Every autumn, billions of birds leave their breeding areas when the temperature drops and food gets scarce to spend the winter in more favorable climes, returning the following spring when warm weather brings food in abundance.
These epic journeys can cover thousands of kilometers, often crossing oceans and deserts where birds cannot stop to rest and eat.
Until now it was thought that migratory birds usually fly at altitudes of below 2,000 meters (1.24 miles), and only in rare extreme cases fly higher than 4,000 meters (2.49 miles). But now my colleagues and I at Lund University in Sweden have observed two different migratory birds — a songbird and a wader — regularly fly at altitudes of between 4,000 meters (2.49 miles) to 6,000 meters (3.73 miles) when they continue their night flights into daytime.
That means between day and night these migratory birds change flight altitude more commonly and dramatically than previously thought, flying quite high during the night and extremely high during daytime.
Technical developments have made new types of bird migration research possible. Until recently, most observations of flying altitudes in migratory birds were made by radar, which could only follow birds for a few minutes.
But technicians at Lund have developed a new type of miniaturized datalogger that attaches to the bird. These loggers make it possible for us to track the birds’ behavior, altitude, and location throughout their journeys.
We used this new type of datalogger to study the migratory behavior of the great reed warbler and the great snipe. Having worked in the field of bird migrations for a long time, we expected this new type of data would add tremendously to our understanding of how birds manage their migrations. But we were not quite prepared for the high-altitude flying behavior we found.
Two different species, same behavior
The tracking of great reed warblers is part of a 40-year long-term project on the species in lake Kvismaren, southern central Sweden, led by Dennis Hasselquist at Lund.
Great reed warblers are songbirds weighing about 30 grams (1.06 ounces). Like most other songbirds great reed warblers normally only fly during the night and spend the days resting or feeding on the ground, throughout their month-long migration from northern Europe to sub-Saharan Africa.
During crossings of the Mediterranean Sea and the Sahara Desert, the great reed warblers sometimes prolong their night flights either for a few hours into the following day or for the full day and next night, lasting up to 35 hours.
Great snipes, meanwhile, are waders weighing about 200 grams (seven ounces) and breed in the mountains of northern Sweden. An international team of researchers led by Åke Lindström at Lund has been tracking these birds for the last decade.
Studies have revealed that great snipes have developed a migratory strategy where most of the 6,000-kilometer (3,728 miles) journey to their wintering ranges in sub-Saharan Africa is performed in one long non-stop flight, lasting 60 to 90 hours.
During the long journeys, which include night and day-flying, we found that great reed warblers and great snipes both flew at around 2,000 meters (1.24 miles) throughout the night, before steeply climbing often more than 3,000 meters (1.86 miles) at dawn. Both species regularly flew at altitudes above 5,000 meters (3.11 miles) to 6,000 meters (3.73 miles), which is far above what we would have expected based on what we knew before about flying altitudes.
One of the great snipes stayed above 8,000 meters (4.97 miles) for more than five hours one day, rising to a maximum altitude of around 8,700 meters (5.41 miles). This is the highest altitude ever recorded for an identified migratory bird.
When the birds climb between 3,000 meters (1.86 miles) to 4,000 meters (2.49 miles), the surrounding temperature drops by about 20 degrees Celsius (68 degrees Fahrenheit). This could be the reason for their behavior. We think the birds may be seeking very cold temperatures to counteract the heat caused by sunlight.
The exertion of flapping their wings during flight produces a lot of heat, and when the heat of a blazing Sun is added to that, the birds must find a way to cool themselves. They stay cruising at these extremely high and cold altitudes until either landing during the day or steeply descending at dusk to fly at altitudes of 2,000 meters (1.24 miles). We found that the great snipes repeated this 24-hour cycle, swapping altitudes throughout several days of long flights.
At this point, we cannot say for sure if it is the heat from the Sun that makes lower altitudes unfavorable in the daytime. The birds may also fly higher to avoid predators found at these altitudes or for orientation purposes.
Yet, the pattern was very consistent. The birds we studied climbed very high every time they carried on flying into daytime, so it is clear that the behavior is important.