Alternative life-history styles of South African birds

Abstract

The amount and reliability of rainfall covary across South Africa from the arid northwest to the humid east. This is taken as a measure of environmental unpredictability to predictability. Dry, intermediate and wet regions are recognized on this basis. Volant terrestrial bird species breeding in South Africa were allocated to the region or regions in which they breed. Comparisons were usually made between species breeding only in the dry or dry and intermediate regions with those breeding only in the wet or wet and intermediate regions. Available data (mean female body mass, modal clutch size, mean incubation and fledging periods in days) were assembled to examine to what extent environmental unpredictability correlated with variation in these parameters, the expected variation being lower mass, larger clutch size and shorter incubation and fledging periods with increasing unpredictability. Mass averages lower but not by a significant amount. Clutch size does not differ at all. However, in insectivorous birds with breeding ranges extending from dry to wet regions there is some significant evidence for larger clutch sizes in the dry region. This does not hold for granivorous birds. Incubation and fledging periods are significantly shorter in passerine birds in the more unpredictable dry region but this does not hold in nonpasserine birds. The results are discussed and it is concluded that experimental work, for which birds are not well suited, is needed to test the effects of different environments on life-history parameters, rather than further correlation studies.