The effect of body mass and fat reserves on the choice of the stopover place and on the stopover period was investigated in 3 species of passerine birds migrating through the Sahara in Egypt. Birds grounded in an oasis with food and water were more than 10% lighter than those from a desert stopover place which offered shade only. Stopover period was in general one day except for the light portion of oasis birds which stopped for up to three weeks during which they regained fat. A calculation of the maximum range which can be covered with the available fat depots revealed that nearly all birds from the desert stopover place should be able to cross the desert whereas about 60% of the oasis birds should not. An intermittant migratory strategy for the desert crossing is proposed with flight during the night and stopover during the day, even without drinking and feeding.

Radar observations of the diurnal timing of bird migration in the Sahara Desert are presented for autumn migration. Study sites were on a transect along the north-south migratory direction. Three groups of birds migrating either during day, evening or night in the northern part of the Western desert in Egypt were identified. The maximum of day and night groups occurred later the further south the study sites were. Based on the distance between sites and the timing of peak migration, birds were flying at an estimated ground speed of about 20 m/s. The maximum of the evening group was at about 2 1 :00 h at all sites. The three groups were classified according to three different strategies of migration across the Mediterranean Sea and the Sahara Desert: (1) the day group of birds performed a non-stop flight across the sea and at least the northern part of the desert;(2) the night group performed an intermittent migratory strategy with stopover at the coast of Egypt to continue migration the next evening; (3) the evening group birds were also intermittent migratory fliers, but they stopped somewhere in the desert after a continuous flight across the sea and part of the desert. About 20% of all migrants are involved in non-stop migration and 80% in intermittent migration with stopover at the coast (7OYo) or with stopover in the desert (10%). It is argued that any species of small passerine has the option to use any of the three strategies.

The temporal pattern of migration by passerine birds during the night, and their arrival during the day at the Egyptian coast and in the northern Sahara Desert was investigated. The mean direction of nocturnal migration at the coast was south-southeast, while at all desert sites it was south-southwest.Birds arrived at the Egyptian coast only during the second half of the night which is explained by the fact that n o birds could have taken off from the Mediterranean Sea. At least some of the birds landed at the coast where they spent the day before taking off shortly after sunset. These birds passed the desert sites at the expected time of day assuming a ground speed of 18 m per second. However, the origin of the birds passing the desert sites early a t night is unclear. They must either have spent the day in the desert north of the study sites or they had overflown the Egyptian coast in the afternoon without landing.The landing of birds during the day a t the desert sites was bimodal. This pattern of arrival is explained either by some birds having landed a t the Egyptian coast in the early morning before continuing, or by deteriorating conditions later in the day during flight or when resting in the desert, that obliged them to seek shelter at the desert sites.A correlation between the number of migrants observed during the night and the number of resting birds in the desert on the following day suggests that an unknown proportion of birds might regularly use an intermittent migratory strategy that includes rest periods by day when crossing the desert, whereas others might adapt a non-stop migratory strategy.In Europe, many migrating bird species, especially longdistance migrants, are known to migrate exclusively during the night (Dorka 1966). A number of ultimate factors have been proposed to explain this behaviour, most prominently the possibility of feeding during daylight and the avoidance of predators during the day (Palmgren 1936). better temperature regulation and water balance (Dorka 1966) and the calm atmospheric conditions that prevail at night (Kerlinger & Moore 1989). Knowledge of the temporal pattern of migratory behaviour under extreme conditions, such as a desert crossing, might give further insight into the adaptive significance of the general phenomenon of nocturnal migration. In addition it might suggest physiological constraints that limit migration.Two strategies for crossing deserts have been proposed: a non-stop migratory strategy with flight during day and night (Moreau 1961) and an intermittent migratory strategy with flight during the night and rest during the day (Biebach