Strategies of passerine migration across the Mediterranean Sea and the Sahara Desert: a radar study

Abstract

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.

Heterothermy in small, migrating passerine birds during stopover: use of hypothermia at rest accelerates fuel accumulation

“…During migration, most of these normally diurnal birds change their activity rhythm and fly at night (Berthold, 1996). If a bird's fuel reserves are adequate, it takes off at dusk and flies non-stop, often for several hundred kilometers at a stretch (Biebach et al, 1986;Biebach et al, 2000). However, this daily rhythm changes when fuel stores run low, and the bird lands to refuel.…”

“…Each year, billions of birds fly in autumn from their breeding grounds in Eurasia to wintering grounds in Africa and return in spring, often covering many thousands of kilometers (Moreau, 1972;Biebach et al, 2000). In small passerine birds, fuel for migratory flight consists mainly of fat (between 85 and 95%) and to a lesser extent of protein (5-15%) (Klaassen and Biebach, 1994;Klaassen et al, 2000).…”

Spring passerine migrants stopping over in the Sahara are not fall-outs

“…They are therefore not ''fall-outs'' as hypothesized by Moreau (1972). Non-stop and intermittent strategies were not seen as mutually exclusive (Bairlein 1992;Bruderer 1994;Biebach 1995;Biebach et al 2000).…”

Expected Effects of Offshore Wind Farms on Mediterranean Marine Life

“…High collision levels of migrating terrestrial birds at well-lit observing platform during periods of bad weather and poor visibility [42] indicate that wind farms located near the coast, or prominent migration bottlenecks, may pose a significant risk to migrating birds. More recent evidence also shows alternative crossing options for some passerine species, including non-stop crossings over the Mediterranean Sea [43]. This indicates that species-specific migrations are not fixed either temporally or spatially, and individual route decisions are due to risk analysis of many parameters including energy reserves, weather conditions, and genetic disposition [44,45].…”