July 04, 2016
(Image: Henri Weimerskirch/CEBC CNRS)
If there were an endurance record for bird species, the little-known frigatebird would win.
A new study shows that frigatebirds routinely fly for months at a time without landing, skimming the sea surface only occasionally to catch floating prey. Previously, scientists thought these birds and another bird species, known as swift birds, only remained aloft for many days at a time.
The study was published Thursday in the journal Science.
Frigatebirds accomplish such a feat by serving as nature's most intuitive weather forecasters (sorry, Weather Channel). The birds, as if preprogrammed with a particular flight plan, make giant loops around the tropical Indian Ocean, skirting the edges of calm areas known to ancient mariners as the doldrums.
Frigatebird paths (orange) and wind currents in the Indian Ocean. (Image: CEBC CNRS/Earth.nullschool.net)
They hopscotch through upward air currents in cumulus clouds in order to reach higher altitudes, from which they can glide a longer distance, the study found.
"From our previous studies on frigatebirds, it was known that they can stay in flight continuously for several days, but not for several months, which implies quite a lot of questions about energetics, availability of convection, or sleep for example," said lead author Henri Weimerskirch, a researcher at CNRS in Villiers, France, in an email.
To stay aloft for so long, the study found, these birds use some of the same techniques that glider pilots do. They take advantage of upward-moving air underneath tropical cumulus clouds — the clouds that often look like popping popcorn kernels in the sky.
If you've ever flown through a cumulus cloud and felt a push down into your seat, followed by a roller-coaster like drop in the aircraft, then you too have felt the effects of an updraft followed by a downdraft.
By outfitting 24 adult birds and 25 juveniles with a variety of instruments, from GPS trackers to loggers measuring the beating of the birds' wings as well as heart rate, the researchers were able to gain insight into the phases of flight that are most and least energy intensive.
What they found is that they have a specialized ability to soar underneath, and oftentimes within, tropical cumulus clouds that contain strong updrafts.
In fact, the study found that some frigatebirds reached altitudes as high as 13,500 feet, where the air is below freezing and the amount of oxygen is quite low. This suggests the birds are adapted to extreme environments.
"Frigatebirds are generally flying with the wind around the oceans and presumably use this to reduce the overall journey costs but also to maximize coverage of the ocean surface in search of feeding opportunities," said Charles Bishop, a scientist at the school of biological sciences at Bangor University in Gwynedd, Wales.
Tropical cumulus clouds seen near Cuba in 2002. (Image: Roberto Machado LightRocket via Getty Images)
"They are looking for fish and squid that have been forced to the surface by predators such as tuna fish and dolphins and then they attack from above."
The authors describe the birds' technique for flying across entire oceans as "wind-drift-circling-soaring," since they ride low-level winds around the doldrums, drift for long distances with little energy exertion, and fly in circles to higher altitudes as they ride the updrafts in cumulus clouds.
Using such techniques, the study shows that these birds can travel for up to 260 miles in a single day for up to 48 straight days or more.
It's fortunate that the frigatebirds have such amazing abilities, because they are otherwise ill equipped for life as a seabird. For one thing, unlike all other marine birds their feathers are not waterproof, forcing them to go after prey that is on the surface of the ocean, rather than underneath like many other birds, such as cormorants. They also have rather small legs, which prevents them from landing on the sea surface to rest.
In addition, they have longer wings than many other birds, which would make it harder to generate the lift required for takeoff.
Since these birds are so highly dependent on weather patterns, as the climate warms in response to human-caused global warming and atmospheric circulation is altered, frigatebirds may have different flight paths in the future, the study notes.
By Henri Weimerskirch/CEBC CNRS
News date : 07/04/2016 Category : Animals/Pets