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Summary. For the present purpose Africa is divided into everyreen, semi‐arid and intermediate (deciduous) types of count?, all of which occur in all latitudes south of the Sahara. “Breeding season” is limited to the months during which eggs are laid by the species concerned in the given area and all records are interpreted accordingly. As a basis for ascertaining the curve of breeding activity in a given area through the year the number of species laying in each month has been ascertained and calculated as a percentage of the total for all species for the whole year. A definite peak in the curve of breeding activity is evident everywhere except in certain areas within about four degrees of the Equator. In one part of this inner tropical belt there may be no distinct breeding season for must groups of birds (Congo), but in East Africa a double breeding season is the rule, with peaks coinciding with the two rainy seasons. Even so close to the Equator as 5°s. the (single) breeding season in evergreen forest is as restricted as in other types of country and its time‐relation to the rains varies locally. In the “ intermediate” type of country characterized by 4–6 months drought each year the timing of the peak breeding season varies from the end of the rains, at Cape Town, to the start of the rains in Natal and several weeks before the rains in areas 23° ‐ 10° S. The key to this local difference is that at Cape Town the rains fall in the cold season, so that vegetation and insects are slow to flush. In the warmer conditions in which the rains begin in Natal the flush comes at once. And further north the dominant vegetation and its associated insects flush towards the end of the drought and well in advance of the rains. From Natal northwards the breeding season for all birds combined shows a progressively less marked peak. The reason is that the seasons of certain ecological categories (1) water birds, (2) raptors and scavengers, (3) ground birds, (4) grass birds, (5) the other birds, tend to diverge. The raptors and scavengers are everywhere the earliest breeders, the biggest species laying by the middle of the dry season. The water birds lay to a large extent towards the end of the rain, and after. The ground birds tend to lay as soon as the grass fires are over and before the heavy rains have induced a lush growth of herbage. The grass birds lay later than most‐others, when the grass has grown high. In semi‐arid areas the breeding seasons are on the whole similar to the foregoing, with most birds breeding when the vegetation flushes, whether just before or after rain has fallen. But the “semi‐and” birds are notably sensitive to rainfall; breeding that has begun is checked if the rains are interrupted. In some communities and categories of birds the “reasons” for the observed breeding seasons are intelligible, the best food‐supply or the safest nesting apparently being secured. In others the reasons are not obvious; and the degree to which the breeding seasons are restricted is often incomprehensible. The observed ...

Summary It is estimated that birds travelling to winter south of the Sahara must on average enter Africa at the rate of 250,000 per mile of longitude, which over a two‐month period gives an average daily entry of about 4,000 birds per mile. Only a small fraction of these come within the range of observation. There are reasons to suppose that the main directions may be southwest in autumn and northeast in spring, involving somewhat diagonal crossings of the desert, rather than directly north and south. The geography, ecology and winds of the Mediterranean and Sahara are described. The average width of the Sahara from north to south is at least 900 miles, but owing to the aridity on the southern edge in spring (mitigated in autumn) many birds probably start their flight up to 1,200 miles south of the Mediterranean. There are indications that migrants do not concentrate on oases. Wind data are given for altitudes up to 3,000 m. (10,000 ft.), which on the whole are much more favourable to birds in autumn than in spring, but with less difference in the sectors Algeria‐Tripoli than elsewhere and with the proviso that in spring at 2,000 m. upwards the winds are favourable to diagonal flight, northeastwards. Hypothetical flight‐performances are estimated and discussed; and it is concluded that ability to fly some 50–60 hours without refuelling is essential for most migrants in spring. The physiological implications are considered. If as much pre‐migration fat is put on in autumn as seems to be needed in spring, then many birds should be able to fly non‐stop from Europe to the tropics, some 1,300‐1,500 miles. Ringing in Tunisia and subsequent recoveries there have provided evidence of accurate navigation directed to a point half‐way through the birds' journeys. The evidence for general abundance of birds in spring and autumn is assembled sector by sector. In the northern deserts of Algeria, in Tunisia and on the coasts of Tripoli and Cyrenaica far more migrants are seen in spring than in autumn, while further east and also in Morocco, the disparity seems to be less. It is concluded that, at least in the central sectors named, a much larger proportion of birds makes a more or less continuous flight, without coming under observation, in autumn than in spring. The information regarding the incidence of 57 species of trans‐Saharan migrants is summarized and discussed for each. A number of species have almost entirely escaped detection, especially in autumn and especially in the eastern half of the area. Most species of migrant are shown to travel over various sectors of the Sahara, irrespective of opportunities to refuel, at both seasons and it seems that many birds fly non‐stop from Europe to the tropics in autumn. Indications that some species or populations travel further east at one season than at another are discussed. “Migration divides” which have been detected in parts of Europe are as a rule not reflected by differences in abundance on the south side of the Mediterranean.