Why Do Vultures Circle in the Sky? Thermals Explained

How thermals lift a vulture

On a sunny day, the ground does not warm evenly. Plowed fields, roads, rocky slopes and parking lots heat up faster than forests or lakes, and the air above them warms, expands and starts to rise in a column or bubble. That column of rising air is a thermal.

A thermal is usually only tens of meters to a few hundred meters wide. To stay inside it, a soaring bird has to turn in tight circles. As long as the air rises faster than the bird sinks while gliding, the bird climbs. When it reaches the top of the thermal, where the air stops rising, it glides off in a straight line, slowly losing height, until it finds the next thermal and starts circling again.

This pattern of circling up and gliding down lets vultures cover huge distances while spending very little energy. Flapping flight is expensive for a big bird, while soaring costs little more than sitting still.

Vultures also use other free lift. Wind blowing against a hillside, cliff or ridge is pushed upward, creating ridge lift. On windy days, turkey vultures often glide along ridges or coastlines for kilometers without circling at all.

Why vultures are built for soaring

Vultures have long, broad wings with a large surface area compared with their body weight. That design gives them a slow sinking rate, so even weak thermals can lift them. The wingtip feathers spread apart like fingers, which reduces drag and helps with control at slow speed.

The turkey vulture holds its wings in a shallow V and rocks from side to side, which helps it stay stable in bumpy air close to the ground. Larger relatives such as the California condor and the Andean condor rely on soaring even more. A tracking study of Andean condors in Patagonia, published in 2020, found that the birds flapped for only about 1% of their time in the air, and one bird flew for more than five hours without flapping. More on that is in do condors flap their wings?.

Because thermals need the sun to warm the ground, vultures have late mornings. They stay on their roosts at dawn, often with wings spread in the sun, and take off once the air starts to rise, usually from mid-morning. On cold, cloudy or rainy days, when thermals are weak, they may hardly fly at all.

Circling to search for food

Circling also has a hunting purpose. From high above, a vulture can scan a huge area of countryside for carcasses. Black vultures and condors find food mainly by sight, while turkey vultures add a sense of smell so good that they can detect dead animals hidden under trees; see can birds smell? for how that works.

Vultures also watch each other. Birds spread out across the sky, each within sight of a few others. When one vulture spots food and starts to drop, its neighbors notice and follow, and their neighbors follow them in turn. Scientists call this local enhancement. It explains how dozens of vultures can appear at a carcass within minutes, even though only one bird found it. Black vultures, in particular, often follow turkey vultures to food.

A vulture that has found something may also circle lower over the spot for a while before landing, checking for danger. That final circling is the one people sometimes notice over a field or roadside.

Do vultures circle over dying animals?

The idea that vultures circle over a dying animal, waiting for it to take its last breath, is a myth popularized by cartoons and Western films. Vultures have no way of knowing that an animal is about to die. Most of the time, a circling vulture is simply gaining height in a thermal, and many vultures may share the same thermal because good lift is scarce.

Vultures do sometimes gather near sick or injured animals, and black vultures occasionally attack weak newborn livestock. But the familiar sight of several vultures turning slowly overhead is almost always about air, not death.

Kettles, roosts and migration

A group of birds circling together in one thermal is called a kettle, because the swirling mass looks like water boiling in a pot. Kettles can be spectacular during migration. In autumn, turkey vultures from Canada and the United States travel south in large flocks, joining Swainson’s hawks and broad-winged hawks in narrow flyways through Mexico and Central America. Counters at hawk-watch sites in Veracruz, Mexico, have often recorded more than a million turkey vultures passing in a single season.

At night, vultures leave the sky and gather in communal roosts in tall trees, on cliffs or on towers, sometimes hundreds of birds together. Roosts may also help birds share information: a vulture that ate well the day before may be followed by others the next morning.

Which birds besides vultures circle?

Many large birds use thermals in the same way. Hawks, eagles, storks, pelicans and frigatebirds all soar in circles, and in spring and autumn they may share the same kettles as vultures. You can tell a turkey vulture apart by its two-toned wings held in a V and its wobbling glide, while hawks and eagles usually hold their wings flatter and steadier.