Hummingbirds have a very unique respiratory system that allows them to meet their high metabolic demands. Their breathing and gas exchange strategies are adapted specifically for hovering flight and feeding on nectar.
Overview of Hummingbird Respiration
Hummingbirds, along with swifts and some insects, are among the only animals capable of sustained hovering flight. Hovering flight requires an immense amount of energy and therefore oxygen. To meet these metabolic demands, hummingbirds have evolved a respiratory system capable of delivering oxygen at very high rates.
Some key features of hummingbird respiration include:
- Very high breathing and metabolic rates – Hummingbirds take around 250 breaths per minute while at rest. During flight, their breathing rate can reach 500 breaths per minute. Their metabolic rate is the highest of all animals relative to their size.
- Highly effective air sac system – Hummingbirds have a system of nine interconnecting air sacs that function to keep oxygenated and deoxygenated air completely separated during both inhalation and exhalation. This allows them to take in fresh oxygen during each rapid breath.
- Thin, stiff lungs – A hummingbird’s lungs are small and do not flex like mammalian lungs. They are shaped like elongated bags that do not change volume during respiration.
- Extensive capillary beds – Hummingbirds have very dense networks of capillaries surrounding their air sacs and lungs to facilitate rapid gas exchange of oxygen and carbon dioxide.
- Countercurrent heat exchange – Cooler incoming air and warmer outgoing air pass closely parallel in the birds’ trachea, allowing heat exchange and reduction in water loss.
Together, these adaptations allow hummingbirds to meet the respiratory demands of hover-feeding on nectar, a lifestyle with one of the highest mass-specific metabolic rates of any animal.
Breathing Rate and Metabolism
One of the most astounding features of hummingbird respiration is their extremely high breathing rate, even at rest. When perched, a hummingbird takes around 250 breaths per minute. This is more than 50 times higher than the average human breathing rate. There are only a few other animals, like some rodents, that take over 200 breaths per minute at rest.
During the energetic hovering needed to feed, a hummingbird’s breathing rate can reach an average of 500 breaths per minute. However, breathing rates have been measured at up to 1260 breaths per minute during maximum exertion. This is among the highest breathing rates ever recorded in vertebrates.
Coupled with this rapid breathing is an extremely high metabolism. Hummingbirds have the highest metabolic rate relative to their size of any animal. At rest, their metabolism is about 10 times higher than expected based on body mass alone. During hovering flight, their metabolic rate can reach up to 34 times the expected level. This immense energy requirement is fueled almost exclusively by nectar sugars.
The rapid breathing provides the oxygen needed to metabolize energy at these extreme rates. Hummingbirds also take relatively large tidal volumes with each breath. This combination allows their respiratory system to deliver oxygen at rates 20 times higher per gram of tissue than in elite human athletes. Overall, hummingbirds have the highest mass-specific rates of gas exchange ever measured.
Air Sacs and Parabronchial Lung Structure
In order to deliver oxygen at such high rates, hummingbirds have evolved a uniquely structured respiratory system. One of the main adaptations is their elaborate system of nine interconnecting air sacs.
Air sacs are thin-walled chambers that connect to the lungs. In birds, parabronchi (the small tubes that make up the lungs) open into air sacs rather than a single cavity like in mammal lungs. Hummingbirds possess seven cervical (neck) air sacs and two abdominal air sacs.
This arrangement allows hummingbirds to completely separate oxygenated air coming in during inhalation from deoxygenated air going out during exhalation. Even with their rapid breathing rate, they take in a full fresh breath of air with each inspiration. Bird air sacs also aid in respiration by keeping the lungs continuously inflated.
The structure of hummingbird lungs contributes to their respiratory efficiency. Their lungs are small, elongated, tube-like structures made up of several pairs of parabronchi. They do not change volume during breathing since the parabronchi open into air sacs rather than a central cavity. The lung wall is extremely thin but stiffened by bone plates called trabeculae.
Having small, rigid lungs allows them to breathe very rapidly without the elastic resistance of expanding and relaxing a soft lung tissue. Hummingbirds also tend to have fewer parabronchi compared to other bird species, reducing dead space and tracheal volume.
Table 1. Comparison of hummingbird and human respiratory structures
| Respiratory Structure | Hummingbird | Human |
|---|---|---|
| Lungs | Small, rigid, do not change volume during breathing | Large, soft, expand and relax during breathing |
| Air sacs | 9 interconnecting cervical and abdominal air sacs | None |
| Breathing rate at rest | ~250 breaths per minute | 12-18 breaths per minute |
Respiratory Surface Area
In order to facilitate the extremely high rates of oxygen uptake and carbon dioxide release needed during hovering flight, hummingbirds have evolved to greatly increase the respiratory surface area inside their lungs.
The interior of the parabronchi are lined with numerous thin membranes stacked closely together like the pages of a book. These are called the paraepithelial surfaces. A dense network of microscopic blood capillaries, called the parabronchial capillary network, closely surrounds the paraepithelial surfaces.
The large combined paraepithelial and parabronchial surface area provides an immense area for gas exchange between the tiny air capillaries and tiny blood capillaries. This allows the rapid transfer of respiratory gases needed to support their unique lifestyle. The density of their respiratory surface area is among the highest of any animal.
Countercurrent Heat Exchange
Hovering flight comes at an energetic cost partly due to the challenge of thermoregulation. Hummingbirds can lose heat rapidly due to their small size and high surface area to volume ratio. Developing heat through muscle exertion helps counterbalance this heat loss.
As an additional adaptation, hummingbirds exchange heat between inhaled and exhaled air. As inhaled air travels down the trachea towards the lungs, it passes very close to adjacent exhaled air traveling in the opposite direction towards the mouth or nostrils. This countercurrent arrangement allows heat exchange.
The cool inhaled air absorbs heat from the warmer exhaled air. This helps conserve heat and reduces respiratory water loss, allowing hummingbirds to maintain their high metabolism and activity levels.
Oxygen Binding Proteins in Blood
Once oxygen is transferred across the respiratory surface of the lungs, it needs to be carried through the bloodstream and delivered to tissues. Hummingbirds have adaptations to enhance this oxygen transport as well.
Their blood has a relatively high hemoglobin concentration, the protein that binds oxygen in red blood cells. The hemoglobin itself also has a high binding affinity for oxygen. This helps maximize oxygen loading in the lungs so it can be efficiently delivered to metabolizing muscles.
Hummingbirds and other small birds lack a specialized high-oxygen affinity hemoglobin found in mammals (called myoglobin) in their thigh muscles. However, in compensation, they have a large total volume of capillaries surrounding each muscle fiber. This ensures rapid delivery of oxygen even without myoglobin’s oxygen storage capability.
Respiratory Control
The extreme elevation in metabolism and gas exchange during hover-feeding implies a parallel increase in respiratory control and oxygen sensing. However, the mechanisms of this sensory and muscular control are not entirely understood.
Hummingbirds appear to lack carotid bodies, sensory organs near the carotid arteries that detect blood oxygen levels. Instead, they may rely on widespread oxygen sensory cells found in avian air sacs and lungs. The pattern of breathing during flight also suggests a role for sensory input from the wings and chest muscles.
Rapid breathing in hummingbirds is controlled by their highly modified vertebrate breathing muscles. These include hypaxial muscles around their spine and abdomen. They also have elastic connective tissues that can act as springs to store energy and drive rapid breathing cycles. The structural adaptations in their lungs also likely reduce resistance and allow rapid airflow.
Unique Respiratory Adaptations
In summary, hummingbirds have a respiratory system exquisitely adapted for hovering flight and feeding on flower nectar. Their small yet highly-efficient lungs, expansive air sac system, enhanced gas exchange surfaces, and heat exchange mechanisms allow an oxygen consumption rate unmatched by any other animal relative to their tiny body size.
This respiratory system powers their unique lifestyle characterized by sustained metabolic expenditures up to 20 times higher than equivalent sized mammals. Hummingbirds provide a fascinating example of evolution shaping the form and function of the respiratory system to meet extreme ecological demands.
Key Adaptations of Hummingbird Respiration
- Extremely high breathing rate – up to 500 breaths/min during hovering
- Elaborate system of 9 air sacs to enhance oxygen intake
- Small, rigid lungs with thin gas exchange membranes
- Dense network of capillaries in lungs for gas exchange
- Countercurrent heat exchange between inhaled and exhaled air
- High oxygen binding affinity of hemoglobin
These specializations give hummingbirds the highest mass-specific metabolic rate of any animal, fueled almost exclusively by the sugars in flower nectar.
Conclusion
In conclusion, hummingbirds have a uniquely adapted respiratory system that provides them with the oxygen needed to hover and feed. Their rapid breathing, air sacs, enhanced gas exchange surfaces, and heat recycling allow them to support record high metabolic rates. Hummingbird respiration provides a fascinating example of evolution optimizing the breathing system to facilitate an extreme lifestyle.
