Medium Ground Finches (Geospiza fortis)

  1. In 1973, Peter Grant started going to an isolated island called Daphne Major in the Galápagoes.
  2. Camped with Rosemary Grant and their daughters Nicola and Thalia.
  3. They marked and measured every finch on the island, for thirty years !
  4. They followed individual birds and their offspring from nestlings to death.
  5. Made many measurements including the depth of the bills.
  6. During the wet years (El Niño) many small seeds are produced on the island.
  7. The finch population tended to have smaller bills that were more efficient eating smaller seeds.
  8. During La Niña-related droughts all the small seeds are eaten and the birds are forced to eat large hard seeds.
  9. As many as 80% of the finches die!
  10. The next year the nestlings tend to have larger bills. Why?
  11. The larger-billed birds have greater feeding success and so live to raise young.
  12. Why don't the birds tend to get larger and larger bills?
  13. During wet years the smaller-billed birds are more successful.
  14. This is a directional selection of the population.
  15. What is the difference between life or death ?
  16. A difference in bill depth of 0.2 mm !