WATER‐BIRDS OVER THE SAHARA

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Abstract
Summary: An investigation is made of the extent to which Palaearctic birds belonging to typically water‐bird families occur south of the Sahara in areas remote from both the sea and the Nile, namely, Darfur, Northern Nigeria and the Inundation Zone of the Niger, and hence may be presumed to cross the Sahara. Fifty‐seven species fall to be considered and for these occurrences within the Sahara are also collated. The results owe a great deal to work done in the last few years, especially ringing.Most of the 57 species dealt with occur regularly in appreciable numbers in one or more of the areas under consideration and some of the species are enormously common. In general there is evidence to show that many birds winter in Africa far to the west of their Palaearctic breeding grounds, from which much diagonal passage of the Sahara is to be inferred.The results are considered in relation to the ecological conditions on the northern and southern edges of the desert and in relation to the meteorological conditions over it. A remarkable circumstance is that few water‐birds indeed of any species have been observed in the oases of the Sahara,so that their crossing of the desert is clearly very efficient, although so much of it is by lengthy diagonal and although many birds entering Africa in the middle longitudes meet with no suitable habitat between Europe and the tropics. Except perhaps for storks, which soar, and certainly for herons, which proceed by heavy flapping, the air speed of water‐birds is much greater than that of the small passerine migrants, the necessary potentiality of which has been calculated as 50–60 hours of flight without refuelling; but too little is known of the aerodynamics and physiology involved for it to be calculated whether water‐birds need to start with the comparable accumulations of fat and no definite information exists except for the Wood Sandpiper, which has been shown to put on up to 32% of fat. By comparison with passerines, more individual water‐bird species winter partly north and partly south of the Sahara; the factors affecting each bird's behaviour in this respect are unknown.