Do experimental methods affect estimates of pollen digestion by birds?

Pollen protoplasts may supply important nutritional resources for birds; however, they are locked up within the mechanically strong and biochemically complex pollen wall. Previous studies of pollen digestion in birds have yielded highly variable and often contradictory results. We tested whether these differences could reflect the vastly different methodologies that have been used. We used a standard method to investigate digestion of Banksia grandis (Proteaceae) pollen in New Holland honeyeaters (Phylidonyris novaehollandiae). Four types of B. grandis pollen were examined: fresh, frozen for a week, stored in conditions to stimulate pregermination for 24 h, or collected by honeybees. Our data indicate that although pollen treatment may influence digestibility of the pollen grains, these differences do not reach statistical significance because they are dwarfed by a high degree of variability between birds fed the same diet as well as variability in gut transit time (generally more pollen grains were digested over longer transit times). Similar patterns were observed for red wattlebirds (Anthochaera carunculata) fed bee-collected pollen. We believe that feeding behaviour or gut transit time may explain the marked differences between previous studies of pollen digestion by nectarivores, particularly the conflicting results for New Holland honeyeaters.