Hawks circle in groups mainly because they are all exploiting the same invisible resource: a thermal, which is a column of warm air rising from the sun-heated ground. When multiple hawks converge on the same thermal and spiral upward together, the swirling formation birders call a “kettle” is not really coordinated group flight in the way a flock of geese flying in formation is. It is dozens of individuals independently riding the same atmospheric elevator. But the story does not stop there, because hawks also gain real advantages from having other hawks nearby, from finding the next thermal faster to giving inexperienced juveniles a flight school they never signed up for.
What a Thermal Actually Is
Thermals form when the sun heats the ground unevenly. A plowed field, a parking lot, or a rocky hillside absorbs more heat than the shaded forest next to it, and the air above the hot spot warms, expands, and rises. That rising bubble or column of air can move upward fast enough to carry a soaring bird with it, eliminating the need for energy-expensive flapping. Research on thermal soaring describes thermals as “localized, warmer regions in the atmosphere moving upward with a speed exceeding the descent rate of birds and planes.”1IOP Publishing (Bioinspiration & Biomimetics). Thermal soaring flight of birds and unmanned aerial vehicles In other words, as long as the air goes up faster than the bird sinks, the bird gains altitude for free.
Hawks circle because thermals are roughly cylindrical. To stay inside that rising column, a bird has to turn continuously, tracing tight spirals. A straight-line path would carry it out of the thermal in seconds. The circular flight path is geometry, not social behavior. When a hawk reaches the top of a thermal, it peels off and glides in a straight line toward its destination, slowly losing altitude, until it hits the next thermal and starts circling again. This alternating pattern of spiraling up and gliding forward is the basic locomotion strategy for almost all large raptors during migration and daily travel.
Why So Many Hawks End Up in the Same Thermal
Thermals are not evenly distributed across the landscape. They pop up over specific terrain features and shift as the sun moves, which means a good thermal is a scarce and valuable find. When one hawk locates a strong thermal and begins circling upward, the rising bird becomes a visible beacon to every other hawk in the area. A raptor gliding along scanning the horizon can spot another bird spiraling upward from kilometers away and adjust its course to join. This is a phenomenon researchers call “local enhancement” or “social facilitation,” and it is one of the main reasons hawks end up circling together rather than alone.
The clustering is especially dramatic during migration, when thousands of hawks travel roughly the same route on the same schedule. Geographic bottlenecks like mountain ridges, coastlines, and narrow land bridges funnel migrating raptors into concentrated streams. At these sites, dozens or even hundreds of hawks can pile into a single strong thermal because there simply are not enough thermals in the immediate area for each bird to find its own. The result is the spectacular kettles that hawk-watchers gather to see at sites like Veracruz, Mexico, or Eilat, Israel.
The Energy Equation Behind Soaring
The reason hawks go to all this trouble is energy. Flapping flight for a large, heavy bird is enormously costly. A comparative study of migrating raptors found that eagles and buzzards used soaring and gliding flight for more than 95% of their observation time, reserving flapping for brief transitions or emergency maneuvers.2Ibis. Flight strategies of migrating raptors; a comparative study of interspecific variation in flight characteristics Only smaller species like sparrowhawks, harriers, and small falcons relied on flapping and gliding flight for a substantial share of their travel, ranging from about 9% to 33%.2Ibis. Flight strategies of migrating raptors; a comparative study of interspecific variation in flight characteristics
This makes intuitive sense. The heavier the bird, the more energy each wingbeat costs, and the greater the payoff of finding a way to fly without flapping. The same study found that in soaring-and-gliding flight, cross-country speed relative to the air increased with body mass, because heavier species glide faster and at shallower angles.2Ibis. Flight strategies of migrating raptors; a comparative study of interspecific variation in flight characteristics A large eagle or vulture, in other words, gets more forward distance per meter of altitude lost than a small falcon does. The physics rewards bigness, which is part of why the largest raptors are the most committed thermal soarers and the ones most often seen kettling in groups.
One interesting detail from the same research: climbing rate inside a thermal did not differ between species. The strength of the thermal itself determined how fast birds went up, not the birds’ size or wing shape.2Ibis. Flight strategies of migrating raptors; a comparative study of interspecific variation in flight characteristics This means a small hawk and a large eagle sharing the same thermal gain altitude at roughly the same rate. The advantage of being big shows up later, when they leave the thermal and glide, because heavier birds can convert that altitude into more forward distance.
Bigger Flocks, Less Flapping
Group circling is not just a passive convergence on the same resource. There is evidence that hawks flying in larger groups actually fly more efficiently than solo birds or birds in small groups. A study comparing broad-winged hawks migrating through Costa Rica and Pennsylvania found that birds in Costa Rica flew in larger flocks, and birds in those large flocks flapped less than those flying alone or in smaller groups.3BioOne Complete. SOARING AND GLIDING FLIGHT OF MIGRATING BROAD-WINGED HAWKS: BEHAVIOR IN THE NEARCTIC AND NEOTROPICS COMPARED The researchers concluded that birds migrating in large flocks do so more efficiently than those flying alone.
Why would a bigger flock reduce flapping? The most likely explanation loops back to social information. When you are flying alone and your thermal dies, you have to search for the next one, which often means flapping to maintain altitude while you scan. When you are flying in a group of 30 or 40, chances are good that another bird will find the next thermal before you lose too much height. You just follow. The collective search is faster and more reliable than an individual one, which translates into less emergency flapping between thermals. The Costa Rica flocks also benefited from stronger, more reliable thermals in tropical latitudes, so part of the difference was environment, but the flock-size effect on flapping frequency held even after accounting for that.
Not All Hawks Are Equal Flockers
If you have ever seen a kettle, odds are good it was mostly broad-winged hawks. A radar-tracking study of migrating raptors in central New York found that although most hawk species were seen in small flocks at some point, only broad-winged hawks could genuinely be called flocking migrants, with most traveling in groups of fewer than 40 individuals.4Canadian Journal of Zoology. Comparative flight behaviour of migrating hawks studied with tracking radar during autumn in central New York Other species like red-tailed hawks, sharp-shinned hawks, and Cooper’s hawks were primarily solo travelers that occasionally shared a thermal with other birds but did not maintain cohesive flocks between thermals.
This distinction matters for understanding what “circling in groups” really means. For broad-winged hawks, the group has some genuine social cohesion. Birds that rise together in a thermal tend to glide together toward the next one, roughly maintaining the flock. For most other raptor species, a kettle is more like a crowd at a bus stop: everyone happens to be using the same service at the same time, but they are not traveling together in any meaningful sense. Once the thermal tops out and the birds peel off to glide, the “group” dissolves as each bird heads in its own direction at its own speed.
Species differences in flocking tendency partly reflect differences in migration strategy. Broad-winged hawks are long-distance migrants that travel from eastern North America to Central and South America, covering thousands of kilometers almost entirely by soaring. Their route takes them through tropical latitudes where thermals are strong and predictable, and the energy savings of group thermal-finding compound over such long distances. Hawks with shorter migrations or more varied flight strategies have less to gain from sticking together.
Juveniles Learn by Following
One of the most compelling reasons hawks benefit from circling in groups involves experience. Young hawks making their first migration have never encountered the full range of atmospheric conditions they will need to navigate. Research on age-related differences in broad-winged hawks found that following experienced adults helps juveniles in two ways: it allows them to find thermals more quickly, and it helps them navigate to appropriate wintering areas.5The Wilson Bulletin. Follow Your Elders: Age-related Differences in the Migration Behavior of Broad-winged Hawks at Hawk Mountain Sanctuary, Pennsylvania
The performance gap between adults and juveniles shows up clearly in soaring ability. A study of Eurasian griffon vultures, which use the same thermal-soaring strategy as hawks, found that adults outperformed juveniles in their ability to adjust fine-scale movements under challenging conditions. Juveniles had lower climb rates when wind shear was tricky, especially on the downwind side of thermal columns, and were less efficient along their routes in both time and energy.6PubMed Central. Adult vultures outperform juveniles in challenging thermal soaring conditions Under easy conditions with strong, steady thermals, the difference was minimal. It was when conditions got difficult that experience mattered most.
This suggests that group circling functions as an informal apprenticeship. A juvenile flying alone in challenging wind conditions might waste energy searching for thermals, lose altitude unnecessarily, or get stuck on the wrong side of a thermal column. A juvenile following an experienced adult can essentially copy the adult’s decisions about when to circle, when to leave a thermal, and which direction to glide next. Over a migration of thousands of kilometers, those accumulated micro-decisions add up to a significant energy and survival advantage.
When Circling Serves Hunting, Not Travel
Migration is not the only context where you see hawks circling in groups. Hawks also soar over foraging areas, and here the group dynamics serve a different purpose. When one hawk spots prey from a thermal and dives, nearby hawks can see the dive and pay attention to the area, a form of information sharing that researchers sometimes call local enhancement in a foraging context.
Soaring turns out to be an excellent hunting platform. A study of wintering Swainson’s hawks preying on insects found that aerial hunting while soaring was the most successful method based on the number of prey captured per unit of energy expended. These hawks caught prey on about 65% of aerial attempts, compared with 42% of ground-based attempts.7Canadian Journal of Zoology. Hunting success of wintering Swainson’s hawks: environmental effects on timing and choice of foraging method Soaring gives a hawk altitude, a wide field of view, and the ability to stoop on prey with gravity-assisted speed, all without the energetic cost of hovering or powered searching flight.
When several hawks circle over the same area, each bird’s hunting dives provide information to the others about where prey is concentrated. This is not cooperative hunting in the way wolves or dolphins hunt together. Each hawk is acting purely in its own interest. But the presence of other hunters inadvertently improves everyone’s ability to locate productive patches. For species that feed on swarming insects or rodents in open fields, where prey density varies sharply over small distances, this kind of passive information sharing is valuable.
The Role of Geography and Weather
Thermals do not exist everywhere at all times, and the places and hours where they are strongest heavily shape when and where hawks circle in groups. Thermals typically begin forming mid-morning as the sun heats the ground, peak in early to mid-afternoon, and fade in the evening. Hawks match their daily schedule to this pattern, sitting idle in the morning, traveling or foraging during the thermal peak, and roosting early when thermals weaken. On overcast or rainy days, thermals can be weak or absent, and hawks either stay grounded or resort to more energetically expensive flapping flight.
Geography matters too. Mountain ridges produce a different kind of updraft called ridge lift or slope soaring, where wind hitting a mountain face is deflected upward. Hawks can soar along ridgelines without circling, flying in a straight line on the rising air. This is why hawk-watch sites on ridges like Hawk Mountain in Pennsylvania see streams of raptors cruising past in a line rather than kettling overhead. The dramatic kettles tend to form in flat or gently rolling terrain where thermals are the only available lift. Coastal areas can produce thermals when land heats faster than water, drawing hawks along shorelines.
The tropical versus temperate difference is relevant here as well. In tropical latitudes, stronger solar heating produces more powerful and more predictable thermals, which is part of why the broad-winged hawks studied in Costa Rica flew in larger flocks and flapped less than those in Pennsylvania.3BioOne Complete. SOARING AND GLIDING FLIGHT OF MIGRATING BROAD-WINGED HAWKS: BEHAVIOR IN THE NEARCTIC AND NEOTROPICS COMPARED Better thermals attract more birds and allow bigger, more efficient flocks. At higher latitudes, where thermals are weaker and less reliable, hawks spread out more and the spectacular kettles are rarer.
Misunderstandings About Hawk Kettles
The most common misconception is that circling hawks are “hunting together” in a coordinated way, like a pack of wolves closing in on prey. While hawks do sometimes hunt from thermals, as described above, the circling itself is almost always about locomotion, not predation. A kettle of hawks spiraling upward over a highway is not targeting anything on the ground. They are gaining altitude to travel.
Another frequent misunderstanding is that hawks circle because they have spotted a dead or dying animal. Vultures do this, and the confusion between hawks and vultures is understandable since both are large, dark soaring birds. Turkey vultures in particular are often mistaken for hawks, and when a group of turkey vultures spirals down toward carrion, it can look like a hawk hunting party. Actual hawks in a kettle are almost always spiraling upward, not downward. The direction of the spiral is a quick way to tell the difference between thermal-riding and food-targeting.
A subtler misconception is that the birds are deliberately flying in formation, adjusting their spacing and speed the way geese do in a V. Thermal kettles are far more chaotic than that. Birds enter and leave at different altitudes, circle at slightly different radii, and do not maintain fixed positions relative to each other. The apparent order of a kettle comes from the geometry of the thermal itself, not from bird-to-bird coordination. That said, birds do avoid collisions, and experienced soarers can position themselves in the strongest part of the thermal’s core, so there is some real-time adjustment happening. It is just less choreographed than it looks from the ground.
Hawks as a Model for Autonomous Drones
The efficiency of thermal soaring in groups has not been lost on engineers. Researchers working on autonomous unmanned aerial vehicles have studied hawk and vulture soaring behavior as a model for extending the flight endurance of glider-type drones. The core idea is that a fleet of solar-powered or unpowered gliders could stay aloft for extended surveillance or environmental monitoring missions by finding and exploiting thermals the way hawks do, including using the positions of other drones in the fleet as social cues about thermal locations. One recent framework proposed a behavioral management approach for deploying soaring-capable UAVs that mimics the way raptors share thermal information.8arXiv. SKYSURF: A Self-learning Framework for Persistent Surveillance using Cooperative Aerial Gliders
The engineering appeal is straightforward. A battery-powered drone that has to fly under its own power might stay aloft for 30 minutes to a few hours. A glider that can find and ride thermals, topping up its altitude repeatedly for free, could theoretically stay in the air for an entire day. Adding the social-information component, where one drone finding a thermal broadcasts its location to nearby drones, mirrors what hawks do visually and could multiply the endurance gains across a fleet. Hawks, in a sense, solved the persistent-surveillance problem millions of years before anyone built a drone, and their solution was remarkably simple: go where the warm air is, and watch what the other birds are doing.