Hummingbirds are amazing creatures that have evolved specialized adaptations for feeding on the nectar from flowers. Their long, slender beaks and tongues allow them to access nectar that other animals cannot. But how do hummingbirds know if a flower’s nectar is worth visiting? Can they somehow detect poor quality or “bad” nectar? This article will explore the latest scientific research on hummingbird nectar detection abilities.
What is nectar?
Nectar is a sugary liquid produced by plants in flowers to attract pollinators. It is made mostly of three simple sugars – sucrose, glucose, and fructose – in different ratios depending on the plant species. The concentration of sugars in nectar varies from about 5-80%, with an average around 25%. Nectar also contains other substances like amino acids, proteins, vitamins, and secondary plant compounds. The nutritional quality of nectar differs between plant species and even individual flowers based on age and environmental factors. High quality nectar has a balance of sugars, amino acids, and other nutrients that pollinators need. Lower quality nectar may have less total sugar, an imbalance of sugars, or contain toxic secondary metabolites.
How do hummingbirds detect nectar?
Hummingbirds have excellent vision and spatial memory that helps them remember flower locations and track nectar replenishment rates. However, they cannot assess nectar quality at a distance using vision alone. To determine if a flower’s nectar is worth feeding from, hummingbirds use two other senses: taste and smell.
Taste
Hummingbirds have taste receptors on their tongue and in their throat that allow them to detect sugars as well as toxins in nectar. They are sensitive to the concentration and mix of sugars in nectar, preferring higher sugar concentrations up to about 25-35%. This corresponds to the natural range found in hummingbird-pollinated flowers. If nectar does not contain enough sugar, they will often reject it and search for a new flower. Hummingbirds can also taste alkaloids and phenolic compounds that are toxic or unpalatable at high doses. They tend to avoid nectar or flowers containing excess amounts of these compounds. Their taste perception of nectar is so refined that hummingbirds can learn to associate specific taste profiles with certain flower species or locations.
Smell
In addition to taste, hummingbirds use their sense of smell to assess potential nectar meals. Their large olfactory bulbs and high density of olfactory receptors allows them to detect many different compounds in minute concentrations. When approaching a flower, hummingbirds will hover near it and sample the scent emanating from it. They can smell the nectar itself as well as floral scent compounds that may indicate nectar quality. For example, they can smell fermented nectar which contains ethanol alcohol. Rotting or yeasty nectar produces other characteristic scent compounds that hummingbirds likely avoid. By smelling nectar before tasting it, hummingbirds can quickly move between flowers to find the highest quality nectar.
What chemicals do hummingbirds detect in nectar?
Here are some of the key compounds hummingbirds can detect with taste and smell that influence their feeding decisions:
Sugars
Sucrose, glucose, fructose – hummingbirds taste sugar levels and ratios to select flowers with their optimal nectar concentration and sweetness.
Amino acids
Such as proline and alanine – these provide protein nutrition to hummingbirds. Insufficient amino acids may cause rejection.
Ethanol
Produced by yeast fermentation of nectar sugars. Hummingbirds avoid nectar with high ethanol levels.
Phenolics
Includes tannins, flavonoids, anthocyanins – plant defense compounds that deter herbivores. Hummingbirds avoid nectar with high total phenolic content.
Alkaloids
Toxic compounds found in some nectar. Even low amounts cause hummingbirds to reject the nectar. Examples are nicotine and gelsemine.
Terpenes
Volatile organic compounds that hummingbirds can smell. Certain terpenes may indicate lower quality nectar.
Acetic acid
Vinegar-like smell of fermented nectar. Hummingbirds avoid nectar with high acetic acid levels.
How do these chemicals affect nectar quality?
The primary components hummingbirds require in nectar are water and sugars. Amino acids, vitamins, and other nutrients are beneficial but not strictly required. On the other hand, nectar with elevated levels of toxins, fermentation products, or plant defensive compounds is harmful and avoided by hummingbirds. Here is a summary:
| Chemical | Low amount | High amount |
|---|---|---|
| Sugars | Poor nutrition | Optimal nutrition |
| Amino acids | Lower quality | Higher quality |
| Ethanol | Fresh nectar | Fermented, avoided |
| Phenolics | More palatable | Toxic, deterrent |
| Alkaloids | Safe to consume | Toxic, deterrent |
| Acetic acid | Fresh nectar | Fermented, avoided |
How do hummingbirds respond to poor quality nectar?
Hummingbirds have several behavioral strategies to avoid consuming excess amounts of poor quality nectar:
Rejection
Hummingbirds will reject flowers and refuse to feed on nectar that tastes or smells spoiled, fermented, or toxic. They eject the nectar from their beak and search for a new flower.
Early departure
At flowers with slightly inferior nectar, hummingbirds may drink some nectar but leave sooner than if the nectar was high quality. This minimizes their exposure to poor nutrition or toxins.
Traplining
Hummingbirds may regularly visit sets of known flowers, avoiding ones with poor quality nectar in previous visits. This traplining behavior helps them remember flower locations and nectar properties.
Discrimination
Individual hummingbirds can learn to associate visual or olfactory flower cues with nectar quality. They preferentially visit flower species or individuals that provide better nectar.
Yard hopping
In residential areas, hummingbirds rapidly sample feeders and move on if nectar is not optimal, indicating they quickly reject poor solutions.
Nectar robbery
Hummingbirds sometimes chew holes at the flower base to steal nectar, avoiding the flower opening. This allows access to nectar while avoiding potential toxins in other floral tissues.
How do we know hummingbirds can taste and smell chemicals in nectar?
Scientists have conducted controlled experiments that demonstrate hummingbirds detect and respond to nectar chemicals:
Feeding preference tests
By creating artificial nectar solutions with varying sugar content, researchers showed hummingbirds preferentially feed on higher sugar concentrations up to 30-40%.
Color dye assays
Adding harmless red dye to nectar shows that hummingbirds limit feeding more on red dyed solutions, indicating taste aversion.
Behavioral observations
Field studies reveal hummingbirds reject flowers injected with alkaloids or with high ethanol and acetic acid from fermentation.
Electrophysiology
Monitoring nerve signals shows taste receptors in hummingbird brains respond to varying nectar sugar content and specific toxins.
Scent choice tests
Hummingbirds were given a choice between scented and unscented feeders. They showed greater preference for feeders with sweet floral odors, avoiding non-floral or fermented scents.
Genetic studies
Genetic analysis reveals hummingbird taste receptor genes are tuned to detect sugars, toxins, and nutrients in nectar.
Conclusion
In summary, hummingbirds have a suite of behavioral and physiological adaptations allowing them to carefully assess nectar quality before feeding. Using their senses of taste and smell, they can detect levels of sugars, amino acids, fermentation products, and plant secondary compounds in nectar. Hummingbirds preferentially feed on nectar with an optimal sugar concentration and balance of nutrients. If nectar contains toxins or lacks critical nutrients, they reject it and move on to new flowers. Their ability to discern high quality nectar sources helps hummingbirds meet their high energy demands while avoiding toxins or nutrient deficits. Continued study of hummingbird nectar detection will reveal more about their impressive sensory capabilities and coevolution with nectar-producing flowers.
