Effects of nectar volume and concentration on sugar intake rates of Australian honeyeaters (Meliphagidae)

Abstract

Sugar intake rates of captive Australian honeyeaters (Meliphagidae) feeding at artificial flowers varied across species, and as a function of nectar volume and concentration within each species. Red Wattlebirds (Anthochaera carunculata, 110 g), achieved higher intake rates than New Holland Honeyeaters (Phylidonyris novaehollandiae, 20 g), and both achieved higher rates than Eastern Spinebills (Acanthorhynchus tenuirostris, 10 g). These results reflect differences in bill and tongue dimensions as well as in body mass. Sugar intake rates for all three species increased with volume (5-50 μl) at any given concentration (10-60% mass/mass sucrose). For a given volume, sugar intake rates peaked at intermediate concentrations: 40-50% for the two larger species, and 30-40% for the smallest species. Published studies for other nectarivores foraging at unlimited volume feeders also show optimal nectar concentrations of 30-50%. However, biophysical theory predicts optima at 20-26% for small volumes, and plants presumed to be adapted for bird-pollination often have dilute nectar (20-30%). To explore this discrepancy further, we presented New Holland Honeyeaters with a range of sucrose concentrations (10-50%) using two presentation schemes. In the first we varied concentration but kept volume constant, thus varying gross sugar reward available in each concentration. This gave maximum sugar intake rates at 50%. In the second we varied both volume and concentration so that gross sugar rewards were equal for all solutions, decoupling high concentrations and large sugar rewards. This gave optima at 20%. We argue that variation among plants in nature more closely resembles the latter, "equal sugar presentation" scheme, and therefore, that dilute nectars may indeed represent adaptations for bird pollination.