same way as thermals. Around the Great Lakes, updrafts may form adjacent to the shoreline as air temperatures over the land exceed those over or near the water and the wind direction is seaward; that is, warm air moves up and over the cooler air near water. Updrafts also occur when winds blowing over broad floodplains of rivers strike the bluffs that parallel the river valleys. Because the Mississippi River is oriented from north to south, it may present a useful leading line as well as good soaring conditions from bluff updrafts.
Raptors respond to landscape features and weather in a similar way year after year. This predictability helps explain why a distributional pattern occurs, showing why migrating hawks may be seen in one given area (river bluff) and not another (open water). When the time dimension is added from observations made over the course of a year, other patterns emerge.
[Note: This story originally appeared in slightly different form in the Spring 1995 issue of Illinois Audubon magazine, Number 252.]
