Diet Variation in a Population of Darwin's Finches

I studied the diets of a banded population of Darwin's Medium Ground Finches (Geospiza fortis) on Isla Daphne Major, Galapagos, between 1979 and 1981. I was able to show how an individual's diet was constrained to a subset of the foods that the population as a whole exploits, and considered how an individual selected from among the foods available to it. I examined both the evolutionary and short—term constraints on diet in the population. During times of food shortage there are essentially three food types on the island. These are small seeds, derived from several plant species, which are soft and easy for finches to handle, and two large seeds, derived from the cactus Opuntia echios and a semiannual plant Tribulus cistoides. The smallest finches are unable to crack the large seeds, while among those that crack large seeds, there are positive associations of seed handling ability and beak size. Generalization on all three seed types at the population level and some specialization of individuals on large or small seeds help to explain the high level of morphological variability observed in the population. Within the large—seed category some individuals specialized on Opuntia or Tribulus seeds, whereas others consumed both. Energy rewards from foraging on either seed appeared to be similar. Individuals did not behave in a manner suggesting that they were attempting to maximize energy returns when foraging on these seeds. Specialization on Opuntia or Tribulus is attributable to the patchy distribution of seeds on the island, and to a host of constraints on foraging, including microclimate and territorial behavior. If feeding abilities on each of the three seed types are considered as separate characters, then the joint evolution of the three feeding behaviors is tightly constrained through beak morphology, known to be highly heritable. This in turn implies high genetic correlations between the three feeding behaviors.