Black-tailed Godwits crossing the Tibetan Plateau adjust flight altitudes in response to topography rather than temperature variations

In the migratory bird pathways crossing Central Asia (the Central Asian Flyway), the Himalayas and Tibetan Plateau –known as the roof of the world– rise to more than 4.5 km above sea level (asl) for over 2,500–3,000 km east–west and 1,000–1,500 km north–south, forming the world’s most extensive high elevation barrier to migratory birds. In other flyways, lowland-living birds climb to altitudes of 5-8 km asl even over low-elevation terrain. However, they typically sustain high altitude-flight for a limited period during daytime, with daytime ascents to more than 5 km asl occurring mostly over hot and arid regions. By comparison, crossing the roof of the world likely requires migrants to endure more extreme high-altitude conditions for longer periods. To examine how topography and day-night conditions shape flight altitudes in the Central Asian Flyway, we collected 86 tracks (49 northward and 37 southward flights from 31 individuals) of Black-tailed Godwits Limosa limosa migrating between non-breeding grounds in Bangladesh and breeding grounds in northern Asia. We found that godwits routinely crossed the Himalayas and Tibetan Plateau at altitudes of 6 km – 7.5 km asl, involving a high proportion of nocturnal high-altitude flight. Topography explained by far the most variation in flight altitudes. Nighttime flight altitudes were on average ~0.6 km lower than daytime altitudes across low as well as high elevation terrain, with no differences between northward and southward migration flights. However, logger temperatures indicated considerable northward-southward, regional and day-night differences in altitudinal temperature profiles. The lack of concomitant patterns in godwits’ regional and northward-southward flight altitudes offers a challenge to the heat-avoidance theory. We propose the Central Asian flyway –with its varied topography across distinct bioclimatic regions– offers an exciting theatre to investigate the causes and consequences of high-altitude flight in comparison with other flyways.