Predicting migratory flight altitudes by physiological migration models

Using the altitudinal profiles of wind, temperature, pressure, and humidity in three flight models, we tried to explain the altitudinal distributions of nocturnal migrants recorded by radar above a desert in southern Israel. In the simplest model, only the tailwind component was used as a predictor of the most preferred flight altitude (T model). The energy model (E model) predicted flight ranges according to mechanical power consumption in flapp ing flight depending on air density and wind conditions, assuming optimal a djustment of airspeed and compensation of crosswinds, and including the inf luence of mass loss during flight. The energy-water model (EW model) used t he same assumptions and parameters as the E model but also included restric tions caused by dehydration. Because wind was by far the most important fac tor governing altitudinal distribution of nocturnal migrants, differences i n predictions of the three models were small. In a first approach, the EW m odel performed slightly better than the E model, and both performed slightl y better than the T model. Differences were most pronounced in spring, when migrants should fly high according to wind conditions, but when climbing a nd descending they must cross lower altitudes where conditions are better w ith respect to dehydration. A simplified energy model (Es model) that omits the effect of air density on flight costs explained the same amount of var iance in flight altitude as the more complicated E and EW models. By omitti ng the effect of air density, the Es model predicted lower flight altitudes and thus compensated for factors that generally bias height distributions downward but are not considered in the models (i.e. climb and descent throu gh lower air layers, cost of ascent, and decrease of oxygen partial pressur e with altitude). Our results confirm that wind profiles, and thus energy r ather than water limitations, govern the altitudinal distribution of noctur nal migrants, even under the extreme humidity and temperature conditions in the trade wind zone.