SMELL & TASTE: Preference of hummingbirds for nicotin or benzylacetone
Nicotin 
'Bitter-tasting Nectar and Floral Odors Optimize Outcrossing for Plants' is the title of an article in ScienceDaily (http://www.sciencedaily.com/release/2008/08/080828162552.htm)
“Scientists at the Max Planck Institute for Chemical Ecology, Jena, have discovered that the chemistry of floral scents and nectar enforces good pollinator behavior and allows plants to optimize their ability to exchange gametes with each other.
In a paper published in the journal Science, the sceintists report the results from field experiments with genetically modified wild tobacco plants that show that particular components of the floral fragrance (benzylactone) attracted pollinators, while bitter tasting and poisonous components (nicotine) of floral nectar enforced modest drinking behavior.
Conclusion: Floral visitors are principally attracted by the scent, and they drink more nectar if it doesn't taste bitter. Using video cameras, the researchers confirmed this result: Both attractant-deficient lines were rarely visited by hummingbirds (e.g. Archilochus alexandri) and white-lined sphinx moths (Hyles lineala).
When visitors took nectar from flowers which contained a natural amount of deterring nicotine, they stayed for a short time only, while they enjoyed the nicotine-free nectar of corresponding trangeneic lines.
Consequently, these flowers were visited for longer periods, especially by hummingbirds.