The response of captive silvereyes (Zosterops lateralis) to the colour and size of fruit

Abstract

Birds are important consumers and dispersers of the seeds of fleshy fruit and some have been shown to be selective in their choice of fruit. However, our knowledge of how birds respond to a variety of fruit characteristics is poor. Some birds are known to avoid green fruit or consume them less than fruit of other colours. The fruit of many plant species are green when they are unripe and contain low concentrations of sugars and high concentrations of secondary compounds. In this study, captive silvereyes (Zosterops lateralis) were presented with a choice of red, white and green artificial fruit. Half of them were given these fruit with equal sugar concentrations (15%) and the other half were given the choice but with the green fruit having a sugar concentration twice that of the other two colours (30%). Green fruit were not strongly avoided by silvereyes and were actually preferred when they had a higher sugar concentration than the other two fruit types (red and white). Sugar concentration was therefore a more important determinant of fruit choice than colour. Fruit size is also known to affect fruit choice in some bird species. Small fruit are easier to consume than larger fruit, but contain less pulp. To maximise energy gain, birds should consume fruit that are large, but not large enough to incur high handling costs. Silvereyes in this study were presented with a choice between 20 artificial fruit 4 mm in diameter (large fruit) and 20 artificial fruit 2 mm in diameter (small fruit), both of which they are able to consume. In this experiment, silvereyes exhibited a strong preference for large fruit over small fruit. In general, larger fruit contain more pulp and therefore more energy than smaller fruit. However, other fruit traits, such as seed load, are likely to influence fruit choice by silvereyes in the wild and result in a trait hierarchy.