A new study has found that a rare and wide-ranging North Atlantic seabird — Desertas petrel (Pterodroma deserta) — demonstrates unique foraging behaviors that involve following tropical cyclones during hurricane season in order to exploit the storms’ dynamic conditions.
Unlike other seabirds who spend the majority of their lives flying above the open ocean, these petrels not only do not steer clear of intense tropical cyclones, but seek them out, a press release from Woods Hole Oceanographic Institution (WHOI) said.
“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,” said Francesco Ventura, lead author of the study and a WHOI postdoctoral investigator in biology, in the press release. “When we saw the data, we nearly fell off our chairs. This is the first time we have observed this behavior.”
Desertas petrels nest on Portugal’s Bugio Island, off North Africa’s western coast. The island has the world’s only documented nesting colony of the species on the planet, with fewer than 200 pairs living on a plateau encompassed by steep cliffs.
Ventura pointed out that, because this is the first study to examine birds who follow storms like the Desertas petrels do, it is conceivable that other species of pelagic birds could use the same strategy.
“Our study is the first one documenting this ‘wake-following’ behaviour, so we don’t really know whether other birds do the same. Previous studies, mostly focussing on coastal settings, almost exclusively investigated the avoidance strategies implemented by birds,” Ventura told EcoWatch in an email. “It is possible, I would say even likely, that other pelagic birds (such as Desertas petrels), which forage in the open ocean, thousands of kilometers away from land (where they are not at immediate risk of being blown inland) could do the same. In fact, it is possible that pelagic predators other than seabirds could do the same, e.g. sharks, tuna, marine mammals.”
During the seabirds’ six-month breeding season, Desertas petrels go on incredible foraging expeditions, frequently spending weeks crossing the Atlantic on roundtrip journeys as long as 7,500 miles, according to WHOI.
“We correlated the birds’ locations with intensifying storm conditions, including waves up to 8 meters high and wind speeds of 100 kilometers per hour,” Ventura said in the press release. “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.”
So how long do Desertas petrels stay aloft for such long periods of time?
“During the breeding season seabirds act as central place foragers, meaning that when they go out at sea in search for food they have to depart from and return to their nest. They are ‘tied’ to their central place. However, despite this constraint, some species carry out very long foraging movements, regularly covering thousands of kilometers before returning to the colony. Desertas petrels are one of the most incredible examples of this hypermobility. Adopting a flight behaviour known as ‘dynamic soaring,’ these birds extract energy from the wind to fly at low cost, without flapping their wings. During incubation, while their partner is sitting on the egg, a foraging Desertas petrel can cover basically the entire North Atlantic, traveling up to 12,000 km in 2-3 weeks skillfully exploiting the wind. They are not in flight 100% of the time: during their journeys, they can rest from time to time, sitting on the surface of the ocean,” Ventura told EcoWatch.
At night it’s a different story.
“Not at night though, when the petrels are most active, hunting for mesopelagic prey (species that live in the depths that come up to the surface at night during their diel vertical migration). Desertas petrels don’t rely on predictably good foraging grounds along the continental shelf or shelf-break. Rather, their ‘opportunistic’ foraging strategy is to maximise the distance covered to increase the probability of encountering prey in the open ocean,” Ventura said.
The petrels — whose genus Pterodroma means “wings on the run” — hunt for small fish, crustaceans and squid who normally dwell 600 to 3,000 feet below the surface, the press release said. Desertas petrels are not able to dive to such depths, so they wait until after dark when their prey come closer to the surface.
“Desertas petrels feed upon mesopelagic prey. Their diet is quite varied, comprising squid and several species of lanternfish and hatchetfish. The petrels also target injured/dead prey floating on the surface. Desertas petrels’ prey usually live thousands of kilometers below the surface, well beyond reach of the surface feeding petrels, but can become available for the petrels mostly at night when they migrate towards the surface,” Ventura told EcoWatch.
The study demonstrated that the wakes of tropical cyclones provide improved conditions for foraging, with much lower sea surface temperatures and large increases in surface chlorophyll, according to the press release. The changes point to increased ocean mixing and productivity, with a presumption of more plentiful abundance of prey and accessibility for petrels to feed on the surface.
“Tropical cyclones exert a very strong mechanical energy input due to hurricane-force winds, which can mix ocean waters from the surface and further down in the water column beneath that sharp temperature gradient,” Ventura explained to EcoWatch. “By mixing these waters, the hurricane stirs up some of the cooler waters from further down up into the surface layer (and inversely also mixes some of the warmer surface waters further down). The end result is a more well-mixed water column, with an overall cooler and more nutrient rich upper surface layer than before the storm’s passage.”
“In our study, we found that the strong winds and enhanced vertical mixing brought phytoplankton toward the surface. Intense mixing driven by tropical cyclones could also bring zooplankton and other weakly swimming species to the surface which, in turn, can attract fish and squid,” Ventura added.
The study, “Oceanic Seabirds Chase Tropical Cyclones,” was published in the journal Current Biology.
It has been established that cyclones drastically impact coastal and oceanic ecosystems, but their effects on pelagic marine species are not well understood, the press release said. This new research illuminates how predators such as Desertas petrels adapt their foraging tactics in the ocean environment and use changes induced by cyclones to their advantage.
“We now have a fresh perspective on hurricanes’ impact on marine ecosystems through the eyes of an apex predator,” Ummenhofer said in the press release. “This study provides valuable insights into the resilience and foraging strategies of pelagic seabirds in the face of extreme weather events.”
Ventura said that the effects of climate change on the foraging opportunities for Desertas petrels are quite complex.
“Very optimistically, one may think that the increased storminess due to climate change may improve the foraging efficiency of [these] pelagic petrels, but I fear that this would be quite an oversimplification. First, it’s important to highlight that while we found a ‘positive’ effect of tropical cyclones opening up foraging opportunities in the open ocean, they have well-known devastating impacts on coastal settings, including the destruction of breeding colonies and the so-called seabird ‘wrecks’ (mass mortality events),” Ventura told EcoWatch.
Desertas petrels are practiced at navigating the weather dynamics of tropical cyclones, but how predicted shifts in the storms’ patterns might affect these remarkable seabirds is not yet known.
“Desertas petrels, like other pelagic tube-nosed seabirds species (belonging to the order Procellariiformes) are supremely adapted to exploit energy from the wind to sustain their oceanic journeys. This reliance on the winds also makes them quite vulnerable to the forecasted changes in atmospheric circulation brought by climate change. Furthermore, we still know very little about the effects on the abundance and potential shifts in distribution of prey (e.g. linked to changes in sea temperatures), so… it’s very difficult to say. The best we can do is to continue with the long-term monitoring of the movement ecology and demography of these true gems of biological diversity. An improved understanding means a more robust basis upon which to build predictions of what will happen under climate change,” Ventura explained.
This article by Cristen Hemingway Jaynes was first published by EcoWatch on 1 August 2024. Lead Image: The seabird Desertas petrel exploits dynamic conditions during cyclones to find food. Kirk Zufelt / Woods Hole Oceanographic Institution.
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