Wind-governed flight altitudes of nocturnal spring migrants over the Sahara

Abstract

Flight costs make up a large proportion of energy expenditure during migration and are strongly dependent on atmospheric conditions aloft. Birds crossing the Sahara can take advantage of the fairly reliable trade-wind regime. In our study, we investigated whether birds adapt flight altitude to minimise energy or water loss. Data from free-flying birds were collected during spring migration with a tracking radar in an oasis in Mauritania, about 500km east of the Atlantic coast. Density measurements revealed the height distribution of the ongoing migration up to 4km above ground level (agl). Daily radiosondes provided information on temperature, humidity, pressure and wind profiles up to 4km agl. We compared height distributions of nocturnal migrants with predictions based on the atmospheric conditions. The two models used predicted maximum flight ranges and maximum flight durations either for energy or water constrained birds. Nocturnal migrants were flying mainly 2km agl, where predicted maximum flight ranges were generally largest. There was little difference between the two models, whereas both models were strongly shaped by the tailwind component. In addition, altitudes where water loss per flight time was minimal could explain some of the remaining variance. The results confirm that wind is the most important factor determining flight altitudes in bird migration and, at least in spring, water stress above the desert seems to play a minor role.

Ostrich 2007, 78(2): 337–341