A new study has found that a rare and wide-ranging North Atlantic seabird \u2014 Desertas petrel (Pterodroma deserta) \u2014 demonstrates unique foraging behaviors that involve following tropical cyclones during hurricane season in order to exploit the storms\u2019 dynamic conditions.<\/p>\n
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.<\/p>\n
\u201cInitial 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,\u201d said Francesco Ventura, lead author of the study and a WHOI postdoctoral investigator in biology, in the press release. \u201cWhen we saw the data, we nearly fell off our chairs. This is the first time we have observed this behavior.\u201d<\/p>\n
Desertas petrels nest on Portugal\u2019s Bugio Island, off North Africa\u2019s western coast. The island has the world\u2019s only documented nesting colony of the species on the planet, with fewer than 200 pairs living on a plateau encompassed by steep cliffs.<\/p>\n
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.<\/p>\n
\u201cOur study is the first one documenting this \u2018wake-following\u2019 behaviour, so we don\u2019t really know whether other birds do the same. Previous studies, mostly focussing on coastal settings, almost exclusively investigated the avoidance strategies implemented by birds,\u201d Ventura told EcoWatch in an email. \u201cIt 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.\u201d<\/p>\n
During the seabirds\u2019 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.<\/p>\n
\u201cWe correlated the birds\u2019 locations with intensifying storm conditions, including waves up to 8 meters high and wind speeds of 100 kilometers per hour,\u201d Ventura said in the press release. \u201cUpon 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.\u201d<\/p>\n
So how long do Desertas petrels stay aloft for such long periods of time?<\/p>\n
\u201cDuring 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 \u2018tied\u2019 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 \u2018dynamic soaring,\u2019 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,\u201d Ventura told EcoWatch.<\/p>\n
At night it\u2019s a different story.<\/p>\n
\u201cNot 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\u2019t rely on predictably good foraging grounds along the continental shelf or shelf-break. Rather, their \u2018opportunistic\u2019 foraging strategy is to maximise the distance covered to increase the probability of encountering prey in the open ocean,\u201d Ventura said.<\/p>\n
The petrels \u2014 whose genus Pterodroma means \u201cwings on the run\u201d \u2014 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.<\/p>\n
\u201cDesertas 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\u2019 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,\u201d Ventura told EcoWatch.<\/p>\n
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.<\/p>\n
\u201cTropical 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,\u201d Ventura explained to EcoWatch. \u201cBy 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\u2019s passage.\u201d<\/p>\n
\u201cIn 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,\u201d Ventura added.<\/p>\n
The study, \u201cOceanic Seabirds Chase Tropical Cyclones,\u201d was published in the journal Current Biology.<\/p>\n
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.<\/p>\n
\u201cWe now have a fresh perspective on hurricanes\u2019 impact on marine ecosystems through the eyes of an apex predator,\u201d Ummenhofer said in the press release. \u201cThis study provides valuable insights into the resilience and foraging strategies of pelagic seabirds in the face of extreme weather events.\u201d<\/p>\n
Ventura said that the effects of climate change on the foraging opportunities for Desertas petrels are quite complex.<\/p>\n
\u201cVery 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\u2019s important to highlight that while we found a \u2018positive\u2019 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 \u2018wrecks\u2019 (mass mortality events),\u201d Ventura told EcoWatch.<\/p>\n
Desertas petrels are practiced at navigating the weather dynamics of tropical cyclones, but how predicted shifts in the storms\u2019 patterns might affect these remarkable seabirds is not yet known.<\/p>\n
\u201cDesertas 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\u2026 it\u2019s 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,\u201d Ventura explained.<\/p>\n