Seabirds may find food at sea by flying in a massive, kilometers-wide arc

Radar shows that groups of birds can form a giant, coordinated line over the ocean’s surface

Black noddies (Anous minutus) are one of the bird species thought to be involved in constructing “rake” formations over the ocean surface.

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Food is scattered thinly in the open ocean, and seabirds often need to search far and wide to find sustenance. Now, researchers think they’ve found a new cooperative strategy among the birds, one that may help them survive on the high seas. Multiple groups of flying seabirds can arrange themselves into massive line formations that “rake” over the ocean’s surface, possibly to more efficiently search for food. 

Far from land, the ocean can be unforgiving, but seabirds have a remarkable suite of adaptations for finding food in this blue desert. For example, seabirds can work together when foraging on a chance school of fish, boosting catch rates by synchronizing their diving. 

But the shape and scale of the cooperative “rake” formations — with multiple bird groups stretching kilometers across the sky — have never before been scientifically observed in seabirds, researchers report March 23 in the Journal of Avian Biology.“To our knowledge, no such flight pattern … had ever been observed and described,” says Camille Assali, an animal ecologist at the Marine Biodiversity Exploitation and Conservation research unit in Sète, France.

To track the birds’ movements, Assali and colleagues looked at a year’s worth of radar images from a tuna fishing boat hundreds of kilometers off of Africa’s coastline, near the Gulf of Guinea. Fishing boats use radar to find seabirds, which can hint at the whereabouts of roiling fish activity. 

Analyzing the radar images for movement patterns in groups of seabirds, the scientists found that multiple groups would fly in a parallel sweep as often as 19 times a day. The birds also seemed to be adjusting their speed and position relative to one another so that they formed an evenly spaced line over the top of the ocean. Some of these formations, with groups spaced half a kilometer apart and reaching four kilometers across, would last nearly 20 minutes before collapsing or merging together. 

Assali thinks the raking patterns likely result from behavioral coordination between groups of seabirds maintained by keeping track of each other by sight, rather than being caused by environmental factors such as headwinds or the angle of the sun. Neither of those factors seemed to have an effect on the movements.

The birds may be using rake formations to scan for signs of food, the researchers say. By distributing themselves in a line, the birds may avoid missing sections of ocean surface or searching the same area twice. But more research is needed to confirm any link to foraging, Assali says.

“We would need to measure the foraging success of raking versus nonraking birds and highlight potential costs and benefits of these different foraging strategies,” she says.

It’s also not yet known which bird species, or how many, are involved in these flight patterns. Assali says the best candidates are sooty terns (Onychoprion fuscatus) or noddies (Anous spp.). 

And more field observations, or even video footage taken with a portable camera from inside the formations themselves, could help scientists figure out why the birds form rake patterns in the first place, Assali says. 

Sasha Dall, a mathematical ecologist at the University of Exeter in Cornwall, England, says that if these seabird rakes are some kind of coordinated, prey-finding method, it’s “something that I haven’t really heard anybody document before.” Dall points out that research showing rake formations collapsing around a source of prey would help establish the link between the flight patterns and food finding. 

The uniform nature of open water may also lend itself to this search strategy, which could also help the seabirds spot other birds and animals hunting fish, Dall says.

Whatever the rakes are for, it’s clear they’ve already uncovered a trove of new questions.

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