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.
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