The ecology of clutch size variation in Darwin's Small Ground Finch Geospiza fuliginosa: comparison between lowland and highland habitats




This study examines the ecology of clutch size variation between two contiguous populations of Small Ground Finch Geospiza fuliginosa that breed in geographically distinct habitats (lowlands, highlands) on Santa Cruz Island of the Galapagos Archipelago. Small clutch size is considered an adaptive response to high predation risk (to reduce behavioural conspicuousness at the nest and lower the energetic costs of re-nesting), whereas high food abundance is predicted to favour large clutch size. This study has two main objectives: (1) to present descriptive data on clutch size variation in G. fuliginosa between habitats, and (2) to test life-history predictions for clutch size variation based on food abundance and nest predation hypotheses. The highlands on Santa Cruz Island were characterized by high nest predation (52%), high annual rainfall and high insect abundance across the year, whereas the lowlands were characterized by low nest predation (5%), seasonal rainfall and high insect abundance during the breeding season. The highland birds had smaller clutch size and a shorter re-nesting interval. Patterns of nest attendance differed significantly between habitats. The percentage of time incubating per hour was comparable between habitats, but the mean duration per incubation event was longer in the highlands (with fewer events per hour), which is consistent with the prediction of reduced behavioural conspicuousness at nests under conditions of high predation risk. There were no significant differences in nestling feeding levels between habitats, probably because G. fuliginosa is a multiple prey loader.