Monthly rainfall has been registered at eight sites on four of the Galápagos Islands for a variable number of years. The most extensive, continuous records are from a coastal site on San Cristóbal for the period 1950-67. Analysis of all these data establish the following characteristics: (1) Although rainfall is seasonal, it is very variable, particularly in the wet-season months of January to May. (2) Annual variation is very large. The amount of rainfall tends to alternate in successive years, and a 4-yr periodicity in rainfall is indicated. (3) There is also variation on a longer time scale; the 1950's were 50% wetter than the 1960's. (4) There is more precipitation per year at high elevations than at low elevations, it is distributed more evenly throughout the year, and it varies less among years. Finch demography, particularly reproductive aspects, is suggested to be influenced by rainfall through the effect of rainfall upon food supply. Several qualitative predictions about reproduction, mortality, and dispersal are made on the basis of the established patterns of variation in rainfall. The meagre data on finch population dynamics are consistent with these predictions, but future research is required to test them adequately.
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Grant et al. (1980) studied this question.