Theories and Methods
Great raptor migration occurs in the Great Lakes spring and fall. Here we do our best to cover some high-level concepts of hawk migration and how they may improve your in-field experiences. While it's fun to keep these factors in mind when planning a day of birding, please keep in mind that there are many variables are hard to predict - and there is no better way to see birds than just getting out there to see what is happening!
Sources for weather information can be found here. Common/simplified migration routes in spring (left) and fall (right) are shown below.
Further Notes on Concentration
There is some misconception around what "ideal" migratory conditions are for raptors. Birders associated high counts at hawkwatches with these suitable conditions, yet it would be better to consider these ideal conditions for the birders, because they concentrate hawks along flightlines suitable for our observation. Hawks migrate on a variety of conditions, generally only stopping due to adverse precipitation or significant headwinds (unless you're a Peregrine).
Generally, birders look for offshore winds to push raptors close to shorelines. Two examples of how wind and geography can affect concentration are provided below (using buteos such as Broad-winged Hawk or Red-tailed Hawk) as example species.
In the east wind example, winds are onshore for most of Lake Erie, allowing raptor migration to "fan out" inland with little concentration over any particular location. One exception would be the Holiday Beach Hawk Watch in the extreme SW corner of Ontario, where winds are breifly offshore, forcing more birds to "round the corner" in greater concentrations than elsewhere. Once they cross the river, birds are again free to "fan out" as the east winds push them offshore again in Michigan.
In the southwest wind example, all of Lake Erie in Ontario is experiencing onshore winds, again allowing the flight to "fan out", including at holiday beach. In this scenario however, once birds cross the river, the wind direction allows for concentration along the lakeshore which could create a more concentrated flightline for those watching at Lake Erie Metropark.
Two interesting caveats (not shown) would include:
A moderate concentration could occur along the south shore of Lake St. Clair under these conditions.
Strong southwest winds may be prohibitive to buteo migration, ending the potential for a flight.
Forward Speed of Migration
The forward speed of migrating raptors, particularly those which kettle such as eagles and buteos, is affected by wind direction. Two examples are provided below for extreme southwestern Ontario - northwest winds and northeast winds. Both would be considered suitable conditions for fall migration, but each has advantages and disadvantages.
Kettling behaviour on northwest winds will push birds "backwards" and also closer to shore, so more time is spent moving inland or course-correcting. This could lead to lower daily counts of raptors (everything else being equal) as the total distance they are able to cover is lower than the northeast wind example, where birds are able to continue moving "forward" (southwest) either while kettling or gliding. Birds moving forwards "faster" could lead to higher total counts.
Counts aren't everything however! Raptors may spend less time during each individual attempt at kettling in the northwest than the northeast wind example, instead kettling more frequently reaching lower maximum heights. Kettling during northeast winds still allows for forward progress, encouraging birds to continue doing it and achieving significant heights. At face value, higher birds may be harder to identify - but also many birds during northeast may simply glide high overhead without flapping or circling with a tailwind before they are lost from sight - providing even less clues for identification (flapping or soaring) than you would get on other conditions.
Lake Breeze
Calm sunny weather may be the most ideal conditions for raptors to migrate in the Great Lakes, yet many hawkwatches will report low numbers. This is due to the movement occurring inland away from shorelines. Observers however can still luck into exceptional migrations if the stars (and the lake breeze) aling just right! Some graphics are provided below to demonstrate the phenomenon.
As solar heating warms the ground, air rises faster here than over the cooler waters of the Great Lakes. This can create an onshore breeze, terminating at a concentrated point of rising air (aka thermals). Due to the shape of the lakes, a visible boundary of rising air can form, and is often visible due to the sharp boundary between puffy clouds and clear skies. Positioning yourself under this boundary can reveal a reasonably concentrated flight line of raptors which are using this rising air for kettling and migration.
These conditions are variable and fickle, so recognizing the cloud conditions is important, but worth it when spectacular flights are encountered at unexpected locations. Keep your eyes peeled on the bases of the southernmost (fall) or northernmost (spring) puffy clouds which identify the best rising air / thermals.
Flight Lines
Raptors may follow similar flight paths during a given day or a set timeframe within a day of hawkwatching. They may be of one species, multi-species or more genus-specific. Recognizing these patterns and lines can improve your day in the field, by focusing your efforts and spotting more birds.
Predicting their locations are very challenging. Smaller, fast moving raptors will move lower and closer to shorelines early in the day or during strong winds. More water-friendly species such as Harriers, Bald Eagle or Osprey may more readily get blown out over the water. Heavier buteos and Golden Eagles may intentionally stay further inland where they can gain height and more readily move between thermals (even higher and more inland than Turkey Vulture migration). Being aware of this phenomenon, and assessing what is happening from day to day, can help you get the most out of each day in the field.
Case Studies:
Bald Eagles offshore, Golden Eagles inland along Lake Huron - Nov 16, 2019
Migratory Routes
One interesting variable with Great Lakes hawkwatching involves the variety of geographic features which can alter the presence or absence of migratory raptors depending on the species, their migration routes, and their tolerance to crossing water. Some examples include:
Bald Eagles, Peregrine Falcons, Osprey, Merlings, etc. crossing water at places like Point Pelee.
Rough-legged Hawks taking an atypical migration route (enhanced east-west movement) compared to our other raptors. This can cause them to appear in variable year-to-year numbers at most locations. (E.g., some years large numbers can pass north of the Great Lakes, whereas other years weather pushes birds south well into Ontario.
Broad-winged Hawk number fluctuations due to weather variability during their narrow migratory window.
Case Studies:
Rough-legged Hawk migration squeezing below Lake Superior, heading west in Michigan on Oct 18, 2023.
Significant year-to-year variability in Rough-legged Hawk numbers along Lake Huron in Ontario.
Bald Eagle counts of 51 east of Point Pelee and 9 west of Point Pelee on Oct 17, 2021.
Inclement Weather
Most local hawk watch locations are affected differently by the variable weather patterns of the Great Lakes. Some common conditions to keep in mind include:
Direction of frontal rains in relation to the movement of birds. (e.g., rain-defined fronts moving west will push westward moving raptors ahead of them. A similar front will dramatically end any eastward movements).
Birds may be more willing to move in snow (fall) than rain.
Fall rain or snow squalls may temporarily or continuously block raptor movements on a given day, resulting in wildly different flights on either side of the line.
Left image: a spring front moving west can bring hawks westward moving raptors both before and after the rain for Braddock Bay or Derby Hill, whereas observers counting hawks moving east at Beamer CA in Grimsby will likely have a significant lull.
Right image: cold air passing over the warm waters of Lake Huron can create significant lake effect rain or squalls. Some days, birds find breaks in the precipitation and continue moving into southwestern Ontario. Other days, the band is too intense and the flight stops (even though it may be sunny in nearby Chatham-Kent!).
Local Geographic Features
Ridgelines, bluff faces, escarpments, etc. can all provide lift under the right weather conditions. There are likely too many examples to note here, but some common examples include:
Peregrine Falcons riding updrafts along bluff faces of the Great Lakes in the fall, often in counter-intuitive winds like strong south or southwest.
Red-shouldered Hawks riding lake-breeze generated updrafts along the Niagara Escarpment in Grimsby in mid- to late-March.
Hawk Ridge in Minnesota.
Seasonality and Species-Specific Behaviours
These are important considerations! We've added some notes on this topic to the hawk watching identification page.
Notes:
Further Case Studies:
