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

The influence of accessibility on the fruit preferences of frugivorous silv ereyes (Zosterops lateralis) was examined in three different contexts: for captive individuals, in a captive group and in a field situation. Individua l silvereyes in small cages were presented with identical artificial fruit that differed only in their accessibility from a perch. The birds removed f ruit that could be obtained by 'picking' rather than by 'reaching up' and a voided 'hanging' to remove fruit. A second experiment tested the response o f 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 sca le on artificial trees. In this experiment, silvereyes again avoided 'hangi ng' 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 mor phology, 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 whi ch the fruit is presented. Properties of the fruiting plant, such as the br anching pattern, perch stability and position of the fruit display, are lik ely to influence fruit preference. These aspects of plant structure should be considered when assessing foraging behaviour and resource use by frugivo rous birds.