Sugar type and concentration preference of the Cape white-eye, Zosterops virens
Differences in nectar characteristics of bird-pollinated plants were originally attributed to whether the plant was pollinated by non-passerine hummingbirds or passerine nectarivores. Subsequently this has been refuted and it is now hypothesized that these differences reflect whether the plants are pollinated by occasional or specialist nectarivores (feeding niche hypothesis). The volumetric intake, sugar preference and sugar concentration preference of the Cape white-eye (Zosterops virens), a small occasional nectarivore that also feeds on fruit and insects, was determined. This was done over a range of concentrations of both hexose and sucrose sugars using equicaloric solutions in pairwise choice tests. Birds exhibited no significant sugar type preference at any sugar concentration, but showed a preference for the lowest concentration (10%) of both hexose and sucrose solutions when presented with a range of sugar concentrations. The birds were able to meet energy demands on a dilute diet by adjusting volumetric intake and did not lose body mass. A significant preference for the 10% hexose and sucrose solutions may suggest that these occasional birds exert a selective pressure on the plants they pollinate to produce dilute nectar.
Key words: bird pollination, Cape white-eye, occasional nectarivore, sugar preferences.