Variability in seed size selection by granivorous passerines: effects of bird size, bird size variability, and ecological plasticity.

The niche variation hypothesis predicts a direct relationship between intraspecific variability in feeding ecology and the variability of the morphological traits related to feeding behaviour. The following study tests this prediction by measuring in captivity the seed size preferences and the morphology of 9-11 individuals of seven specialized granivorous bird species. The average seed size preferences of these birds have previously been shown to be related to components of bill size. The ranges of seed sizes selected were related to the mean bill sizes of birds in a way that paralleled the patterns found when analysing average values. Bill and body size variability were not related, however, to the range of seed preferences after controlling for the significant mean-variance relationship showed by morphological traits. Thus, results do not support the niche variation hypothesis. the significant effect of average bill size on diet variability was consistent with the direct relationship between bird size and ecological plasticity expected on the basis of the shape of the family of functions relating seed size and seed profitability for different-sized birds. These findings suggest morphological mechanisms for ecological plasticity whose generality and evolutionary significance merit further research.