In the column “Since You Asked” in every issue of BirdWatching, Contributing Editor Julie Craves answers readers’ questions about birds and bird behavior. Here is a question from our September/October 2019 issue.
Q: What makes a group of hawks a “kettle?” — Clair Van Buren, Bloomington, Indiana
A: Hawks and other raptors migrate during the day. As the sun heats the ground, warm air rises from the earth. Certain geographic features, including natural topography or human-built areas, can vary the rate and location of heating, creating columns of warm, ascending air. Birds can enter these updrafts, and by flying or soaring in a circle within the column, they can be lifted high into the sky. As the birds reach a height where the column dissipates because it meets increasingly cooler air, they can simply set their wings and glide down into another thermal in the direction they are headed. Using this method, the birds can travel quite far while conserving energy, as it takes far less effort than constant flapping.
The sight of a group of hawks taking advantage of a thermal, all swirling and spiraling, is reminiscent of objects being stirred or boiling in a pot — hence the terms a “kettle of hawks” or “hawks kettling.”
Thermals do not form until the sun is sufficiently high in the sky, usually mid-morning, so observing kettles of hawks doesn’t require rising at the crack of dawn. Nor do thermals form over water. Notably, migrating hawks will lose altitude as they cross a lake or large river, so sites located near shorelines are great places to watch migrating hawks as they arrive onshore at lower altitudes before they’re able to “catch” a new thermal to ride.