Why Are Thousands of Hawks Flying in Circles?

Aug 28

3 min read

Every fall, broad-winged hawks begin leaving the forests of New England and eastern North America. Some will travel thousands of miles to wintering grounds in Central and South America.

And for much of the journey, they barely need to flap their wings.

Their secret starts on the ground.

The sun heats patches of earth, warming the air just above it.

This movement of warm and cool air also helps create a familiar summer sea breeze. Land heats faster than water, warm air over the land rises, and cooler air moves in from over the water.

The hawk circles within the rising air, gaining thousands of feet of altitude while barely flapping its wings. Once the hawk is high enough, it leaves the thermal and glides south, gradually losing altitude as it travels. When it finds another thermal, it circles upward again.

Rise. Glide. Rise. Glide.

A journey of thousands of miles becomes a succession of rides on rising air.

And this isn't just an eastern phenomenon. Across western North America, Swainson's hawks use the same technique during an even longer migration. Some travel from breeding grounds in western North America all the way to the grasslands of Argentina.

Eagles, storks, cranes and pelicans use the same strategy, riding thermals upward before gliding toward their next patch of rising air.

But migrating hawks can turn the process into a spectacle.

When one bird finds a good thermal, others find it too. Dozens begin circling. Then hundreds may join them. At major migration points, thousands of hawks can be spiraling upward through the sky together.

This swirling mass of hawks has a wonderfully appropriate name: a kettle.

When they reach the top of the thermal, the kettle begins to come apart. Hawks stream out and glide toward the next column of rising air. There they gather and begin climbing all over again.

There is one catch to relying on rising air: it is much less dependable over large expanses of water.

Land heats quickly in the sun and produces the rising air the hawks need. Water heats much more slowly, so strong thermals are less dependable over the ocean. Without that lift, these soaring birds have to spend considerably more energy keeping themselves airborne.

So their need for rising air helps determine the route.

Broad-winged hawks moving south through eastern North America generally stay over land, funneling through Mexico and Central America rather than striking out across the Gulf of Mexico. Swainson's hawks arriving from western North America also converge on the journey south.

As North America narrows toward Central America, birds that began their journeys across a huge swath of the continent are squeezed into an increasingly narrow corridor.

By the time the hawks reach central Veracruz, Mexico, the geography has squeezed migrants from across North America into one of the world's great wildlife spectacles. About 4.5 million raptors pass through the corridor each fall, and on peak days more than half a million have been counted overhead. Individual kettles can contain tens of thousands of circling hawks.

Eventually, a broad-winged hawk that spent its summer in a New England forest may end up thousands of miles away in South America. A Swainson's hawk from western North America may continue much farther, eventually reaching Argentina.

Their routes span continents, helped along by something as ordinary as warm air rising.

The sun warms the earth. The air rises. And the hawks go south.