When crossing the Mediterranean Sea and the Sahara Desert, great reed warblers that otherwise conduct their migratory flights strictly at night continue flying by day, but climb to previously unknown daytime cruising altitudes - from roughly 2,000 meters to well above 5,000 meters above the Earth. The findings about the heights these birds reach during daytime flights over geographic barriers reveal a previously unrecognized behavior in migratory birds and might help to explain the evolution of nocturnal migration. Each year billions of migrating songbirds traverse vast, intercontinental distances during their annual migratory sojourns. Most species preferentially fly at night and spend their days on land to rest and refuel before setting off again on the next leg of their nightly journey. However, the migratory tracks of some species involve traversing great distances over inhospitable terrain, like deserts and oceans, where landing options are few. While previous studies have shown that these birds will occasionally prolong their nighttime flights into daytime or even shift into a nonstop flight strategy when crossing formidable barriers, little is known about how these birds manage these extended flights. To better understand the flight behavior of migratory songbirds while crossing barriers, Sissel Sjöberg and colleagues equipped great reed warblers with custom-made, multisensory data loggers and tracked their annual journey between breeding sites in Europe and wintering areas in tropical Africa - a trip that requires crossing both the Mediterranean Sea and the Sahara Desert. Sjöberg et al. found that while crossing boundaries, the warblers extended their flights into the daytime. Unexpectedly, however, the authors discovered that the birds sharply gained altitude at dawn - more than doubling their nocturnal cruising altitudes, reaching upwards of 6,200 meters (roughly 20,000 feet) above sea level. At dusk, the birds rapidly descended to their mean nighttime altitudes. While Sjöberg et al. note that the explanations for the marked increase in altitude between night and day flights are not clear, they suggest that could be related to daily variation in ambient light, thermal regulation, winds, predation, vision range and exposure to solar radiation. This finding, say the authors, may help to explain why the overwhelming majority of nonsoaring long-distance migrant birds generally conduct their migratory flights during the night.
For reporters interested in trends, a 2016 Science study found that frigatebirds, which can stay aloft for months at a time, achieve these long flights by capitalizing on atmospheric conditions; by doing so, they fly over hundreds of miles a day using very little energy.