THE BREEDING SEASONS OF AFRICAN BIRDS— 1. LAND BIRDS.

Abstract

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 timing of breeding seasons can be secured only by external factors regulating an internal rhythm. Day‐length, rainfall and humidity, temperature and visual stimuli are each considered briefly. Each may be effective on some species in some areas, but no one generally.