Importance of seed ingestion to an avian frugivore: An experimental approach to fruit choice based on seed load

Frugivorous birds may be able to reduce the cost of processing seeds by discarding seeds, selecting fruits that have a high pulp-to-seed ratio, or by choosing fruit in which seeds are packaged in a way that the frugivore's gut can void them more rapidly. A preference for fruit based on pulp-to-seed ratio or seed composition within a fruit is likely to have important implications for plants and evolution of seed size. We tested whether captive Silvereyes (Zosterops lateralis) discriminate among artificial fruit on the basis of seed presence by presenting birds with artificial fruit with or without a seed. In the first experiment, fruit were translucent so that birds could see which fruit contained a seed. In the second experiment, the visual cue was removed. When Silvereyes were presented with a choice between translucent, artificial fruit with or without a seed, they showed a strong preference for fruit that did not contain a seed. However, when the visual cue to seed presence was removed, preference for seedless fruit was still significant, but markedly reduced. We also tested seed-size preference of Silvereyes in the field in Victoria, Australia. Seeds from a fruit commonly consumed by Silvereyes, fragrant saltbush (Rhagodia parabolica), were recovered from Silvereye faecal samples and their volumes measured. Comparisons were made between seed volumes of fruit consumed by Silvereyes and those within fruit available on the plant. Silvereyes consumed significantly smaller seeds than the mean size available on saltbush plants. When Silvereyes were presented with a cereal-based diet containing artificial seeds (designed to mimic large fruit containing many small seeds), they avoided seed ingestion and were able to consume proportionally more cereal than seeds, even when on a hi-h seed-load diet (30%). Seed dispersal by Silvereyes may be inefficient for plant species with large fruit containing many small seeds, because Silvereyes in this experiment were able to avoid ingesting seeds.