Migratory Birds Fly Higher Over Deserts and Lower Over Seas -- Anatomy Explains Why

Billions of Birds, Two Impassable Barriers

Every autumn, more than a billion small birds leave Europe and head south toward sub-Saharan Africa. They cross two obstacles that offer no place to land, no food, and no shelter: the Sahara Desert and the open water of the Mediterranean Sea or Bay of Biscay. Both barriers demand sustained non-stop flight, but they present very different physical challenges. The desert is hot and dry, with rising air columns that can provide lift but also stress a bird's thermoregulation. The sea is cooler and humid, with surface winds that can be exploited near the water but variable conditions aloft.

How birds manage these two entirely different environments during what may be the most dangerous stages of their annual journey has been difficult to study, because tracking small birds in three dimensions over remote terrain requires miniaturized technology that has only recently become available. A consortium of researchers from Tour du Valat, CEFE/CNRS, the Museum national d'Histoire naturelle, and the Swiss Ornithological Institute used multi-sensor tracking devices to follow 17 species across both barriers, publishing their findings in iScience.

Altitude as a Thermoregulatory Tool Over the Sahara

The contrast between desert and sea flight was stark. Over the Sahara, birds typically flew at 2,500 to 4,000 meters above sea level during daytime, often extending their nocturnal flights into daylight hours. "These small birds fly almost exclusively at night, but they sometimes extend their night flights into the following day when crossing the desert. When the sun rises, we have found that they fly higher," said Jocelyn Champagnon, senior researcher at Tour du Valat and co-last author.

The interpretation is straightforward thermally. Air temperature drops approximately 6.5 degrees Celsius per 1,000 meters of altitude. A bird at 3,500 meters is exposed to air perhaps 20 degrees cooler than at sea level -- a meaningful physiological advantage during sustained exertion in a hot, sun-exposed environment.

What Anatomy Predicts

The study went further than documenting the altitude patterns: it asked which physical traits predicted which species flew how high. Three relationships emerged from the data.

Birds with larger wings flew higher over both barriers. Larger wing area generates more lift, which may make high-altitude, lower-density air more accessible. Darker-plumaged species flew higher during daytime desert crossings -- probably because dark feathers absorb more solar radiation, making altitude even more thermally necessary. Species with shorter wing bones flew higher during daytime flights; the researchers suggest this reflects a greater need for cool air dissipation because shorter-boned wings have less surface area for radiating heat. Conversely, species with longer wing bones have larger, more vascularized wing surfaces that likely allow more efficient heat release at lower altitudes.

These relationships demonstrate that migration altitude strategies are not arbitrary or purely behavioral -- they are constrained and predicted by anatomy.

The Unexpected Danger in Sea Crossings

Over marine barriers, birds descended dramatically, sometimes to within a few dozen meters of the water surface. The Northern Wheatear was extreme in this regard, with many individuals spending most of their Mediterranean crossing below 50 meters altitude. The researchers speculate this may exploit calmer winds near the surface or aerodynamic benefits of proximity to water.

The practical consequence is significant. "Conversely, when crossing the seas, they descend, which puts them at risk of colliding with future offshore wind developments," said Champagnon. Wind turbines in offshore wind farms typically extend from the surface to 150-200 meters or higher. Species flying at 30 to 50 meters during sea crossings are at direct collision risk from turbine towers and blades near water level. The study's funding -- from French programs specifically designed to assess offshore wind impacts on migratory birds in the Gulf of Lion and Bay of Biscay -- reflects the urgency of this concern.

Long-distance migratory birds are declining across almost all surveyed species. The added pressure of offshore wind infrastructure in migration corridors makes accurate altitude data during sea crossings a conservation priority.