Birds circle for several different reasons, and the explanation depends on the species, the time of day, and what’s happening around them. Most often, circling birds are riding columns of rising warm air called thermals to gain altitude without flapping their wings. But circling can also signal foraging, migration, roosting, or even a defensive response to a predator.
Riding Thermals to Save Energy
The most common reason birds circle is to take advantage of thermals. As the sun heats the ground unevenly, certain patches of earth warm the air above them, creating invisible columns of rising air. Birds enter these columns and spiral upward inside them, gaining altitude with little to no flapping. Vultures, hawks, eagles, and other large soaring birds rely heavily on this technique. Vultures, for example, flap their wings less than 5% of the time they’re airborne. Soaring and gliding uses roughly two to three times less energy than flapping, saving these birds over 60% of the energy active flight would require. In fact, a soaring vulture burns about the same amount of energy as one sitting still on a perch.
The physics works like this: the warm air column pushes the bird upward at speeds around 2 meters per second (roughly 4.5 mph). By flying in tight circles, the bird stays inside the column long enough to climb hundreds or even thousands of feet. Once the thermal weakens at higher altitude where the air cools, the bird stops circling, extends its wings, and glides in a long downward slope toward the next thermal. This cycle of circling up and gliding forward lets large birds cover enormous distances with almost no muscular effort.
Migrating Hawks and the “Kettle” Effect
During spring and fall migration, you might see dozens or even hundreds of hawks spiraling together in a tight, swirling mass. This is called a kettle, named because the birds look like objects swirling in a boiling pot. Broad-winged hawks, red-tailed hawks, and other raptors migrate during the day specifically to use thermals. They gather over the same column of rising air, ride it to the top, then glide off in the direction they’re headed until they find the next one. Geographic features like ridgelines, dark pavement, and open fields create stronger or more predictable thermals, so migration routes often follow these landmarks.
This method lets birds travel remarkable distances while conserving energy. A hawk that would exhaust itself flapping continuously can instead hopscotch from thermal to thermal across entire continents, barely beating its wings along the way.
Vultures Searching for Food
When you see vultures circling, they’re often scanning for a meal. Turkey vultures have an unusually strong sense of smell and can detect the gases released by decaying animals from high in the air. Circling at altitude gives them a wide search area. Black vultures, which have a weaker sense of smell, rely more on their eyesight and on watching other vultures. When one vulture drops suddenly from a circling group, nearby vultures take that as a signal that food has been found and follow it down. So a group of circling vultures doesn’t necessarily mean there’s something dead directly below them. They may still be searching, using altitude as a vantage point.
Seabirds Tracking Underwater Predators
Over the ocean, circling frigatebirds and other seabirds are often watching the water surface for signs of fish. Frigatebirds can’t dive or swim, so they depend on subsurface predators like tuna and dolphins to drive small fish and squid to the surface. When a frigatebird circles, it’s climbing to get a better view of the sea below, scanning for the splashing and disturbance that signals a feeding frenzy underneath. These birds tend to concentrate near thermal fronts, boundaries where water masses of different temperatures meet, because those zones attract the small organisms that draw fish, which in turn attract the larger predators that push prey to the surface.
Swifts Circling Before Roosting
If you see a large swirling cloud of small birds funneling into a chimney or a hollow tree at dusk, you’re likely watching chimney swifts. During fall migration, swifts gather by the hundreds to thousands to spend nights together inside large masonry chimneys. Before entering, they create a dramatic swirling vortex above the roost site, circling tighter and tighter before dropping in. This communal behavior has a practical purpose: swifts are social birds that prefer to nest and roost near other swifts, and gathering in large numbers provides warmth and safety overnight. These displays have become local spectacles in many U.S. cities, drawing crowds of people who come to watch the nightly funnel.
Mobbing a Predator
Sometimes circling isn’t calm or strategic. It’s aggressive. When smaller birds discover a perched hawk, owl, or other predator, they often gather and begin swooping over it repeatedly while calling loudly. This behavior, called mobbing, involves birds flying in repeated loops and passes over the threat. Jays, crows, blackbirds, and many songbirds do this. The mobbing calls appear to be directed at the predator itself, and the strategy works: predators typically give up and move on rather than endure the harassment. If you see a group of smaller birds circling aggressively around a tree or a specific spot, there’s a good chance a raptor or an owl is sitting there.
Responding to Weather Changes
Birds can sense dropping barometric pressure, the kind of pressure change that precedes storms. Research on white-crowned sparrows found that declining pressure in the 12 to 24 hours before a snowstorm triggered increased food intake, suggesting the birds were preparing for bad weather. While this doesn’t cause circling directly, shifting air pressure changes the behavior of thermals and wind patterns, which can alter how and where soaring birds circle. On days with strong, well-formed thermals (typically warm, partly cloudy afternoons), you’ll see more circling birds. On overcast or rainy days, thermals weaken and soaring birds stay closer to the ground or perch altogether.
If you notice birds circling higher and more actively than usual on a warm afternoon, it likely means strong thermals are developing, which often correlates with building cumulus clouds and sometimes approaching thunderstorms later in the day.