FRUIT SIZE IN WILD OLIVES - IMPLICATIONS FOR AVIAN SEED DISPERSAL

1. The response of frugivorous birds to an enlargement of fruit size, and the consequences for both birds and plants, are analysed for the i nteraction between avian seed dispersers and olives (Olea europaea). 2 . The enlargement of fruit size promotes a shift in frugivorous birds' feeding behaviour, from swallowing fruits whole to pecking pieces of pulp. The relative frequency of olive consumption using each feeding b ehaviour was assessed by combining field data on frequency of appearan ce of olive pulp and seeds with data from laboratory trials. 3. Sardin ian Warblers (Sylvia melanocephala) and European Robins (Erithacus rub ecula) were mainly peckers both on cultivated and wild olives. Blackca ps (Sylvia atricapilla) consumed wild olives mainly by swallowing but consumed cultivated olives (larger than the wild ones) primarily by pe cking. Song Thrushes (Turdus philomelos) were primarily swallowers of both types of fruits. 4. Laboratory trials with Song Thrushes, Blackca ps and European Robins showed that: (a) all were able to peck fruits; (b) fruit size determined a shift from swallowing to pecking, as pecki ng frequency increased with the enlargement of the fruit size; (c) all the species had an increased fruit handling failure rate when trying to swallow increasingly large fruits; and (d) from the birds' perspect ive, small shifts in fruit size may have important consequences on fru it profitability. 5. Pecking on olives turns the mutualistic fruit-fru givore interaction into a fruit-pulp predator interaction, thus arisin g a conflict between the plant and frugivorous birds. 6. This study sh ows that heavy dependence on fruit is not always simply related to see d dispersal. The same frugivorous bird species can act as a seed dispe rser or a pulp predator for the same plant species. The threshold betw een these roles is highly influenced by the ratio gape size/fruit size .