Hummingbirds are often thought of as delicate, tiny creatures that flit from flower to flower. Their small size and high metabolism leads some to believe they may be cold blooded, like reptiles. However, this is not the case – hummingbirds are warm blooded. Let’s take a closer look at the evidence.
What does it mean to be cold blooded?
Cold blooded animals, also known as ectotherms, do not use their metabolism to control their internal body temperature. Their body temperature is largely dependent on external temperatures. When it is warm outside, the animal heats up, and when it is cold, its body cools down. This is in contrast to warm blooded animals, also known as endotherms, which use their metabolism to maintain a stable body temperature regardless of external conditions. Mammals and birds are warm blooded, while reptiles, amphibians, fish, and invertebrates are cold blooded.
Why some believe hummingbirds are cold blooded
There are a few reasons why some people mistakenly think hummingbirds are cold blooded:
- They have a very high metabolism – Hummingbirds have extremely fast metabolisms, with heart rates up to 1,260 beats per minute. Their high energy needs lead some to believe they could not possibly maintain a stable body temperature.
- They are small – Many cold blooded animals are small in size, so some assume hummingbirds’ tiny size means they must also be ectotherms.
- They enter torpor – To conserve energy, hummingbirds can enter a hibernation-like state called torpor. Their metabolic rate and body temperature drops during torpor, similar to some cold blooded creatures.
While these factors fuel the misconception, they do not actually mean hummingbirds are ectotherms.
Evidence that hummingbirds are warm blooded
There is an overwhelming amount of evidence that hummingbirds are in fact warm blooded endotherms:
- Stable body temperature – Hummingbirds maintain an internal body temperature of 104-108°F regardless of external conditions.
- High metabolism – Their metabolisms are many times faster than reptiles and other ectotherms.
- Shivering – Hummingbirds shiver when cold in order to raise their body temperature, something ectotherms cannot do.
- Distribution – Hummingbirds live in many cold climates where ectotherms could not survive.
- Torpor is temporary – Periods of torpor are a temporary reduction in their otherwise consistent high metabolism.
- Warm blooded parents – Hummingbirds are born to parents who are warm blooded.
Scientists who have studied hummingbird physiology and metabolism extensively agree they are definitively endothermic. Their bodies operate like other warm blooded bird species.
Hummingbird metabolism
Hummingbirds have the highest metabolism of all animals relative to their size. Their hearts beat up to 1260 times per minute and they take up to 250 breaths per minute. Their high energy needs require they eat often. Hummingbirds will consume more than their own body weight in nectar each day, often feeding every 10-15 minutes. They need this consistent energy supply to power their wings which beat up to 80 times per second.
This extremely fast metabolism generates a large amount of internal body heat. Even when resting, hummingbirds have higher body temperatures than many other birds. In cold climates, they shiver constantly to maintain their high temperature.
This metabolic rate is similar to small warm blooded mammals and far exceeds that of comparably sized cold blooded animals. Lizards, for example, take only a few breaths per minute compared to a hummingbird’s rapid respiration.
How torpor differs from cold blooded animals
When food is scarce or overnight when flowers are not available, hummingbirds will enter a state of torpor. Their metabolic rate slows down dramatically and body temperature drops to conserve energy. This can minimize their energy needs by 50-95%. Torpor allows hummingbirds to survive periods when their high-energy lifestyle is not sustainable.
Some key differences between torpor and cold blooded animals:
- Torpor is temporary – Hummingbirds enter torpor when resting or overnight, but revert back to their active, high-energy state when awake and feeding.
- Self-induced – Hummingbirds consciously lower their metabolic rate and temperature. Cold blooded animals have no control – their body temperature depends entirely on environmental conditions.
- Energy saving mechanism – Torpor allows hummingbirds to minimize energy needs when food is unavailable. For cold blooded animals, their low metabolic rate is the norm.
So while torpor superficially resembles a cold blooded animal, it is in fact a temporary, voluntary state that hummingbirds use to supplement their normal warm blooded metabolism.
Unique adaptations for a high metabolism
Hummingbirds have many unique adaptations that allow them to sustain their high-energy requirements:
- Extremely light bones – Their bones are thin and hollow to minimize weight and energy needs for flight.
- Enlarged hearts and lungs – Up to 2.5% of their body weight is heart muscle, allowing very rapid blood circulation.
- High oxygen capacity – They have proportionately more red blood cells than other birds to deliver oxygen.
- Low glucose levels – Their blood has the lowest glucose levels of all birds, so sugar is quickly used as fuel.
- High efficiency kidneys – Their kidneys function exceptionally well to process large amounts of sugars and expel excess water.
These adaptations allow hummingbirds to meet their extreme metabolic demands. No cold blooded animal could survive with such high energy needs.
Behavioral adaptations to regulate temperature
Hummingbirds exhibit behaviors to maintain their optimal temperature in different conditions:
- Perching in sunlight to warm up
- Seeking shade when overheated
- Fluffing up feathers to trap body heat
- Panting to cool down
- Flying and feeding more aggressively to raise metabolism and temperature
These conscious behaviors illustrate hummingbirds actively regulating their body temperature, a key characteristic of warm blooded endotherms.
Surviving in cold climates
Hummingbirds are found as far north as Alaska, where temperatures regularly drop below freezing in winter. Yet hummingbirds thrive in these cold climates throughout the year, even breeding there in summer.
Their ability to survive and be active in freezing temperatures would not be possible if they were cold blooded. Reptiles and other ectotherms become completely inactive or cannot survive at low temperatures. The fact that hummingbirds can readily endure cold climates definitively shows they are effective endotherms.
High energy diet
Hummingbirds have a diet ideally suited to power their metabolically expensive lifestyle:
- Nectar – Their primary food source is floral nectar, which contains up to 25% sugar, an efficient fuel.
- Insects and tree sap – Additional protein and fat sources provide nutrients not found in nectar.
- Many small meals – They eat up to their body weight in nectar daily, spread across many quick, small meals.
The frequent consumption of energy-dense sugars and other nutrients sustains their intense metabolic rate. Cold blooded animals do not require this specialized high calorie diet.
Cold blooded characteristics hummingbirds lack
There are many key traits exclusive to cold blooded animals that hummingbirds clearly do not possess:
- No natural enzyme antifreeze – Ectotherms that live in cold climates produce antifreeze to survive.
- No brumation – Reptiles brumate, a winter dormancy similar to hibernation, which hummingbirds do not do.
- No poikilothermy – Hummingbirds maintain a stable internal temperature, unlike poikilotherms whose internal temperature varies.
- No ectothermy – They do not rely on external heat sources to control body temperature.
The absence of these features common in cold blooded creatures provides compelling evidence that hummingbirds are endotherms.
Warm blooded parents
Hummingbirds are altricial birds, meaning they hatch helpless and undeveloped. Their parents are warm blooded, and they could not survive without the care of their endothermic parents keeping them warm. The fact they are incubated and reared by birds with high, stable body temperatures clearly indicates hummingbirds are also endothermic from birth.
Conclusion
In summary, extensive evidence conclusively demonstrates that hummingbirds are warm blooded:
- They maintain a constant high body temperature.
- They have an exceptionally fast metabolism compared to ectotherms.
- They shiver to raise their body temperature.
- They thrive in cold climates.
- Torpor is a temporary reduction in their otherwise high metabolism.
- They have adaptations to enable their high energy lifestyle.
- They exhibit behaviors to regulate their temperature.
- They have a diet with adequate calories to support their needs.
- They lack biological characteristics of ectotherms.
- They are born to warm blooded parents.
While they share a few superficial similarities with cold blooded animals, hummingbirds are definitively endothermic. Their extremely high metabolic rate produces enough internal heat to maintain a constant body temperature required for their hover-feeding lifestyle. From their circulatory systems to their nesting behaviors, every aspect of hummingbird biology reflects their classification as warm blooded.