Adaptation to Frugivory of Mediterranean Avian Seed Dispersers

Abstract

Morphological and digestive correlates of seed dispersal were investigated in 41 bird species of mediterranean scrublands of southern Spain. Seed dispersers (feeding on whole fruits and voiding seeds unharmed) are indistinguishable from nonfrugivores and fruit predators (feeding on pulp or seeds and not performing dispersal) in the ratio of gizzard mass, liver mass, and intestine length to body mass, but differ significantly in bill morphology and average gut passage time (GPT). Seed dispersers tend to be flatter and broader billed than other groups, and have a wider mouth relative to bill width. GPT of seed dispersers is significantly shorter than that of other groups. Morphological correlates of seed dispersers suggest that adaptations for insectivory serve as preadaptations for feeding on whole fruits. Shorter GPT's, in contrast, appear to be an adaptation indispensable to sustained, intense frugivory. Seasonal frugivory imposes limits on commitments to permanent structural modifications, and more subtle functional adaptations are to be expected. In the context of plant—disperser coevolution, unapparent adaptations of seed dispersers to plants are as relevant as the more conspicuous structural adaptations reported for year—round frugivores. In mediterranean scrublands they suffice to maintain a high—efficiency seed dispersal system.