Hummingbirds are attracted to the tubular flowers of foxglove for their nectar. However, foxglove contains toxins that can be harmful to hummingbirds if consumed in large quantities. In small doses, foxglove nectar poses little risk, but it’s important to be aware of the potential dangers.
Is foxglove poisonous to hummingbirds?
Yes, foxglove contains cardiac glycosides, which can be toxic to hummingbirds. Cardiac glycosides affect the heart muscle and disrupt normal heart rhythm. The two most common glycosides found in foxglove are digoxin and digitoxin.
All parts of the foxglove plant contain these glycosides, including the nectar. The level of toxins varies depending on the species and individual plant. On average, the nectar contains lower concentrations than other parts of the plant.
How does foxglove poison hummingbirds?
When hummingbirds drink nectar from foxglove flowers, they ingest small amounts of cardiac glycosides. The toxin accumulates in the body with repeated exposure.
As levels build up, the glycosides start to disrupt electrolyte balance and interfere with heart muscle contractions. This causes abnormal heart rhythms, which can be fatal if the dose is high enough. Symptoms include irregular heartbeat, tremors, and collapse.
What is a lethal dose for hummingbirds?
It’s difficult to determine an exact lethal dose because individual sensitivity varies. However, scientists have conducted toxicity testing on domestic chickens, who have heart physiology similar to hummingbirds.
One study found the LD50 (median lethal dose) of digitoxin in chickens to be 4.5 mg per kg of body weight. Extrapolating from this, a 3.5 gram hummingbird would likely have an LD50 around 15 micrograms of digitoxin.
The nectar concentration of cardiac glycosides is estimated to be between 1-100 micrograms per gram. So a hummingbird would need to consume a significant amount of foxglove nectar to reach toxic levels.
Do hummingbirds know to avoid poisonous nectar?
Hummingbirds have no innate ability to identify poisonous nectar. However, they do exhibit learned behaviors to avoid unpleasant or harmful food sources:
- Generalists like ruby-throated hummingbirds will sample many flower species and remember which ones provide the richest, tastiest rewards.
- They tend to return more frequently to flowers they’ve had positive experiences with.
- If they encounter unpleasant or bitter-tasting nectar, they spat it out and avoid returning to those flowers in the future.
So while they can’t detect toxins, hummingbirds can learn through trial and error to identify flowers with distasteful or odd-tasting nectar, including foxglove.
Do hummingbirds get addicted to foxglove nectar?
There is no evidence that hummingbirds can become addicted or dependent on the cardiac glycosides in foxglove nectar. The doses they might ingest from typical feeding are far too low to cause a pharmacological addiction.
However, some scientists speculate that hummingbirds purposefully seek out small amounts of these toxins. That’s because cardiac glycosides can act as a mild stimulant. So they may provide a brief energy boost, similar to caffeine in coffee for humans.
This theory remains scientifically unproven though. And any exposure comes with risks, so it’s likely not an intentional strategy by hummingbirds.
How much foxglove nectar is dangerous?
The exact amount that would be toxic is difficult to define. But it’s estimated that a hummingbird would need to consume nectar from dozens or hundreds of foxglove flowers in a short time span for poisoning to occur.
Drinking nectar from a single foxglove flower is highly unlikely to cause problems. And sampling from a few flowers in a garden probably poses little risk as well. It’s repeated, concentrated exposure that increases danger.
Factors that influence toxicity risk:
- Foxglove species – Some species like Digitalis lanata have higher glycoside content.
- Plant parts – Leaves, seeds, and stems are more toxic than nectar.
- Individual variation – Toxin levels differ among plants.
- Quantity consumed – Danger increases with more consumption.
- Body size – Smaller birds are more vulnerable.
- Existing health issues – Pre-existing heart conditions increase sensitivity.
How can you keep hummingbirds safe around foxglove?
A few foxglove plants in a garden are unlikely to pose a major hazard. But you can take some simple steps to reduce any potential risks:
- Plant foxglove away from hummingbird feeders and preferred flowers. This prevents association between the toxic flowers and food sources.
- Provide many nectar-rich, non-toxic flowers in your garden. Giving hummingbirds alternatives reduces their reliance on any one plant.
- Avoid large plantings of foxglove near hummingbird habitat. The more foxglove available means greater exposure.
- Keep foxglove blooms trimmed. Removing spent flowers prevents seed development which can spread foxglove.
- Consider non-toxic digitalis relatives like molinaria, penstemon or agastache.
Signs of foxglove poisoning in hummingbirds
Look for these symptoms if you suspect a hummingbird has consumed toxic amounts of foxglove nectar:
- Difficulty flying or loss of control in flight
- Irregular wingbeat patterns
- Shaking or tremors
- Lethargy, weakness, or immobility
- Rapid breathing
- Collapsing on the ground
A poisoned bird may die quickly or remain ill for hours or days before death. If you witness clear signs of intoxication, call a wildlife rehabilitator immediately.
How to help a poisoned hummingbird
If you find a hummingbird displaying signs of foxglove poisoning, here are some steps you can take:
- Carefully capture the sick bird and place in a ventilated container indoors. Avoid excess stimulation.
- Contact a wildlife rehabilitation center. They may induce vomiting or give activated charcoal to stop further toxin absorption.
- Provide warmth, dark, quiet space while transporting the bird for expert care.
- Supplemental oxygen from a vet may help stabilize breathing.
- Supportive care like fluids and nutrition are used until the toxins clear the body.
- Monitor the bird’s condition closely for 12-24 hours since death can still occur.
With rapid expert care, some poisoned hummingbirds can fully recover. But prevention is really the key to avoiding this life-threatening risk.
Conclusion
Foxglove contains cardiac glycosides that can be toxic to hummingbirds at high doses. However, hummingbirds are unlikely to ingest dangerous quantities through typical flower feeding. The risk primarily comes from concentrated availability near preferred food sources or an abundance of foxglove in prime habitat. Gardeners can take simple precautions to make foxglove less accessible and reduce reliance by providing many alternate nectar sources. While foxglove does pose some hazard, a few plants are unlikely to threaten hummingbirds that can access a diverse diet.
