Abstract:
The winter-flowering succulent Aloe marlothii provides nectar for many opportunistic
avian nectarivores in southern African savannas. We assessed the importance of A. marlothii
nectar sugar for opportunistic nectarivores by analysing temporal changes in stable
carbon isotope ratios (d13C) in the tissues of birds in Suikerbosrand Nature Reserve,
South Africa. The blood of the 11 most common non-granivorous opportunistic nectarivores
at our site was enriched in 13C by 3.4 ± 1.5& during the flowering period of
A. marlothii, reflecting the enriched crassulacean acid metabolism (CAM) isotopic
signature of nectar ()12.6 ± 0.5&). This relatively small contribution of A. marlothii
nectar to assimilated carbon in whole blood contrasted with that of exhaled CO2 in African
Red-eyed Bulbuls Pycnonotus nigricans and Cape White-eyes Zosterops capensis. In
both these species, the d13C of breath samples was significantly enriched compared with
blood and feathers, and closely resembled that of the nectar, revealing combustion of
ingested nectar rather than assimilation. Although our analysis was complicated by the
presence of C4 grasses, whose d13C values are similar to those of CAM photosynthesizers,
when considered with previously published feeding observations our data reveal that
opportunistic nectarivores feeding on A. marlothii nectar obtain a relatively small fraction
of their assimilated carbon, but most of their metabolized carbon, from this seasonally
available carbohydrate food resource. Because the d13C values of insects associated with
C3 plants also became enriched during the flowering season, some insect-eating opportunistic
nectarivores may have assimilated A. marlothii carbon indirectly from insects. This
study highlights the importance of understanding isotopic routing when assessing the
nutritional significance of specific dietary items to consumer communities.
