The feeding of darwin's finches on Tribulus cistoides (L.) seeds

Food-handling skills of finches vary as a function of bill size. Geospiza magnirostris crush and shatter the woody mericarps of caltrop (Tribulus cistoides) to reach the seeds, whereas G. fortis (a smaller species) bite and tear at them. G. magnirostris accept and crack most of the mericarps picked up; fortis reject most mericarps but often feed on fragments. Energy rewards are greater for magnirostris than for fortis. Intraspecific variation in handling skills parallels interspecific variation. G. fortis individuals with large bills spend more time on mericarps and extract more seeds than do those with small bills, and they reject mericarps with a lower frequency. Those observed feeding on seeds from Tribulus mericarps were larger, particularly in bill size, than those not observed feeding on them.