Why some migratory birds fly higher over deserts and lower over seas
New tracking research reveals that small migratory birds use strikingly different flight strategies over the Sahara and major seas, shaped by wing design, heat management and even plumage colour
Tracking birds across the toughest barriers on Earth
Every year, billions of small migratory birds travel between Europe and Africa, negotiating some of the planet’s most formidable ecological barriers. Vast deserts offer no food or shade, while long sea crossings leave birds with nowhere to land if conditions deteriorate. Until recently, how small birds coped with these challenges was largely guesswork.
Using miniature multi-sensor tracking devices, researchers have now followed 17 species of small migratory birds as they crossed the Sahara Desert, the Mediterranean Sea and the Bay of Biscay. By analysing more than 350 individual barrier-crossing flights, the study reveals that birds do not use a single “best” strategy. Instead, flight altitude, timing and duration vary markedly between species, and these differences are closely linked to wing morphology and plumage colour.
High-altitude tactics over the Sahara
Crossing the Sahara proved to be the most demanding challenge. On average, birds flew at around 1,600 metres above sea level at night, but those that continued flying into daylight climbed much higher, often reaching 2,500–3,000 metres and occasionally exceeding 4,000 metres.
This daytime climb appears to be a response to heat stress. Solar radiation rapidly warms flying birds, and ascending into cooler air may help prevent overheating during prolonged flights. Most species that flew through the day showed this behaviour, although some, such as Northern Wheatear and Tawny Pipit, maintained lower altitudes.
Wing size matters more than wing shape
The study found that birds with larger wing area tended to fly higher over the desert, both at night and during the day. Larger wings generate more lift in thinner air, making high-altitude flight easier to sustain.
Surprisingly, wing aspect ratio - often linked to long-distance flight efficiency - did not strongly influence desert flight altitude. This suggests that the ability to gain lift at high elevations may be more important during barrier crossings than sheer aerodynamic efficiency.
Plumage colour and heat management
Feather colour also played a clear role. Darker-plumaged species flew higher during daytime desert crossings than paler species. Dark feathers absorb more solar radiation, increasing the risk of overheating, and climbing into cooler air may offset this thermal burden.
The structure of the wings mattered too. Species with relatively shorter wing bones tended to fly higher during daytime desert crossings, possibly compensating for reduced heat dissipation by seeking cooler conditions at altitude.
Flying low over the sea
In contrast, sea crossings followed a very different pattern. Flights over the Mediterranean and Bay of Biscay were generally much lower, averaging under 800 metres, with many birds flying just tens of metres above the water.
Some species, particularly Northern Wheatear, spent large portions of their sea crossings extremely close to the surface. These low-level flights may reduce energy costs by exploiting calmer air or aerodynamic ground effects, and could also help limit overheating during daylight crossings.
Different barriers, different solutions
The research shows that small migratory birds tailor their flight strategies to the specific challenges posed by each barrier. Over deserts, altitude appears to be a key tool for managing heat, while over the sea, staying low may reduce energetic costs and thermal stress.
Crucially, these strategies are not random. They are closely linked to physical traits such as wing area, wing structure and plumage colour, highlighting how migration has shaped the evolution of birds in subtle but important ways.
Why this matters
Understanding how birds cross ecological barriers is increasingly important as climate change alters wind patterns, temperatures and the availability of safe stopover sites. Species that already operate close to their physiological limits may be particularly vulnerable to rising temperatures or shifting weather systems.
This study provides a clearer picture of the hidden strategies birds use to survive extreme journeys - and reminds us that even small differences in wing shape or feather colour can mean the difference between success and failure on migration.
January 2026
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