Do experimental methods affect estimates of pollen digestion by birds?

Abstract

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.

The evolution, ecology, and conservation of hummingbirds and their interactions with flowering plants

“…The most common explanations invoke birds' ( i ) high mobility, which may provide long‐distance pollen dispersal and thus higher gene flow (Stiles, 1978a; Cronk & Ojeda, 2008; Krauss et al ., 2017; Pauw, 2019), and ( ii ) reliable flower visitation in areas with flight conditions too cold or wet for insects (Stiles, 1978a; Cronk & Ojeda, 2008), such as high‐elevation cloud forests or deep, shady canyons (Cruden, 1972; Stebbins, 1989; Dalsgaard et al ., 2009). Another hypothesized advantage is reduced pollen loss; unlike bees, which gather pollen to feed their offspring, birds adapted for nectar‐feeding do not visit flowers to consume pollen (Martínez del Rio, 1994; Fleming et al ., 2012) and do not groom pollen grains into pollen‐carrying structures while foraging (Nicolson, 2007b). Thus, birds may transfer more pollen to subsequently visited plants (Thomson & Wilson, 2008; Krauss et al ., 2017), as supported by experiments comparing pollen carryover in hummingbirds versus bumblebees (Waser, 1988; Castellanos, Wilson & Thomson, 2003).…”

Effect of Sweetness Level and Amino Acid Composition of Palm Sugar on Feed Intake of Trichoglossus haematodus in Captivity

“…In the wild, he main protein source in lorikeet diets is pollen (Smith & Lill, 2008). Supplementing their nectar diet with pollen (Fleming & Moore, 2012) insects or spiders is therefore important, especially during seasons when the birds are moulting or breeding (Nicolson & Fleming, 2014). In order to meet the protein requirement for the birds, therefore the palm sugar solution as the primary diet should be added with other food as a protein source.…”

Drinking problems on a ‘simple’ diet: physiological convergence in nectar-feeding birds

“…Nitrogen levels found in most floral nectars are generally insufficient to meet the nitrogen requirements of nectarivorous vertebrates (López-Calleja et al, 2003). Supplementing their nectar diet with pollen (Fleming and Moore, 2012), insects or spiders is therefore important, especially during seasons when the birds are moulting or breeding. A reasonably recent review (Tsahar et al, 2006) concluded that nectarivores and frugivores have nitrogen requirements only ~25% of that of omnivorous birds.…”