Accessibility as a factor influencing frugivory by silvereyes (Zosterops lateralis): field comparisons with aviary experiments

Abstract

The influence of accessibility on the fruit preferences of frugivorous silvereyes (Zosterops lateralis) was examined in three different contexts: for captive individuals, in a captive group and in a field situation. Individual silvereyes in small cages were presented with identical artificial fruit that differed only in their accessibility from a perch. The birds removed fruit that could be obtained by ‘picking’ rather than by ‘reaching up’ and avoided ‘hanging’ to remove fruit. A second experiment tested the response of silvereyes to fruit accessibility in a large aviary where birds fed in a group. The artificial fruit were presented at a larger and more natural scale on artificial trees. In this experiment, silvereyes again avoided ‘hanging’ to obtain fruit; however, these birds showed no preference for ‘picking’ over ‘reaching up’. Foraging observations of silvereyes were recorded for three different plant species in the field. Silvereyes generally avoided ‘hanging’ to remove fruit, although the foraging method used varied with the species of plant on which the bird was foraging. The foraging method used by avian frugivores to remove fruit is likely to be influenced by plant morphology, as well as the morphology of the bird. The strong preference shown by silvereyes for fruit that are more accessible suggests that when other fruit characteristics are equal, accessibility is important in influencing fruit-removal patterns. However, this may be affected by the context in which the fruit is presented. Properties of the fruiting plant, such as the branching pattern, perch stability and position of the fruit display, are likely to influence fruit preference. These aspects of plant structure should be considered when assessing foraging behaviour and resource use by frugivorous birds.