Flight altitude of trans-Sahara migrants in autumn: a comparison of radar observations with predictions from meteorological conditions and water and energy balance models

Radar observations on the altitude of bird migration and altitudinal profil es of meteorological conditions over the Sahara desert are presented for th e autumn migratory period. Migratory birds By at an average altitude of 101 6 m (a.s.l.) during the day and 571 m during the night. Weather data served to calculate Eight range using two models: an energy model (EM) and an ene rgy-and-water model (EWM). The EM assumes that fuel supply limits Right ran ge whereas the EWM assumes that both fuel and water may limit eight range. Flight ranges estimated with the EM were generally longer than those with t he EWM. This indicates that trans-Sahara migrants might have more problems balancing their water than their energy budget. However, if we assume fuel stores to consist of 70% instead of 100% fat (the remainder consisting of 9 % protein and 21% water), predicted flight ranges of the EM and EWM largely overlap. Increased oxygen extraction, reduced eight costs, reduced exhaled air temperature, reduced cutaneous water loss and increased tolerance to w ater loss are potential physiological adaptations that would improve the wa ter budget in migrants. Both the EM and EWM predict optimal flight altitude s in agreement with radar observations in autumn. Optimal flight altitudes are differently predicted by the EM and EWM for nocturnal spring migration. During spring, the EWM predicts moderately higher and the EM substantially higher Eight altitudes than during autumn. EWM predictions are therefore i n better agreement with radar observations on flight altitude of migrants o ver the Negev desert in spring than EM predictions.