Hawks in a kettle

Reader Nilou sent a video of what seems to be a group of hawks (I don’t know the species, nor even if they’re hawks), circling in the sky. The YouTube notes say, “So today after flying I saw this odd formation of over 20-30 hawks that seams like they were having a ceremonial dance in the sky! They were circling around one area for 3 mins til the leader took them all back into the mountains.”

Have a look.

Since the videomaker had no idea what was going on, I consulted our Official Website Evolutionary Ornithologist®, Bruce Lyon, for an explanation. Here’s what he told me, posted with permission:

First, the numbers. There are some famous hawk migration spots where raptors concentrate and sometimes the numbers can be amazing (tens of thousands, hundreds of thousands per day I believe). Some places in Central America can have very impressive numbers of species like broad-winged and Swainson’s hawks and people take tours just to the see the hawks. The photo below was taken in Veracruz:
Second, the wheeling in a circle has a name—kettle. This from wikipedia:
Ornithologist Donald Heintzelman has done more than anyone to popularize the term kettle, using the term at least as early as 1970 in his book Hawks of New Jersey to describe raptor flight, followed by uses in print over four decades. The related terms “caldron” and “boil” are also heard to describe the same sorts of raptor behavior.  Osprey-watcher David Gessner, however, claims a Pennsylvania lowland called the Kettle (“der Kessel” in Pennsylvania Dutch), near Hawk Mountain, is the source of the term.

Kettles are often seen with raptors, but other soaring birds like storks and white pelicans also do this. I have watched 100 white pelicans do this and as the flock wheels in unison, the light on the wings and bodies changes suddenly and dramatically land it can be mesmerizing. I have photos of this somewhere. It often happens in migration.

Third, why circle? I strongly suspect that these birds are all circling to stay in the same thermal (a rising mass of air, often quite localized). These birds use thermals to gain lift which makes long distance flight cheap. Soaring birds often circle like this, as do human glider pilots. However, when we see a single raptor do it we often do not take note; but when you see hundreds or thousands of birds doing it together, it makes an impression.

As a side note, humans are really bad at estimating larger numbers of anything and the video you shared suggests that there were 20-30 birds. I bet it was more like 50 or maybe even 100. I teach a lab on ideal foraging on coots and the students have to count the birds at feeding stations every minute. One year we used photos to check the numbers—about about 40 birds the students began to seriously underestimate the numbers.


  1. CAS
    Posted April 6, 2019 at 2:43 pm | Permalink

    Nice video, picture and explanation.

  2. ThyroidPlanet
    Posted April 6, 2019 at 2:46 pm | Permalink


  3. harrync
    Posted April 6, 2019 at 3:22 pm | Permalink

    I think I now understand the unusual sight I saw a few days ago; it was most likely a kettle. Well, maybe a mini-kettle, since there were only six raptor-type birds involved. They were flying a rather tight circle – maybe 50 feet or so across – and all at same altitude, and pretty evenly spaced around the circle. [When there are only six of you, it’s probably a lot easier to keep organized.] They slowly rose in altitude, and eventually headed off to the higher mountains to the west. I live in Hendersonville, NC – about 2,000 ft elevation, between hills about 3,000 ft. [Growing up in the California, I refuse to call 3,000 ft a mountain.] I suspect, though, that those 3,000 ft hills were enough to create the up-draft the birds were taking advantage of. So I think WEIT has solved one of my puzzlements.

  4. Roger
    Posted April 6, 2019 at 3:23 pm | Permalink

    Glad I read this. Had I seen something like this I probably would have assumed they were buzzards and thought nothing more.

  5. rickflick
    Posted April 6, 2019 at 4:47 pm | Permalink

    I’ve see this kind of flocking in hawks a couple of times, although the numbers were smaller that this film shows. I like the term Kettle. I’ll try to impress my birding friends with it.

  6. Joe Dickinson
    Posted April 6, 2019 at 5:38 pm | Permalink

    I remember my father, an avid birder, talking about Hawk Mountain but never had the chance to visit it.

  7. bPer
    Posted April 6, 2019 at 7:10 pm | Permalink

    I’m a former soaring pilot and I concur that this video shows raptors circling in a thermal, either to gain altitude or to rest before proceeding. In soaring, though, we call this collection of circling flyers a gaggle.

    A bit of explanation. In constant-speed flight, a glider is in a slow, steady descent as it flies forward. In order to stay in the air, the pilot must find a source of rising air to regain altitude. If the air is rising faster than the glider is descending, the net effect is a rise in altitude. A common source of rising air is a thermal. This is a column of warm, rising air, often topped by a cumulus (“cotton-ball”) cloud.

    In order to go a significant distance, a glider pilot thermals up to gain altitude, then glides straight through the still or sinking air in the desired direction of travel for a bit, then thermals again to regain altitude, and so on. Think of a saw-tooth profile.

    From the light and sound in this video, I suspect that this was filmed in an urban area near sunset. I suspect that the raptors found a thermal popping off a warm spot like an asphalt parking lot. If you look closely, you’ll notice that they aren’t flapping their wings. That means they’re gliding, and either they’re circling in the thermal to gain altitude before proceeding or just lolling around in the weak thermal to rest for a bit.

    I’ve actually thermalled with a red-tailed hawk a couple of times. One time, the hawk joined me in the thermal (as opposed to me joining it), and it flew like a human glider pilot, circling in the same direction as me and setting itself up at my altitude and directly opposite me in the circle. It hung around for maybe a handful of orbits, with both of us checking each other out, then it headed off on its way. Magical.


    • rickflick
      Posted April 6, 2019 at 7:53 pm | Permalink

      Consistent with human souring, which is often done where the pilot knows the lay of the land and knows where thermals are likely to be, I’ve heard accounts of birds of prey that know the lay of their territory and often fly at low altitude to specific spots, where they know there will be thermals, to navigate aloft and to the larger world beyond. When I heard about this I considered the possibility that the bird’s mental world is a 3 dimensional landscape of moving air columns.

    • Nicolaas Stempels
      Posted April 7, 2019 at 6:29 am | Permalink

      Yes, when gliding these raptors circling may indicate where there are some good thermals. One has to be careful though, what’s good for a buzzard may be insufficient for a much heavier and larger glider.

      • bPer
        Posted April 7, 2019 at 10:09 am | Permalink

        Indeed! Particularly down low, thermals aren’t really organized into a strong, neat circular airstream yet, and while birds might be able to exploit it, a glider typically cannot.

        Also, it’s well known in the soaring community that you shouldn’t treat circling birds as a guaranteed thermal. That seagull you join may be more interested in the garbage dump below than in thermalling. 🙂


  8. Dave137
    Posted April 6, 2019 at 7:10 pm | Permalink

    A few years back I went on a hike in Nova Scotia, and toward the end of the trail it opened upon a wide cliff.

    Up above were hundreds and hundreds, and hundreds, of blue-jays, which would swoop and dive and then collectively climb high all over again: behavior I’ve never seen before, at least in the eastern US.

    • rickflick
      Posted April 6, 2019 at 7:45 pm | Permalink

      That’s amazing. Blue jays are supposed to be noisy roust-abouts in small groups passing overhead in the trees, scolding every other creature in the forest. Your account does not compute!

      • Dave137
        Posted April 6, 2019 at 9:49 pm | Permalink

        rickflick: I share your confusion! I’ve casually searched for an accounting of that phenomenon but I haven’t found much. Maybe I’ll send some of the photos to Dr Coyne for consideration. It really was a remarkable sight.

        • rickflick
          Posted April 6, 2019 at 10:00 pm | Permalink

          Perhaps we can just accept the phenomenon and hope to see it again. 😎

  9. ladyatheist
    Posted April 6, 2019 at 7:26 pm | Permalink

    I had a murder of crows kettling overhead one day. I have no idea how that relates to raptor behavior, but I’ve never seen it before or since. They looked and sounded like crows, at least. It’s quite a sight.

  10. Lorna Salzman
    Posted April 7, 2019 at 1:47 pm | Permalink

    They look like Black Vultures to me. Short tail, tight circular soaring in large group.

    • rickflick
      Posted April 7, 2019 at 4:58 pm | Permalink

      I was thinking vultures too. It’s kind of hard to tell with such a long shot. The vulture neck is longer and thinner but I can’t make out enough detail to know. The turkey vulture and I think the black vulture have a significant dihedral angle to the wings, which does appear to be the case here.

  11. gravelinspector-Aidan
    Posted April 7, 2019 at 2:45 pm | Permalink

    As a side note, humans are really bad at estimating larger numbers of anything

    Politicians are not immune to this problem, though they mostly think they are.

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