Rainfall as a breeding stimulus and clutch size determinant in South African arid‐zone birds

Rainfall is known to be a stimulus for initiating breeding in arid‐zone birds, but its influence on clutch size is unknown for most southern African birds. The breeding activity of nine out of 11 species studied in the arid Nama Karoo biome of South Africa increased markedly after appreciable rainfall. Some resident insectivores did breed in the absence of rainfall, even under drought conditions, but also laid eggs within one week of small rain showers. Nomadic granivores required more substantial rainfall to stimulate what was usually a synchronized, population‐wide breeding response. The timing and length of the breeding season was dependent on the integrated effect that rainfall and temperature had on the growing season of the vegetation. Over 50% of the 11 species studied showed evidence of an increase in clutch size following large rainfall events. Grey‐backed Finchlark Eremopterix verticalis began laying larger clutches within one week of a rainfall stimulus, illustrating how rapid this response can be.