The nutritional significance of a winter‐flowering succulent for opportunistic avian nectarivores

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 (δ¹³C) 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 δ¹³C 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 δ¹³C 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 δ¹³C 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.