Mystery Solved: How Hummingbirds Taste Sweetness

Have you ever wondered how the hummingbirds that visit your feeder can seem to tell if you use one brand of nectar instead of another? How can they taste the difference if they don’t have tastebuds?

This mystery has been an important area of study for scientists as they are one of a very few species of birds that seek out the sweetest flowers and food sources, rejecting the less sweet varieties in favor of the nectar of others. These birds are some of the most choosy and decisive on the planet. How they determine the sweetest food sources is just now being understood.

Good Genes

According to genetic studies conducted since the first complete bird genome (the chicken) was unraveled within the last decade, birds lost their ability to taste sweetness probably millions of years ago.  The T1R2 gene was identified as the receptor that binds to amino proteins to taste sweetness.This gene is absent in hummingbirds, so how do they do it?

As we see with many instances of adaptation in animals (vertebrates especially), the hummingbird has adapted to thrive on its given food source. The umami receptor which is designed to taste savory and meat (blood), is actually present in hummingbirds but it has been shown that the receptor appears to have adapted to allow them to taste sweetness as well.

Replacing the T1R2 “sweet” gene as the receptor for tasting sweetness certainly did not happen overnight, but is the result of millions of years of evolution. It’s not unusual to find this gene absent, especially in carnivores and omnivores who need the savory and meat taste more than the sweet. The T1R2 gene “breaks” somewhere along the way and disappears throughout subsequent generations.

Scientists studying this phenomena found mutations in the umami receptors in several varieties of hummingbirds, leading them to the hypothesis that those receptors have adapted to receive the sweet taste in those with the mutations. As they unraveled, recreated and studied the genetic evidence (using chickens, swifts and hummingbird DNA), they found 19 DNA mutations that led to the adaptation of the umami receptors. These mutations allows not only the savory but also the sweet taste to be identified in hummingbirds. Maude W. Baldwin co-authored that worked to clone genes for taste receptors from these birds to test how they responded to amino acids and proteins to detect sugar.

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One theory...

One theory of how hummingbirds redeveloped their “sweet tooth” is being widely accepted based on the current genetic research. Ancient hummingbirds probably ate insects, and began to hover around flowers where insects were likely to be found. Over time this led them to the flowers themselves where they developed the mutations to allow them to identify the sweetness of the nectar. As this adaptation became more widespread, behavior in subsequent generations followed the eating of nectar to replace the insects altogether. Hummingbirds developed the mutated genetics that we find today. (Source Material)

An article published in the August 2014 issue of Science Magazine explores the theory in more detail. Studies of the hummingbird’s close relative the Chimney-Swift (an insect-eater), indicated that where the swift and hummingbird species diverged in history, the mutation likely took place. Hummingbirds found themselves in a unique avian niche as nectar-eaters,and propagated that niche with the variety of the species we see today. The fact that there are currently over 300 separate species of hummingbirds leads us to the conclusion the loss of the T1R2 gene is a gain for the hummingbirds who can now taste carbohydrates (sweetness). Hummingbirds require vast resources of carbohydrates to power their energetic flight (they eat their weight in nectar daily), so they’ve adapted the ability to find and identify the flowers that produce the sweetest nectar.  (Source Material) These molecular and genetic studies have been backed up by extensive field research. One study conducted in the Santa Monica Mountains of California and other feeding stations showed that hummingbirds require only a small taste to know if food suits them and is sweet enough. They will often reject food that has been artificially sweetened with Sucralose or aspartame, choosing plain sugar water, pure fructose and pure glucose instead. The behavior of hummingbirds tasting artificial sweeteners or even plain water was observed as “a characteristic behavioral pattern of beak withdrawal, head-shaking and/or spitting”  as they clearly rejected these tastes in favor of the sweeter options. (Source Material)

Over time, as evolutionist Charles Darwin noted, …”taste… in my theory must be acquired by certain food being habitual - (and) hence become hereditary.” Hummingbirds then having acquired the ability to identify those flowers with the tastiest nectar then passed that mutation down through the generations where it has been retained. These birds have honed the ability through the ages to choose the sweetest nectar to fuel their tiny furnaces.

Image by Anne & Saturnino Miranda from Pixabay

Mike Cerio

About Mike Cerio

Mike Cerio founded Sweet-Seed in 2012 with little more than a hunch that other wild bird admirers would, if given the alternative, feed their birds nectars and feeds built around what was being actively sought out by their favorite birds in nature. You can get more from Mike (and Sweet-Seed) on Twitter.