My colleague, Rajesh Tandon (pdf) and his students Vineet (lead author) and Chandan have found that in Tecomella undulata (Bignoniaceae) in Rajasthan, India, sunbirds “rob” flowers of nectar i.e., do not pollinate the flowers they drink from, because they are too small to contact the pollen-containing anthers.
Robber and pollinators of T. undulata. (A) The robber (Nectarinia asiatica) consuming nectar by piercing through the corolla tube, and (B) through the opening of the tube. (C & D) The two pollinators – Pycnonotus leucotis and Pycnonotus cafer legitimately forage the flowers and facilitate pollination. Inset of (A): Note the hole (arrow) in the corolla tube made by the robber. doi:10.1371/journal.pone.0102607.g001
BUT, contrary to what one might expect, this does not necessarily entail a loss to the tree–because the ‘true’ pollinators, two species of bulbul, need more nectar than the robbers leave, the pollinators go from flower to flower on the same or different trees–resulting in ensured seed set.
Read the original, cool paper here:
Singh VK, Barman C, Tandon R. 2014. “Nectar Robbing Positively Influences the Reproductive Success of Tecomella undulata (Bignoniaceae)” PLoS ONE DOI: 10.1371/journal.pone.0102607