Adult Survivorship in Darwin's Ground Finch (Geospiza) Populations in a Variable Environment

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Abstract
(1) Patterns and causes of adult mortality were studied in two species of Darwin''s finches, Geospiza fortis and G. scandens, on Isla Daphne Major, Galapagos, from 1976 to 1983. The climate ranged from a drought (1977) to exceptionally extensive rains (1983). (2) Annual adult survival in both species varied considerably, and was highest during periods of very light or very heavy rainfall. Mean annual adult survival was 0.69 .+-. 0.44 S.D. for fortis and 0.82 .+-. 0.21 S.D. for scandens. Adult mortality in both species was negatively correlated with the abundance of small seeds per bird. Independently, there was also a negative statistical effect of finch density alone on mortality. (3) Survivorship curves of individual cohorts of fortis and scandens varied considerably in shape, and their age structures were highly unstable. These features occurred because cohorts encountered different environmental conditions. (4) Survival rates of adults of both species were independent of age in most years. An exception occurred during and following an 8-month breeding season in 1983, when fortis 7 years or older survived less well than all other birds. (5) Female fortis suffered higher mortality than males in 2 out of 3 dry years. In at least 1 year, the differential survival could be attributed to selection against small size, since females are smaller than males. The sexes of G. scandens survived equally well in all years. (6) Habitat quality and reproductive activity did not influence survival in either species to a detectable extent. There was, however, a complex interaction between age, sex, habitat quality and survival in both species in some years; males or females of particular ages living in certain habitats survived significantly better than other classes of birds (7) Mean annual adult survival in Darwin''s finches is higher than that of many temperate passerines and is more similar to that of tropical rainforest species. Annual variation in survival rates in very high, even in comparison with temperate species, and may have significant indirect effects on reproductive output in these birds. Fluctuating food supplies and population densities appear to be important causes of this variation.