High-altitude shorebird migration in the absence of topographical barriers: avoiding high air temperatures and searching for profitable winds

Nearly 20% of all bird species migrate between breeding and nonbreedingsites annually. Their migrations include storied feats of endurance andphysiology, from non-stop trans-Pacific crossings to flights at the cruisingaltitudes of jetliners. Despite intense interest in these performances, thereremains great uncertainty about which factors most directly influence birdbehaviour during migratory flights. We used GPS trackers that measurean individual’s altitude and wingbeat frequency to track the migration ofblack-tailed godwits (Limosa limosa) and identify the abiotic factors influencingtheir in-flight migratory behaviour. We found that godwits flew ataltitudes above 5000 m during 21% of all migratory flights, and reachedmaximum flight altitudes of nearly 6000 m. The partial pressure of oxygenat these altitudes is less than 50% of that at sea level, yet these extremelyhigh flights occurred in the absence of topographical barriers. Instead,they were associated with high air temperatures at lower altitudes andincreasing wind support at higher altitudes. Our results therefore suggestthat wind, temperature and topography all play a role in determiningmigratory behaviour, but that their relative importance is context dependent.Extremely high-altitude flights may thus not be especially rare, but they mayonly occur in very specific environmental contexts