Birds circling at sunset is a common sight, but this aerial display is a purposeful behavior rooted in their biology and survival strategies. These twilight flights serve several important functions, preparing birds for the night and enhancing their collective safety.
Gathering for the Night
A primary reason for birds flying in circles at sunset is communal roosting, where many species gather in large groups to spend the night. Birds like starlings, crows, and swallows are particularly known for this behavior. Starlings, for instance, form massive, swirling flocks known as murmurations, which can involve hundreds of thousands of individuals coalescing before settling into their roosts. These communal roosts offer enhanced safety and warmth, especially during colder months.
The circling allows incoming flocks to merge and consolidate, ensuring all members are present before descending. This aerial activity can also serve as a social hub, where birds reinforce bonds and exchange information about foraging areas. For example, American crows fly to communal roosts that can contain thousands of birds before reaching their final overnight location.
Defensive Strategies
Flying in large, circling formations at sunset also acts as a powerful defense mechanism against predators. The sheer number of birds and their synchronized movements can create a “confusion effect,” making it difficult for a predator to single out an individual target. This is especially effective against raptors like hawks or falcons, which may hunt during the twilight hours.
A large group provides more eyes to spot potential threats, and a coordinated alarm call can alert the entire flock to danger. The continuous, fluid motion of a murmuration can overwhelm and deter predators, reducing the chance of any single bird being caught.
Environmental and Navigational Factors
Birds may also use these circling flights for orientation and navigation before settling into their nocturnal roosts. The setting sun provides directional cues, with birds potentially using the patterns of polarized light in the sky to calibrate their internal compasses. This is particularly relevant for migratory species that might embark on their journeys shortly after dusk.
Some birds may utilize residual air currents, such as thermals, to gain or maintain altitude with minimal energy expenditure. Thermals, rising columns of warm air, allow birds to conserve energy before descending to their roosting spots. Visual landmarks also play a role, allowing birds to confirm their location and navigate to familiar roosting sites.