HIGH SURVIVAL OF DARWINS FINCH HYBRIDS - EFFECTS OF BEAK MORPHOLOGY AND DIETS

Three species of Darwin's finches (Geospiza fortis, G. scandens, and G . fuliginosa) hybridize rarely on the small Galapagos island of Daphne Major. Following the exceptionally severe El Nino event of 1982-1983, hybrids survived as well as, and in some cases better than, the paren tal species during dry seasons of potential food limitation. They also backcrossed to two of the parental species. This study was undertaken to compare the diets of hybrids with the diets of the parental specie s in order to assess possible reasons for the high hybrid survival. Di ets of F-1 hybrids and first generation backcrosses to G. fortis were intermediate between the diets of the respective parental species. Dis tinctiveness of the hybrid diets was most pronounced where the diets o f the parental species differed most. A strong determinant was beak mo rphology; hybrids inherit beak traits from both parents, and, on avera ge, have intermediate beak sizes. Among the combined groups of species and hybrids, and among the hybrids alone, dietary characteristics cov aried with beak morphology. Hybrids that differ most from G. fortis in beak morphology, the G. fortis x G. scandens F-1 hybrids, experience a feeding efficiency advantage when feeding on Opuntia echios seeds, c ommonly consumed in the dry season. These findings are used to interpr et the higher survival of hybrids after 1983 than beforehand. The El N ino event that year led to an enduring (10-yr) change in the habitat a nd plant composition of the island. A decrease in absolute and relativ e abundance of large and hard seeds apparently caused relatively high mortality among G. scandens and the largest G. fortis individuals. Hyb rids were favored by an abundance of small seeds. The high survival of G. fortis x G. scandens F-1 hybrids may have been due, additionally, to a broad diet and to efficient exploitation of Opuntia seeds. The st udy demonstrates long-term ecological and evolutionary consequences of large-scale fluctuations in climate, and the role of ecological (food ) factors in determining hybrid fitness.