Authors: REY, P., J. and GUTIÉRREZ, J. E.
Published: Volume 44(1), June 1997. Pages 27-39.
Language: Spanish
Original Title: Elección de fruto y conducta de alimentación de aves frugívoras en olivares y acebuchares: una estrategia óptima basada en la razón beneficio/tiempo de manipulación
Keywords: benefit/cost ratio, frugivory, fruit choice, olive orchards, Sylvia atricapilla and Turdus philomelos.
Summary:
This paper focuses on fruit choice criteria by the two most abundant frugivorous birds wintering in olive orchards in southern Spain. We analysed the choice of olive size by Blackcaps Sylvia atricapilla and Song Thrushes Turdus philomelos using both experimental and observational approaches. In the experimental approach, olives were classified in four different size classes and trials were conducted with captive individual birds by offering pairs of fruit sizes. We recorded the number of successful feeding attempts (those in which the whole fruit was swallowed) and the handling time necessary to achieve a successful attempt. For the observational approach, we compared the frequency of occurrence of olive seeds in bird diet samples from olive orchards (large-sized fruits) and from a wild olive shrubland (where olive fruits are considerably smaller). We also compared the frequency of seed sizes found in diet samples with the frequency of occurrence of seed sizes from fruits available in the field. In the trials both bird species choose the smaller fruit class of each paired choice, this tendency being stronger for Blackcaps. Criteria leading to this choice were: a) to maximise the benefit obtained per handling time necessary to achieve success; b) to minimise the ingested seed load. Both in olive orchards and in the wild olive shrubland birds tended to feed on small fruits (at least when fruits were swallowed whole, not pecked). In diet samples from olive orchards, we found a frequent presence of pulp alone, only rarely of seeds, whereas seeds occurred frequently in diet samples from the wild olive shrubland. We hypothesize that this pattern is due to the scarcity of olives small enough to be swallowed whole in olive orchards. This would cause a decrease in the benefit/cost ratio and a subsequent shift to a more profitable feeding behaviour (pecking of the pulp).
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