Sustained bird flights occur at temperatures far beyond expected limits

Migratory birds deposit fat and protein before passing ecological barriers and must economize these during such crossings. Birds crossing the western Sahara during autumn face a trade-off between cold and humid air along with head winds at high altitudes versus warm and dry air along with tail winds at low altitudes. Since water loss rate increases with temperature, migrants should avoid warm and dry air to save water and hence fly at high altitudes. By quantifying nocturnal songbird migration across the western Sahara with radar, we found that more than 60% of the songbirds migrated below 1000 m above ground level. Thus, the majority of songbirds performed sustained migratory flights in much warmer and drier conditions than predicted (weighted means: 30 °C; relative humidity: 27%; water vapour density: 7.8 g/m 3). Based on the metabolically available water from fat and protein catabolism, we estimated the maximum possible overall water loss rate of a flying model bird, the garden warbler, Sylvia borin, for the entire Sahara crossing at 0.29 g/h. This is considerably lower than water loss rates for the same model bird passing our study site, 0.62 g/h at 30 °C, based on applied calculations of physiological studies. Our results clearly show that migrating songbirds can fly at much higher temperatures, and have considerably lower water loss rates, than predicted. This new insight based on observations under natural conditions will have substantial impact on the development of new physiological models for birds and other animals with restricted access to water.