Birds often fly in circles, a common aerial behavior that serves various purposes crucial for their survival, navigation, and social interactions. These include conserving energy during long journeys and effectively scanning for food. Understanding why birds engage in this circling flight reveals insights into their abilities and strategies in the natural world.
Harnessing Air Currents
Many large soaring birds, such as eagles, hawks, and vultures, frequently fly in circles to efficiently utilize atmospheric phenomena known as thermals and updrafts. Thermals are columns of warm air that rise from the ground as the sun heats the Earth’s surface. Birds detect these rising air currents and enter them, allowing them to gain altitude without expending much energy by flapping their wings. By circling within a thermal, a bird effectively rides this invisible elevator of warm air, spiraling upwards. This circular motion helps them stay within the core of the rising air column, where the lift is strongest.
Once a bird reaches a desired altitude, it can then glide long distances, slowly losing height, until it finds another thermal to ride. This technique, known as thermal soaring, allows birds to cover vast territories and migrate over long distances with minimal energy expenditure. Birds can also use updrafts, which are rising air currents created when wind hits obstacles like hills or cliffs, to gain lift.
Seeking Food and Prey
Circling is an effective strategy for many birds to locate food and prey, particularly for raptors like hawks, eagles, and falcons. These birds possess keen eyesight, allowing them to spot small movements or subtle contrasts from significant heights. By circling, a bird gains a wider field of vision, enabling it to systematically scan a broad area below for potential meals. This aerial vantage point provides an ideal position to assess the landscape and identify targets before initiating a swift descent or “stoop.”
For instance, a bald eagle flying at 1,000 feet can survey approximately three square miles for prey. The circling motion allows them to maintain focus on a specific area. Vultures, which are scavengers, also utilize this circling behavior to efficiently search for carcasses over large territories.
Social and Navigational Cues
Beyond energy conservation and hunting, birds circle for various social and navigational reasons. Before long migratory journeys, many species gather in large flocks and may circle together. This pre-migratory circling can serve as a signal for other birds to join the group, fostering cohesion and coordination for the upcoming flight. Such collective circling also makes it more challenging for predators to target individual birds, providing a form of defense.
Birds may also circle as a territorial display, signaling their presence to rivals, or as part of courtship rituals. For example, during the breeding season, male and female red-tailed hawks may soar in wide circles at high altitudes to reinforce their pair bond. Circling can also help birds orient themselves in unfamiliar territory, using landmarks or celestial cues to find their way. This behavior allows them to assess their surroundings and decide on the most appropriate direction for their continued flight.