ALTITUDE CHOICE BY NIGHT MIGRANTS IN A DESERT AREA PREDICTED BY METEOROLOGICAL FACTORS

The height distribution of nocturnal migrants in southern Israel was d etermined by conically scanning the sky with the pencil-beam of an X-b and radar at different elevation angles. Altitudinal profiles of meteo rological parameters were derived from radio sondes launched at midnig ht and from pilot balloons launched every 4 h. A model to predict the height distribution of birds by means of meteorological variables was developed by assuming that the observed proportions of birds within a height zone, compared with the neighbouring height zones, reflect the degree of the birds' preference for that height zone. Only one among t he variables included in the multiple regression analysis proved to ha ve a significant influence on the height distribution of migrants: the difference of tailwind speed between height zones. Simulations with 1 000 birds choosing altitudes by means of the night's altitudinal profi le of tailwind speed closely traced the observed distributions. The fa ct that all the other meteorological factors which were previously sug gested to have an influence on the flight range in trans-desert migrat ion were not selected as relevant factors is discussed. The following basic information on nocturnal bird migration in the Negev is provided as a background for the statistical analysis: Directions of migration are within very narrow limits. During the first hour after take-off 6 0% of autumn migrants and 75% of spring migrants are climbing, with ve rtical speeds of 0.1-2 m per s and 0.1-4 m per s, respectively. During the rest of the night, climbing and descending birds are in nearly eq ual proportions. Thus, there is a high potential of sampling atmospher ic conditions at different altitudes. Height distributions in spring a nd autumn show the influence of the trade wind situation, autumn migra nts making use of the northerly winds at low levels in spite of high t emperatures, while spring migrants tend to reach the southwesterly win ds at higher levels.