1. The response of frugivorous birds to an enlargement of fruit size, and the consequences for both birds and plants, are analysed for the interaction 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 behaviour was assessed by combining field data on frequency of appearance of olive pulp and seeds with data from laboratory trials. 3. Sardinian Warblers (Sylvia melanocephala) and European Robins (Erithacus rubecula) were mainly peckers both on cultivated and wild olives. Blackcaps (Sylvia atricapilla) consumed wild olives mainly by swallowing but consumed cultivated olives (larger than the wild ones) primarily by pecking. Song Thrushes (Turdus philomelos) were primarily swallowers of both types of fruits. 4. Laboratory trials with Song Thrushes, Blackcaps and European Robins showed that: (a) all were able to peck fruits; (b) fruit size determined a shift from swallowing to pecking, as pecking 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’ perspective, small shifts in fruit size may have important consequences on fruit profitability. 5. Pecking on olives turns the mutualistic fruit–frugivore interaction into a fruit‐pulp predator interaction, thus arising a conflict between the plant and frugivorous birds. 6. This study shows that heavy dependence on fruit is not always simply related to seed dispersal. The same frugivorous bird species can act as a seed disperser or a pulp predator for the same plant species. The threshold between these roles is highly influenced by the ratio gape size/fruit size.

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We analyze the effectiveness, biases and limitations of a poorly-known emetic, apomorphine, for studying the diet of granivorous birds. Apomorphine was tested on three Fringillids yielding an effectiveness of 43-78%, comparable to that reported for the most common emetic, tartar. The effectiveness of the emetic varied significantly among species, probably due to different physiological tolerances. Older chicks and adults were equally sensitive to apomorphine, but it was ineffective in young chicks. The action of the chemical was not independent of food type, and certain food items were underrepresented with this method. However, the importance of most food categories closely resembled the one found through dissection. Mortality caused by exposure to apomorphine was low (< 3%) and successive applications of apomorphine proved to be innocuous, thus allowing for long term studies. Yet, the effectiveness of apomorphine decreased when administered repeatedly within a short time. Apomorphine is an advisable tool for laboratory and field work on granivorous passerines because it is effective, practically harmless, provides reliable results, and can be used repeatedly and easily both with nestlings and adults.