Seabirds Chase Tropical Storms, Use Them For Foraging: Study

A study published in Current Biology, titled "Oceanic Seabirds Chase Tropical Cyclones," reveals that the rare Desertas Petrels (Pterodroma deserta), seabirds native to the North Atlantic, exhibit unique foraging behaviors during hurricane season. Contrary to other pelagic seabirds, these petrels do not avoid intense tropical cyclones but instead exploit the dynamic conditions to their advantage, shedding new light on the impact of cyclones on open ocean marine life.

"Initial studies suggested that seabirds either circumnavigate cyclones or seek refuge in the calm eye of the storm. However, the Desertas Petrels we tracked did neither; instead, one-third of them followed the cyclone for days, covering thousands of kilometers,” stated Francesco Ventura, lead author and Postdoc Investigator in Biology at the Woods Hole Oceanographic Institution (WHOI). “When we saw the data, we nearly fell off our chairs. This is the first time we have observed this behavior.”

“It’s striking how well the birds know how to exploit the large-scale wind conditions over the North Atlantic for their travels,” said Caroline Ummenhofer, Associate Scientist in Physical Oceanography at WHOI. “When you overlay the petrels’ foraging trips on top of average winds, it’s a very close match.”

Seabirds' unique foraging behavior

The pigeon-sized seabirds nest on Bugio Island in Portugal off the western coast of North Africa. The island hosts the only known nesting colony of these pigeon-sized seabirds, with fewer than 200 pairs residing on a plateau surrounded by steep cliffs. During their six-month breeding season, they embark on extraordinary foraging journeys, often spending weeks at sea and flying round trips of up to 7,500 miles across the Atlantic. They belong to the genus Pterodroma, meaning "wings on the run."

Researchers correlated the birds' locations with intensifying storm conditions, including waves up to 8 meters high and wind speeds of 100 kilometers per hour. “We correlated the birds' locations with intensifying storm conditions, including waves up to 8 meters high and wind speeds of 100 kilometers per hour,” said Ventura.

“Upon encountering strong winds, the birds reduced ground speed, likely by spending less time in flight to avoid injury to their wings. In addition, the wakes of the storms provided predictably favorable wind conditions with higher tailwind support than alternative routes. Impressively, none of the birds we tracked were harmed by the storms and there was no incidence of nest desertion.”

Despite these conditions, none of the birds were harmed and there was no incidence of nest desertion. The petrels pursue small fish, squid, and crustaceans that typically dwell at depths of 600 to 3,000 feet.

Exploiting storm-induced ocean changes

Desertas Petrels follow hurricanes where prey accumulates closer to the surface. Cyclone wakes provide enhanced foraging conditions, with sharp drops in sea surface temperature and increases in surface chlorophyll, suggesting enhanced ocean mixing and productivity.

This increases prey abundance and accessibility for surface-feeding petrels. The interaction between tropical cyclones and the ocean is interesting due to intense vertical mixing in upper ocean layers caused by strong winds and breaking seas.

“One of the interesting aspects of the interaction of a tropical cyclone and the ocean is the intense vertical mixing in the upper ocean layers caused by very strong winds and huge breaking seas,” said Philip Richardson, Emeritus, Physical Oceanography at WHOI and a co-author on the paper. “The cyclonic winds can cause a divergence in the upper layer that moves cooler, deeper water toward the surface.”

Cyclone winds can cause a divergence in the upper layer, moving cooler, deeper water toward the surface. Desertas Petrels find mesopelagic prey from deep within the vertical column, providing an easy meal at the surface.

“The cyclones present a highly valuable foraging opportunity for Desertas Petrels because the storms churn up mesopelagic prey from deep within the vertical column, giving the seabirds an easy meal at the surface,” explained Ventura. “While storms are typically seen as destructive, particularly in coastal areas, our research reveals that functional perturbance driven by storms can create new opportunities. We’re advancing our understanding of how petrels navigate the open ocean to find food.”

The study by Ummenhofer offers a new perspective on hurricanes' impact on marine ecosystems, revealing how higher predators like Desertas Petrels adapt their foraging strategies in dynamic ocean environments and use cyclone-induced oceanographic changes to their advantage, providing valuable insights into the resilience and foraging strategies of pelagic seabirds.