We have known for some time that New World Vultures have an excellent sense of smell, as do many seabirds (Procellariformes). This is due in part to their large olfactory bulbs-rounded mass of tissue that contains several types of nerve cells that are involved in the sense of smell. The olfactory bulbs receive information about smells from the nose and send it to the brain by way of the olfactory tracts. The size of the olfactory bulbs in hummingbirds is about half that of seabirds and slightly less than that of Rock Pigeons.
However, hummingbirds’ olfactory bulbs are extremely small. Previous studies indicate that the birds did not have a preference for different smelling flowers. Bird-pollinated flowers generally do not have a strong scent, unlike insect-pollinated flowers which generally do.
In a recent Paper in Behavioral Ecology and Sociobiology entitled What is that smell? Hummingbirds avoid foraging on resources with defensive insect compounds in which the authors present evidence that hummingbirds avoid some flowers with defensive chemicals in order to make foraging decisions and avoid conflict with potentially dangerous insects at a flower or feeder. For their experiments, the researchers allowed more than 100 hummingbirds in the field and laboratory to choose between two feeders, either sugar water alone, or sugar water plus one of several chemicals whose scent signaled the presence of an insect. There were no visual differences between the two feeders offered in each of the experiments.
Tests included the scent deposited on flowers by European honeybees, an attraction chemical secreted by Argentine ants, and formic acid, a defensive compound produced by some Formica ants which is known to harm birds as well as mammals. So it’s not the smell of food, nectar, that attracts hummingbirds, but the presence of defensive chemicals that demonstrate that hummingbirds do have a sense of smell, at least for certain substances.
From Science Daily: “Recent behavioral studies have shown that some bird species use their sense of smell to navigate, forage or even to distinguish individuals. Silke Steiger at the Max Planck Institute for Ornithology and her colleagues chose a genetic approach for their study. Their research focused on the olfactory receptor (OR) genes, which are expressed in sensory neurons within the olfactory epithelium and constitute the molecular basis of the sense of smell. The total number of OR genes in a genome may reflect how many different scents an animal can detect or distinguish. In birds such genetic studies were previously restricted to the chicken, hitherto the only bird for which the full genomic sequence is known. They found considerable differences in OR gene number between the nine bird species. The brown kiwi from New Zealand, for example, has about six times more OR genes than the blue tit or canary.”
So genetic studies may solve this question eventually.