Trans-desert bird migration

Thousands of bird species are migratory, performing a north-south, often cross-equatorial, seasonal migration between northern and southern homes (Figure 3.6), of which the one north of the subtropical region is used for breeding. A migratory bird is an integral component of these two distinct ecosystems, linked only by migration routes, used for reaching its destination in the fastest and safest way. Since most deserts are wedged within the subtropical latitudes, since many regions within subtropical latitudes are deserts, and since competition for resources at either end favors early arrival, most migrants select the shortest route, which is very likely to include a desert-crossing section serving as a corridor for the migration of large numbers of birds. Migrating is always costly, but more so on desert crossing, because migrants spend most of their life outside the desert and hence are not as well-adapted to deserts as resident birds. Yet, since crossing deserts rather than circumventing them evolved as an adaptation to reduce travel time, the added cost is reduced by specific adaptations. The fuel used for travel is fat-light in weight and rich in energy -stored under the skin. A 10g warbler needs fat stores approximately a quarter of its body mass to complete the flight across the Sahara Desert, which can take 40 hours (Carmi and others 1992). Burning this fat during the intensive and exhaustive flight generates much heat and the rapid breathing during flight removes body water. To reduce the risks of overheating and dehydration, most small cross-desert migrants fly at night. When the desert tract to be crossed is short, birds may make it in a single flight, but otherwise birds conduct intermittent flights between stopovers (Biebach and others 2000); they alight at dawn, seeking shaded and concealed refuges, even as small as a shading rock or a single bush if nothing better is in sight, where they minimize water loss and rest prior to taking off again when night falls.

Migrants can do even better when stoppingover in desert oases, which function as stepping stones for en route energy replenishment (Safriel and Lavee 1988). But because oases are rare (for example, the combined area of Saharan oases is only 2 per cent of this desert), scattered and hence difficult to spot, many migrants use routes along desert rivers such as the Nile (Box 3.1). Combined, all desert oases and riparian corridors comprise a critical habitat for a hugely disproportionate number of migratory species. Yet, the richer an oasis or a riparian tract is, the more it is likely to be impacted by people (two-thirds of the human population of the Sahara concentrate in oases) who can pose a threat to the migrants; the intensification of irrigated agriculture in natural and man-made oases, for example, may add open water and some locally abundant sources of food, which does attract migrants, but agrochemicals and other pollutants may harm the migrants, either directly, or through damage to the insects they might feed on (e.g., Evans and others 2005).

Cross-desert bird migration is also being impacted by global climate change. Cues such as day length, which are independent of climate, are involved in determining the onset of migration. These have evolved to synchronize with changes in seasonal abundance of plant and insect food resources in the two far-apart "homes" of the migratory birds; in contrast with the day-length cues, these changes in food availability are highly dependent on climate. Climate change will decouple the synchrony between non-climatic cues and climatic events. This synchrony is critical for cross-desert migrants, many of which time their migration such that they arrive at their feeding ground, just prior to entering, or just after exiting the desert when the food supplies there peak, which enables them to replenish their fat reserves for the remaining journey (Vickery and others, 1999). Compounding that problem is the projected "expansion" of deserts, which would increase the distance of that flight, perhaps beyond that which birds are already adapted to, or to which they might rapidly adapt. Furthermore, even if climate change does not jeopardize the voyage itself, it may reduce the benefits of migration. Since migrants have only a small margin of safety of energy reserves during migration, the condition in which they arrive at their destination, where they may encounter intense competition, substantially determines their survival.

To conclude, the cross-desert migration of birds, many of which are both familiar and important to people living far from deserts, is sensitive to human impact, climate-change included. Since this migratory network can only be as strong as its weakest link, the conservation of desert sites used by alighting migrants, as well as off-desert ones on which the success of cross-desert migration depends, is urgently required (hutto 2000).

Locusts moving through deserts

Though there are many more insect species than bird species, the number of migratory insects is smaller than that of migratory birds and only one small group of migratory insects is associated with deserts - the locust. Unlike migratory birds which cross deserts in a regular, seasonal two-way migration, locusts cross the desert in a unidirectional, irregular pattern. And, whereas the arrival of migratory birds is often welcome, the sighting of locust swarms is always ominous. Several locust species spend part of their lives in deserts and many locust swarms cross deserts. Most significant is the desert locust (Schistocerca gregaria) of the least dry parts of deserts - arid (but not hyperarid) regions in 25 countries of the Sahel (including Burkina Faso, Chad, Mali, Mauritania and Niger), the Arabian Peninsula, along the coast of the Red Sea, and along the coast of ROPME Sea Area (Regional Organization for the Protection of the Marine Environment - Kuwait Regional Convention, 1978) up to the India-Pakistan border (Simpson and others 1999). The desert locust in the solitary phase poses no threat to crops; its small populations are dispersed in patches of suitable habitat, with little movement between them (Ibrahim and others 2000).

When spells of good rainfall occur in several successive rainy seasons, the soil becomes moister and vegetation grows more quickly. In response the animals also grow more quickly and egg-laying in holes dug in the soft, moist soil intensifies. Each of the formerly isolated populations increases, and their movement downwind leads to a concentration of several crowded, fast-growing populations (Despland and others 2004). Once the aggregated population is large and crowded, the individuals change in colour, physiology and behaviour which, combined, helps them to aggregate and reproduce intensively (Pener 1991) especially when rains persist, thus increasing pressure on resources. When, eventually, vegetation is decimated at the source, a large-scale directional, downwind flight of whole swarms is initiated (Culmsee 2002).

This movement can bring the swarms to other desert arid or hyperarid areas, or to non-desert areas (Figure 3.7) - the "outbreak areas" (as distinct from the areas they inhabited prior to the massive movement - the "recession areas", Pedgley 1981). Carried by the wind which takes them to where it takes the rain too (Waloff 1960), swarms that may contain 50 000 million individuals, migrate from deserts into non-desert areas, where they can spread over more than 20 per cent of the land surface of up to 65 countries including in the Sahara and the Arabian Desert. They can reach southern Spain, Turkey, West Africa, India, Bangladesh, Tanzania and the Democratic Republic of Congo, consuming 100 000 tons of vegetation a day. Swarms originating in deserts can even cross oceans; during the 1986-89 plague, swarms escaped extermination in western Africa and crossed 5 000 km over the Atlantic Ocean, reaching the West Indies and the eastern coast of Venezuela. Swarms migrating to outbreak areas rarely return to the original recession areas (Ibrahim and others 2000): they die out, or are destroyed by cold weather or pest control measures. Since it is the desert edges that are often the source of locust plagues, it is there, in countries such as Algeria, Somalia, Sudan and Iran, where the battle against locusts can be most-effectively fought, in order to save crops in other areas, in countries such as Mali, Niger, Chad and Yemen (Showler 2002).