Science

20 Days, 7,500 Miles: Desertas Petrels’ Mind-Boggling Foraging Trips

Using regional winds to their advantage, these rare seabirds fly as far and fast as they can to search out patches of food.

On Portugal’s uninhabited Bugio Island, off North Africa’s western coast, nests one of Europe’s rarest seabirds, the Desertas Petrel. Here, at the only known nesting colony in the world, fewer than 200 pairs nest on a plateau atop rugged cliffs. But it’s not just their rarity that makes these pigeon-size birds intriguing: During their six-month-long breeding season, parents undertake remarkably long foraging trips, sometimes spending weeks at sea traveling many thousands of miles across the Atlantic Ocean to find food.

Desertas Petrels belong to the genus Pterodroma, meaning wings on the run, and they chase after small fish, squid, and crustaceans that live 600 to 3,000 feet below the ocean surface. The petrels can’t dive that deep, so instead, they wait until nightfall when these ocean critters migrate vertically through the water column to shallower waters. But if surface waters are murky, and the temperature and oxygen levels aren’t just right, the marine prey may not surface on schedule. This introduces some unpredictability into the seabirds’ food-gathering routine, says Francesco Ventura, a doctoral candidate in quantitative biology at the University of Lisbon in Portugal. 

This unpredictablity made Ventura and his colleagues curious how Desertas Petrels managed such uncertainty in their food supply; how do they decide how far to travel in search of food and what routes to take?

“We knew from other species belonging to this genus that Pterodroma deserta would be excellent flyers,” Ventura says. But how excellent surprised even him. Turns out, Desertas Petrels undertake foraging roundtrips up to 7,500 miles long—one of the longest recorded for any animal during breeding—and they opportunistically feed along the way, according to Ventura’s study published in Proceedings of the Royal Society B in January. They do this skillfully, expending barely more energy than if they were sitting on the nest, by taking advantage of regional winds.

To uncover the extent of Desertas Petrels’ incredible flights, Ventura needed data on their movements. Previously his supervisor Paulo Catry, an ornithologist at the Marine and Environmental Sciences Centre based in Lisbon's ISPA – Instituto Universitário, and his team had chosen 20 birds nesting in burrows atop Bugio Island cliffs and taped GPS trackers weighing as little as six paper clips onto their tail feathers.

Back in the lab, Ventura and the research team watched the birds virtually zip in and out of Bugio, receiving location data every two hours for three years between the 2015 and 2017 breeding seasons“It became a usual routine to download the GPS data and see these fantastic trips to the other side of the Atlantic,” Ventura says.

The GPS data showed that Desertas pairs took turns. One parent stayed in the nest burrow to incubate their single egg, while the other spent time feeding at sea. Although these birds sometimes made short, 75-mile trips near the colony, 90 percent of the time they undertook food voyages that lasted 14 days and covered 5,000 miles on average—not just once, but at least four times in their July to December breeding season. One bird with particular stamina flew 7,500 miles in 20 days.

A single roundtrip for these petrels is almost equivalent to taking an airplane ride from Rio de Janeiro to New York. And as one parent makes this enormous 14 to 20-day journey, the other petrel on nest duty lives off of energy reserves it accumulated during its previous foraging trip.

Few birds travel such long distances to acquire food, particularly during breeding. The Wandering Albatross, for example, holds the record for a 33-day, 9,300-mile foraging trip. Sooty Shearwaters travel for 11 to 14 days and up to 7,800 miles in search of food. Short-tailed Shearwaters also undertake 17-day, 7,600-mile feeding trips.

But how do Desertas Petrels do it? Knowledge of regional winds is paramount, Ventura says. To see how the petrels used wind to power their flights, he matched real-time wind direction to their flight speeds during the birds' long journeys. The birds consistently used tail winds blowing at a 60-degree angle, which allowed them to soar with minimal flapping and maximize travel speed without wasting energy. In fact, it takes practically no energy at all, Ventura says. He also calculated that the birds took the fastest possible paths given the conditions, inferring that the petrels are capable of choosing the most efficient travel routes.

To him it suggested that the birds take advantage of winds to traverse favorable flyways, and feed opportunistically rather than rotating between select food hotspots. “Their strategy is to maximize distance because by covering more ground they increase their chances of finding food along their tracks,” he says.

Even after spending years studying these birds, the researchers remain impressed with the birds’ stamina and range. To think that a bird can cross almost the entire span of the Atlantic Ocean and come back to its nest during a single foraging trip fills us with wonder,” Catry says.

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