Search party flock formations in migrating birds
This paper reports the discovery of a previously overlooked "search party" flock formation in migrating terns, characterized by sparse, orthogonal side-by-side arrangements that facilitate a unique fly-and-forage strategy combining migration with surface feeding.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Birds have long captivated human imagination, from the iconic V-shapes of geese cutting through the sky to the swirling, chaotic clouds of starlings at dusk. For scientists, these gatherings are more than just spectacles; they are complex puzzles involving energy conservation, predator avoidance, and the spread of information about food. We know that many birds fly in tight formations to save energy or in dense clusters to stay safe. However, a different kind of behavior has emerged from the open waters of the Baltic Sea, challenging our understanding of how birds move while searching for a meal. This new observation focuses on a strategy where birds fly side-by-side in a wide, sparse line, covering a vast area as they travel. It suggests that for some species, the journey and the hunt are not separate activities but a single, coordinated effort to sweep the ocean for food.
In May 2024, researchers conducted a survey from a ship anchored forty kilometers off the coast of Sweden, between the islands of Öland and Gotland. Their goal was to monitor bird movements as part of an environmental assessment for a potential wind farm. Using a combination of human observers with binoculars and high-resolution marine radar, they tracked groups of migrating terns. These are small seabirds that feed by skimming the surface of the water. The team was looking for standard migration patterns, but what they found was a distinct and unusual formation. Instead of flying in a tight group or a staggered line, the terns spread out horizontally, perpendicular to their direction of travel. They moved like a search party, with individual birds separated by distances of up to 300 meters, creating a line that could stretch nearly one kilometer wide.
The data collected over five days revealed thirty-four groups of these birds. While the weather was calm enough to track twenty-five of these groups with radar, the pattern was consistent across the sightings. The birds, identified mostly as common or arctic terns with a few black terns mixed in, flew at low altitudes, roughly five to ten meters above the sea surface. They maintained this wide, side-by-side formation for several minutes, covering distances of seven kilometers or more. The radar images showed the birds as a broad band of echoes, moving steadily northeast. In some instances, when the flock passed close to the ship or the buoys marking the observation zone, the formation would briefly tighten into a swarm before spreading out again into the wide search line. During these close passes, the birds also increased their speed, suggesting that the formation was a deliberate choice for searching rather than a reaction to immediate danger.
This behavior represents a specific type of foraging strategy known as network foraging, where individuals search independently but remain in visual contact to react to the success of their neighbors. In this case, the birds were not just migrating; they were actively hunting while they flew. By spreading out over such a wide area, a flock of ten birds could scan a much larger section of the ocean surface than a single bird or a tight cluster could. The researchers argue that this "fly-and-forage" approach allows surface-feeding birds to efficiently locate patches of fish. If a food source is large enough to be comparable to the distance between the birds, the entire group benefits from the discovery. The spacing of the birds, often around 100 meters in larger flocks, appears to be a calculated balance: wide enough to cover new ground, but close enough that if one bird finds food, the others can see and join in.
The study highlights that this formation is distinct from the dense flocks seen in other species or the energy-saving V-formations of waterfowl. It is a specialized method for birds that feed on the surface of the water, allowing them to combine the long-distance travel of migration with the immediate need to find food. While the researchers noted that rougher seas might make this strategy less effective, the calm conditions during the survey allowed them to clearly document this behavior. The findings suggest that what might look like a loose scattering of birds is actually a highly organized system, a living net cast across the water to ensure that no patch of food slips through unnoticed. This discovery adds a new layer to our understanding of bird migration, showing that for some species, the path to their destination is also a path to their dinner.
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