The frigate bird, sometimes called the “pirate of the sea,” is a large seabird known for its striking appearance and massive wingspan, which can reach over seven feet across. These magnificent creatures spend almost their entire lives in the air above tropical oceans, but they possess a significant limitation: their feathers are not waterproof. Because of this, frigate birds cannot land on the water to rest or forage like most other seabirds, which makes continuous flight a necessary mode of survival.
Documenting Extreme Aerial Endurance
Scientific tracking has revealed that these birds can remain continuously airborne for up to two months, or approximately 56 straight days, over the open ocean. This extraordinary feat is not just limited to adults on long migrations, as juvenile frigate birds have been recorded flying almost non-stop for over a year after leaving the nest. These transoceanic journeys are a regular part of their life cycle, with foraging trips frequently lasting for weeks at a time. During these flights, the birds cover vast distances, often traveling more than 400 kilometers every single day. They manage to sustain this travel by minimizing the energy cost associated with powered flight, effectively turning their wings into specialized sails.
Physiological Secrets of Non-Stop Flight
The secret to this unmatched aerial endurance lies in the frigate bird’s exceptional physique, which is perfectly adapted for soaring and gliding. They possess the lowest wing loading of any bird, meaning their body weight is distributed over the largest possible wing area. Their skeleton is remarkably light, with hollow bones and a frame that weighs less than their total feather mass.
This highly efficient structure allows them to minimize the use of flapping, which is energetically expensive. Frigate birds sustain their long flights by relying on atmospheric lift, circling to gain height in columns of rising warm air known as thermals. They also expertly utilize the updrafts found underneath cumulus clouds, reaching altitudes of nearly 4,000 meters (about 2.5 miles).
By maximizing their gliding time, the birds only need to flap their wings for a tiny fraction of their overall flight time. This passive flight strategy allows them to conserve energy, enabling them to glide for hundreds of miles while flapping as infrequently as once every six minutes.
Sleeping While Airborne
Frigate birds have been confirmed to sleep while flying, a behavior made possible by unihemispheric slow-wave sleep (USWS). This allows one half of the brain to enter a state of rest while the other hemisphere remains awake to maintain aerodynamic control. They also engage in short bouts of bihemispheric sleep and rapid eye movement (REM) sleep. Even with these mid-air naps, the birds sleep for less than an hour per day during their long foraging flights.
The Role of Tracking Technology in Discovery
The incredible claims about the frigate bird’s flight endurance were confirmed through the use of highly specialized, miniaturized tracking technology. Researchers, notably a team led by Henri Weimerskirch at the French National Center for Scientific Research, attached tiny, solar-powered devices to the birds. These lightweight instruments, weighing only about ten grams, were equipped with a suite of sensors.
The devices included GPS loggers to pinpoint the bird’s location and altimeters to track their vertical movements. Crucially, they also incorporated accelerometers and heart rate monitors to record the bird’s activity level on a minute-by-minute basis. By monitoring these data points—especially wing-beat frequency, heart rate, and acceleration—scientists could definitively distinguish between flapping flight, passive gliding, and resting periods.
This detailed data collection provided the first tangible evidence of the frigate bird’s non-stop, two-month voyages. The tracking confirmed that the birds were actively using atmospheric conditions, such as riding the upward currents in clouds, to propel themselves across the ocean.