Scientists have tracked the eye movements of a bird in flight for the first time, revealing that pigeons in the air lock their eyes in place rather than looking around. The behaviour may help them control their flight, but it could also leave them more vulnerable to predators.

If animals on the ground want to look at something, they move their head or eyes to fix their gaze on it, then use rapid and sometimes wide-ranging movements of the pupil, known as saccades, to give a stable view of the object relative to its surroundings. But no one really knows what happens when birds are flying.

To find out, at the California Institute of Technology and his colleagues designed a lightweight rig of mirrors and cameras that can be attached to the head of a common pigeon (Columba livia) as it flies, as well as a small backpack that houses a camera control board and battery.

They then trained six pigeons to fly between two perches about 20 metres apart indoors, and three to fly some 25 metres outdoors to return to a coop.

During test flights in both environments, the head-mounted eye-tracking system revealed that after take-off, the birds increased their pupil size and adopted a fixed and consistent eye position in their heads, essentially locking their eyes in place.

“Whenever they start flying, the eyes rotate forward on average,” says Ros.

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If their heads moved, their eyes moved in synchrony with them. The fixed eye position aligns with the primary horizontal axis of the birds’ vision and their vestibular system – the sensory network that controls balance and spatial orientation.

“Pigeons have been shown capable of moving their eyes independently and that they can be moved by a maximum amplitude of about 15 degrees,” says at the University of Birmingham, UK. “Therefore, to show that, during flight, eye movements are less than 1 degree does suggest that the birds are actively stabilising the position of their eyes when in flight.”

Why they are locking their eyes isn’t certain, says Ros. He thinks the alignment with the vestibular system suggests the behaviour may help pigeons distinguish their own motion from external motion – such as the movement of a tree’s branches, or a car or predator – to help them balance and navigate.

It’s also possible that reducing eye movements minimises the computational load on the brain. “The world during flight moves a lot faster than it does during non-flight,” he says.

Eye movements give pigeons a , but Ros says locking their eyes into a forward-facing position is likely to reduce this, leaving a larger blind spot behind them where they couldn’t see predators.

He is curious about what pigeon eyes would do in other situations, because all the tests were done when the birds were low to the ground. “It might be different if pigeons were flying higher up, where there aren’t lots of objects rushing past,” says Ros. He also wonders what would happen when pigeons fly in flocks. “Would they look at other pigeons? At predators? Or at something on the horizon?”

Martin thinks other birds might also stabilise their eye position during flight, including predators. When in pursuit of prey, , he says. “This presumably would require the peregrine to fix the position of their eyes rather than move them about.”

Birds today have dinosaur DNA. They have dinosaur blood in their veins. Birds evolved directly from Velociraptor-type animals and are the only true dinosaurs that still exist. Steve Brusatte is a palaeontologist at the University of Edinburgh, UK, and has been studying 150 million years’ worth of bird evolution, from surviving the asteroid that wiped out their contemporaries to today’s adaptation into almost every niche. Brusatte introduces elephant birds, terror birds, demon ducks and penguins the size of gorillas, and why “when gulls dive bomb you and try to steal your pasty or your chips”, says Brusatte, “you really can sense in the nastiness, the cunning, the agility, the feistiness. You can sense that inner Velociraptor.”

Read more: The evolving science of dinosaurs

Powerful fertiliser based on seabird droppings may have fuelled the rise of a Peruvian agricultural kingdom 900 years ago and helped drive its eventual takeover by the Incas.

Chemical analyses of ancient maize cobs from southern Peru show unusually high nitrogen isotope levels – substantial signs that the plants were fertilised with a mix of seabird excrement, feathers and carcasses known as guano. The findings provide the strongest evidence yet that indigenous Chincha farmers, fishers and merchants harvested this nutrient-rich fertiliser from nearby islands to enhance inland crop fields – and strengthen their socioeconomic position, says at the University of Sydney.

“Privileged access to a crucial resource is a pathway to power – which the Chincha Kingdom had in this case, and the Inca did not,” he says. “Social change may have arisen from a surprising source: bird poop. It’s a fascinating story.”

Between AD 1000 and 1400, the wealthy and densely populated Chincha Kingdom controlled one of Peru’s most productive coastal valleys before being incorporated into the Inca Empire in the 15th century.

The Chincha valley lies just 25 kilometres from the Chincha Islands – home to vast colonies of Peruvian pelicans (Pelecanus thagus), Peruvian boobies (Sula variegata) and guanay cormorants (Leucocarbo bougainvilliorum), along with penguins and gulls. These so-called Guano Islands , largely thanks to their exceptionally high nitrogen content.

The Incas’ use of guano is well documented in early colonial accounts, which describe strict state control over the islands and heavy penalties for harming the birds. But until now, scientists have lacked firm archaeological evidence that their Chincha predecessors were already exploiting the resource. Many historians have long argued that they were – and that access to seabird fertiliser fuelled the kingdom’s economic success, says Bongers. Seabird imagery carved into ceremonial objects and depicted on textiles, ceramics and architectural friezes further suggests the birds held special significance for the Chincha.

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Bongers had been collecting dozens of ancient maize cobs – “perhaps food for the dead” – from Chincha tombs and wondered if they could help solve the mystery.

He teamed up with at the Smithsonian Institution in Washington DC to analyse 35 maize cobs from 14 cemeteries in the Chincha valley, measuring their carbon and nitrogen isotope ratios. Bongers, Milton and their colleagues also analysed collagen from 11 ancient seabird bones from the region – including pelicans, boobies, cormorants, a gull and a penguin – to establish a local isotopic baseline for guano.

The ancient seabird bones showed elevated nitrogen-15 values typical of marine birds. Many of the maize cobs showed even more extreme nitrogen isotope ratios, a hallmark of seabird guano fertilisation.

The findings point to the Chinchas’ use of the island resource by at least 1250, says at Texas A&M University.

Guano may have supported the kingdom’s economic expansion and strengthened its bargaining power when it was later incorporated into the Inca empire – with broader implications for how marine fertilisers shaped social change across the Andes, the researchers say.

“It makes a lot of sense that ancient Peruvians used guano as fertiliser,” says at the University of Maine, who wasn’t involved in the study. “It was a significant expedition to get down there to the islands – but you do that for high-value things!”

The Chincha Island guano is particularly valuable, probably because of the limited rainfall, he says, which allows the nitrogen to stay intact, rather than getting leached out. “This Peruvian guano was the real stuff.”