Abstract:
Nectar-feeding birds generally demonstrate preference for hexose solutions at low sugar
concentrations, switching to sucrose/no preference at higher concentrations. Species vary in
the concentration at which the switch from hexose preference occurs; this could reflect
physiological constraints that would also influence nectar selection when foraging. We
recorded concentration-dependent sugar type preferences in three opportunistic/generalist
Australian nectarivorous species: Dicaeum hirundinaceum, Zosterops lateralis and
Lichenostomus virescens. All three preferred hexoses up to sugar concentrations of 0.25
mol·L-1 and switched to sucrose/no preference for higher concentrations. Using these and
literature records, we investigated physiological mechanisms that may explain the
concentration-dependence of sugar type preferences and compared diet preference data with
foraging records. We measured sucrase activity in Z. lateralis and L. virescens as well as
three specialized nectarivorous species (Anthochaera carunculata, Phylidonyris
novaehollandiae and Trichoglossus haematodus) for comparison with published
concentration-dependent sugar preference data. Sucrase activity varied between these species
(P=0.003). The minimum diet concentration at which birds show no sugar preference was
significantly correlated with sucrase activity for the eleven species analyzed (P=0.005). Birds
with the lowest sucrase activity showed hexose preference at higher diet concentrations and
birds with the greatest sucrase activity either showed no hexose preference or hexose
preference on only the most dilute diets. Foraging data compiled from the literature also
support the laboratory analyses, e.g. T. haematodus (preference for hexose over a wide range
of diet concentrations, low sucrase activity) also feed primarily on hexose nectars in the wild.
Intestinal sucrase activity is likely to contribute to diet selectivity in nectarivorous bird
species.
