Anyone who has seen a hummingbird in action will know that its wings beat so rapidly that their precise movement cannot be discerned by the human eye, blending into a blur as they hover mid-air.

Hummingbirds can tell the difference between nectar and other liquids that don't contain sugar. They only need to lick the liquid three times before they determine that it is not nectar and reject it, pulling away from the substance and spitting it out. They can even tell the difference between true nectar and artificial sugar substitutes.

A new research paper published in the journal Science delves into the way hummingbirds developed their knack for finding sweet foods. The birds' "sweet tooth" comes from a taste receptor that can sense flavor in amino acids. Complicated receptors that have developed over many, many generations of hummingbirds allow them to recognize sugar.

The research team consisted of scientists from all over the world. They also did field work in California and at Harvard University to study hummingbirds.

Hummingbirds live in North and South America. There are over 300 species of hummingbird. The bird has been evolving and expanding their habitat for about 40 to 72 million years.

"It's a really nice example of how a species evolved at a molecular level to adopt a very complex phenotype," said Stephen Liberles, one of the members of the research team. "A change in a single receptor can actually drive a change in behavior and, we propose, can contribute to species diversification."

Liberles is an associate professor of cell biology at Harvard Medical School. He previously published a research paper about the molecular basis of instinctive animal behavior.

The study of hummingbirds was inspired by a previous study of chickens that showed that the birds taste food in a very different way than mammals. It had been assumed that birds had sensory receptors for different tastes the same way that mammals do, with different receptors for savory, salty, bitter and sweet flavors. However, a study of chickens showed that they had no sign of a sweet flavor receptor.

Maude Baldwin, one author of the paper on chickens, wondered what this discovery meant for hummingbirds, who subsist mostly on nectar. Were they, like chickens, missing a sweet receptor? If they were, what did that mean for them? How were they sensing sugar?

The study showed that although hummingbirds have no sweet receptor, their umami, or savory, receptor responds to carbohydrates, or sweet things.

"This is the first time that this umami receptor has ever been shown to respond to carbohydrates," Baldwin said.

They further found that the birds rejected sweet foods that were low in carbohydrate content, like artificial sweetener. This suggests that their taste can actually tell the chemical makeup of a food, not the flavor as humans would perceive it.

This is an interesting look into the sensory systems of birds, and shows how much more we have to understand about the way animals experience the world.