Do floral traits of Strelitzia reginae limit nectar theft by sunbirds?

The shape of flowers frequently corresponds to the morphology of pollinators but some floral traits may also function to prevent non-pollinating flower visitors from stealing flower rewards. Despite the presence of such structures few studies have demonstrated their efficacy in limiting the nectar intake by nectar thieves. The flowers of Strelitzia reginae are regularly visited by sunbirds that do not effect pollination and act solely as nectar thieves. In this species, the nectary is covered by the convoluted bases of the petals (“nectar barriers”). In this study we investigate how non-pollinating sunbirds interact with these nectar barriers and whether nectar barriers play a role in limiting the amount of nectar sunbirds can steal. We quantified the volume of nectar that sunbirds consume while visiting flowers where nectar barriers were present and in flowers where these were experimentally removed. We found that sunbirds consume a median of 106.8 µl of nectar when visiting flowers with nectar barriers present and consumed a significantly greater volume of nectar (median = 158.03 µl) in flowers without nectar barriers. These results suggest that the convoluted petals that cover the nectary of S. reginae may function to reduce nectar theft but are likely to be more effective against insect nectar thieves. This is one of the first studies to quantitatively demonstrate the role of flower features that may function to limit nectar theft.