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 profiles of meteorological conditions over the Sahara desert are presented for the autumn migratory period. Migratory birds fly at an average altitude of 1016 m (a.s.l.) during the day and 571 m during the night. Weather data served to calculate flight range using two models: an energy model (EM) and an energy-and-water model (EWM). The EM assumes that fuel supply limits flight range whereas the EWM assumes that both fuel and water may limit flight range. Flight ranges estimated with the EM were generally longer than those with the 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 flight costs, reduced exhaled air temperature, reduced cutaneous water loss and increased tolerance to water loss are potential physiological adaptations that would improve the water budget in migrants. Both the EM and EWM predict optimal flight altitudes 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 flight altitudes than during autumn. EWM predictions are therefore in better agreement with radar observations on flight altitude of migrants over the Negev desert in spring than EM predictions.

Western Sahara : international law, justice and natural resources

As the Spanish were preparing to leave colonized Western Sahara in 1975, Morocco invaded, sparking a war with the Western Saharan Polisario Front. About 70% of Western Sahara was occupied by Morocco, which stations up to 140,000 soldiers in the territory, primarily along a 1700 kilometre long sand berm that is protected by one of the world’s largest fields of landmines. In 1991, Morocco and the Polisario Front agreed to a truce ahead of a referendum on Western Sahara’s future. However, Morocco has since refused to allow the referendum to take place, and has begun the extensive exploitation of Western Sahara’s non-renewable natural resources. This has both highlighted the plight of the Saharawi people who live in refugee camps in Algeria and in occupied Western Sahara, and pushed the Polisario Front back to a position where it is openly canvassing for a return to war.