Kettling is also what birds do when they wheel in a circle on a thermal column of rising air. It’s like a free elevator for them. From Wikipedia:
A kettle is a term that birdwatchers use to describe a group of birds wheeling and circling in the air. The kettle may be composed of several different species at the same time. Nature photographer M. Timothy O'Keefe theorizes that the word derives from the appearance of birds circling tightly in a thermal updraft "like something boiling in a cauldron." Osprey-watcher David Gessner, however, cites a Pennsylvania lowland called the Kettle ("der Kessel" in Pennsylvania Dutch), near Hawk Mountain, as the source of the term.
In some species—e.g. the terns of Nantucket—kettling behavior is evidently a way of "staging" a flock in readiness for migration. Pre-migrational turkey vultures kettle by the hundreds in the thermals that rise over Vancouver Island before they venture across the Strait of Juan de Fuca toward Washington State. At Hawk Mountain, broadwinged hawks form kettles in September before flying south. Kettling apparently serves as a form of avian communication—an announcement of imminent departure—as well as a way of gaining altitude and conserving strength. Because of this they kettle a lot.
You see turkey vultures kettling all the time, circling on updrafts. Terns and gulls do this a lot. If you’re lucky you’ll see hawks, kites, or even sandhill cranes doing it when they’re in or prepping for migration. It’s an awesome sight. Google for
kettling hawks or
kettling birds to see what I mean.
At some level, probably that of the underlying Kessel, this seems somewhat related to the policing tactic.