In birdwatching, what is meant by the term "kettle" or "kettling"?

Birds of prey like hawks or carrion birds like vultures are often called kettle birds as they use thermals to keep themselves hovering in the air. The act of circling on the thermals resembles a pot boiling, so the term kettle was used to describe the birds. The term kettling refers to a group of birds who are riding the thermals together. Of course, riding the thermal allows the birds to save energy by not having to flap their wings.

https://en.wikipedia.org/wiki/Kettle_(birds)
https://www.birdsoutsidemywindow.org/2008/09/22/kettles/