On all the southern continents, the north-south migrations of the local breeding birds (austral migrants) more or less coincide with the north-south movements of the intercontinental migrants from the northern hemisphere, the movements of both groups being driven by the same seasonal changes in climate and food supplies. However, away from the equatorial rainforests, as mentioned above, bird movements are linked not so much to temperature, but to the corresponding wet-dry seasons, and the predictability or otherwise of rainfall. In addition, most parts of the southern continents escape cold winters, and span a wide enough range of latitude to accommodate the native bird species year-round. Apart from the seasonal influx of birds from the northern continents, therefore, the southern continents have self-contained migration systems. The migrations of many species are relatively short-distance and partial, so are often hard to detect without special study. Nevertheless, some species cross the equator on seasonal journeys exceeding 1000 km. Differences in the ecological circumstances prevailing between the three southern continents impose broad-scale variations on their overall migration patterns.
The rainfall and vegetation zones in Africa mirror one another on both sides of the equator, progressing from rainforest in the wettest equatorial regions, through deciduous woodland to increasingly dry savannahs and grasslands (Chapter 24). In consequence, many birds can find equivalent habitat on both sides of the equator, and because the wet seasons are reversed between north and south sides, they can, by migrating between the northern and southern tropics, benefit from the wet seasons in both. Some such species breed in the northern tropics and spend their non-breeding season in the southern tropics. Others breed in the southern tropics and spend their non-breeding season in the northern tropics. Yet others have separate breeding populations both north and south of the equator, each crossing to the other side on migration (Figure 13.11).
Overall, more than 500 African breeding species are known to perform migrations within the continent (Curry-Lindahl 1981). Most move entirely within the northern tropics, or entirely within the southern tropical and temperate zones. In each case, the general trend is for species to move towards wetter (lower latitude) areas for the dry season (Figure 13.11, for raptors see Figure 13.12). Those that cross the equator to equivalent habitats on the other side are relatively few in number. In addition, in the mountainous areas of the east and south, many species make seasonal altitudinal movements. For example, in Natal in southern Africa, no less than 76 species have been described as 'altitudinal migrants' (Johnson & MacLean 1994).
In Africa, as in Europe, the proportions of species in each region that are migratory can be predicted fairly accurately from the average temperature of the coldest month (Hockey 2000). In regions where this temperature exceeds 20°C, less than 10% of species are migratory. The likelihood of any one species being migratory again depends on its diet, with frugivores being mainly sedentary and insectivores migratory. This applies particularly to those that eat aerial insects (swallows, swifts, nightjars), large active insects (halcyonid kingfishers and rollers) or larvae of flying insects (cuckoos). Over the entire latitudinal range from northern Europe to southern Africa, a strong linear relationship exists between the proportion (P) of birds that are resident and the mean temperature (T) of the coldest month (P = 1.92T + 53.66, r2 = 0.96, df = 19, P < 0.001, Hockey 2000). It extends the relationship discussed above for European birds alone.
The mirror-image symmetry of vegetation zones north and south of the equator, which is so marked in Africa, is less prominent in South America, where altitude
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