The effect of fruit abundance, conspicuousness and sugar concentration on fruit colour choice by captive silvereyes

Wild-caught, captive silvereyes (Zosterops lateralis, Zosteropidae) were given choice tests to determine how some traits of fleshy fruits interact to influence fruit selection by this facultative frugivore. When given a choice between red and white fruits of the polymorphic Fragrant saltbush (Rhagodia parabolica, Chenopodiaceae), the silvereyes' preference for the red fruits was enhanced when they were twice as abundant, but reversed when they were only half as abundant, as the white fruits. Given a choice between red and white artificial fruits, silvereyes showed a 2.3-fold preference for the latter when both were equally conspicuous; this preference was significantly enhanced when the white fruits were relatively more conspicuous (greater contrast with the background colour), but eliminated when they were relatively more cryptic (less contrast with the background colour). A third experiment showed that the sugar concentration of artificial fruits influenced the birds' fruit colour preference, enhancing it by a Factor of 8 when the preferred colour was relatively sugar-rich, but eliminating it when the preferred colour was relatively sugar-poor. The results are discussed in the context of current hypotheses about fruit selection processes in birds.