How Do Birds Breathe? Air Sacs, Lungs and One-Way Airflow

Birds breathe with small, rigid lungs and a set of thin-walled air sacs, usually nine, that work like bellows. The sacs push air through the lungs in one direction, so fresh air crosses the gas-exchange surfaces on both inhale and exhale. Birds have no diaphragm, and avian veterinarians note that every breath is an active muscle movement.

That system is unlike ours. A single breath of air takes two full breathing cycles to travel through it, and birds move air by moving the ribs and breastbone.

This arrangement is one of the most efficient breathing systems among vertebrates, and it helps explain how birds can sustain flight and how some species cross the highest mountains on Earth.

Quick answer: how do birds breathe?

Birds breathe in through the nostrils at the base of the bill, and muscles moving the ribs and breastbone expand and compress the air sacs. The sacs pump air one way through stiff lungs full of tiny tubes, where oxygen enters the blood. Each breath takes two inhale and exhale cycles to pass all the way through.

• Air sacs do the pumping. Balloon-like sacs in the chest and abdomen expand and contract to move air.
• Lungs stay nearly still. Bird lungs are compact and attached to the ribs, so they barely change volume.
• Air flows one way. Inside the lungs, air moves through tiny tubes called parabronchi in a single direction.
• Two cycles per breath. A given breath of air is inhaled, moved through the lungs and exhaled over two inhale and exhale cycles.
• No diaphragm. Muscles move the ribs and breastbone to expand and compress the body cavity.
• Cross-current exchange. Blood flows across the air stream in a way that extracts oxygen efficiently.

What are the parts of a bird’s respiratory system?

A bird’s respiratory system has four main parts: the nostrils and trachea that carry air in, the syrinx where the airway splits and sound is made, two small, rigid lungs where gas exchange happens, and a set of usually nine air sacs that store and pump air. Some air sacs even extend into the bones.

How is air taken in?

Air enters through the nostrils, which birds call nares, usually located at the base of the upper bill, or through the mouth. It passes down the trachea, the windpipe, which in birds is supported by complete rings of cartilage or bone. At the bottom of the trachea sits the syrinx, the voice box of birds, where the airway splits into two primary bronchi, one for each lung.

Birds often have a relatively long trachea for their size. Some cranes and swans have tracheas that loop inside the breastbone, which likely affects the resonance of their calls. A longer windpipe adds “dead space” that holds air without exchanging it. Birds compensate in part with a wider trachea and by breathing more deeply.

What do the lungs look like?

Bird lungs are small, dense and spongy, tucked against the ribs and spine along the back of the body cavity. Unlike mammal lungs, which inflate and deflate, they change volume very little. They are made of many parallel tubes called parabronchi. Branching off each parabronchus is a mesh of very fine air capillaries, intertwined with blood capillaries. That is where oxygen enters the blood and carbon dioxide leaves.

The barrier between air and blood in birds is extremely thin, thinner on average than in mammals, which helps gas move quickly.

What are the air sacs?

Air sacs are thin, transparent, balloon-like structures that fill much of the body cavity. Most birds have nine:

The sacs have few blood vessels and play little direct role in gas exchange. Their job is to store and move air. Some of them extend into bones, filling them with air, as explained in do birds have hollow bones.

How does air move through a bird’s lungs?

Air moves through a bird’s lungs in one direction only, from back to front. Fresh air goes first into the rear air sacs, then through the lungs on the next exhale, then into the front air sacs, and finally out. A veterinary review in dvm360 calls it a two-breath system, because each breath needs two cycles.

This is the heart of the story. Follow one breath of air through the system:

1. First inhale. The bird expands its body cavity. The new air travels down the trachea and mostly bypasses the lung, flowing into the rear air sacs (the caudal thoracic and abdominal sacs).
2. First exhale. The body cavity compresses. The rear sacs push that fresh air forward into the lungs, where it flows through the parabronchi and oxygen is picked up by the blood.
3. Second inhale. As the bird inhales again, the now oxygen-poor air moves from the lungs into the front air sacs (the cervical, clavicular and cranial thoracic sacs). At the same time, a new breath fills the rear sacs.
4. Second exhale. The front sacs push the used air out through the trachea, while the rear sacs send the new breath through the lungs.

In practice, all four steps overlap. On every inhale and every exhale, air is flowing through the lungs in the same direction. That is why bird lungs receive fresh air almost continuously.

How does air “know” which way to go?

There are no valves in the bird lung. The one-way flow comes from the shape and angles of the airways, which steer air along particular paths depending on pressure, a mechanism researchers call aerodynamic valving. Studies with airflow measurements and models have supported this explanation.

How is this different from human breathing?

Mammal lungs end in millions of tiny dead-end sacs called alveoli. Air goes in and comes back out the same way, so some stale air always remains and mixes with fresh air. Birds avoid this problem because air passes through the lungs rather than into dead ends.

Why is the bird lung so efficient?

The bird lung is efficient for three reasons. Air flows through it in one direction, so there is no stale air left in dead-end sacs. The barrier between air and blood is extremely thin. And blood vessels cross the airflow at an angle, a cross-current arrangement that lets the blood pick up more oxygen than tidal breathing allows.

What is cross-current exchange?

In the bird lung, blood capillaries run across the parabronchi at an angle to the airflow. As air moves along a parabronchus, it meets a series of blood vessels, each picking up some oxygen. Because each blood vessel meets air of slightly different oxygen content, the blood leaving the lung can end up with a higher oxygen level than the air being exhaled. This cross-current arrangement is generally considered more effective than the mammalian arrangement, although fish gills, with their countercurrent flow, are more efficient still.

Why does flight need such good lungs?

Flapping flight is one of the most energetically demanding activities in the animal world. Flight muscles need a steady, high supply of oxygen. The combination of one-way airflow, a very thin gas barrier and cross-current exchange helps birds meet that demand. Hummingbirds, which hover with very rapid wingbeats, have among the highest metabolic rates of any vertebrates. For more, see how hummingbirds hover. This one-way airflow is part of what powers the fastest birds on Earth, from swifts to Peregrine Falcons.

How do birds breathe at high altitude?

Birds cope with thin air through efficient one-way lungs plus extra adaptations. Bar-headed Geese, for example, have hemoglobin that binds oxygen more strongly than that of lowland geese and breathe deeply. A US Geological Survey summary of a 2011 tracking study reports that they climb between 4,000 and 6,000 meters in 7 to 8 hours to cross the Himalayas.

How high can birds fly?

Some birds fly remarkably high. Bar-headed Geese migrate across the Himalayas between Central Asia and South Asia. A 2011 study in PNAS used satellite tracking to follow their crossings and found they climbed to elevations of around 6,000 meters (roughly 20,000 feet), often flying at night and early morning, when air is colder and denser. Anecdotal reports of them flying over the highest peaks are harder to confirm.

The most famous high-altitude record belongs to a Rüppell’s Vulture that collided with an airliner over Abidjan, Ivory Coast, on 29 November 1973 at an altitude Guinness World Records gives as 11,300 meters (37,000 feet). It is a single event, but it shows that some birds can reach extreme altitudes.

What makes high-altitude flight possible?

The bird lung’s efficiency is part of it, but several other adaptations contribute. Bar-headed Geese have hemoglobin with a higher affinity for oxygen than that of lowland geese, larger lungs relative to body size than many other waterfowl, and deep, efficient breathing. Birds also tolerate low carbon dioxide levels in the blood better than mammals, which allows them to breathe hard at altitude without the dizziness a person would suffer.

Do other animals breathe like birds?

Some do. One-way airflow in the lungs is not unique to birds: Farmer and Sanders reported it in American Alligators in 2010, and similar findings followed in some lizards. Alligators lack bird-style air sacs, which suggests one-way airflow evolved before birds, possibly in a shared ancestor of crocodilians and dinosaurs.

Birds are not the only animals with one-way airflow in their lungs. A 2010 study in Science by Farmer and Sanders found unidirectional airflow in the lungs of American Alligators, even though alligators do not have air sacs like birds. Similar findings have since been reported in some lizards. These discoveries suggest that one-way airflow evolved earlier than birds, possibly in a common ancestor of crocodilians and dinosaurs, and was later refined in the bird lineage.

Fossil evidence from dinosaur bones supports this history. Many theropod dinosaurs had air-filled vertebrae with openings that match the positions of bird air sacs.

How do birds use breathing for other jobs?

Birds use their breathing system for much more than oxygen. Air pushed from the air sacs through the syrinx powers song, and many songbirds take quick mini-breaths between notes. Panting and gular fluttering cool birds on hot days, since they cannot sweat. In winter, the nasal passages recover heat and moisture from outgoing air.

How does breathing help birds sing?

Bird song is powered by the respiratory system. Air pushed from the air sacs flows through the syrinx, where membranes vibrate to produce sound. Because the syrinx sits where the airway divides, many songbirds can control each side separately, producing two notes at once. Songbirds also take quick “mini-breaths” between song notes, allowing long, complex songs without running out of air. Read more in how birds learn songs.

How does breathing help birds keep cool?

Birds do not sweat. On hot days, they pant with an open bill, which increases evaporative cooling from the moist surfaces of the airway. Many species also use gular fluttering, rapidly vibrating the thin skin of the throat to increase evaporation. The air sacs may add to cooling by increasing the moist surface area exposed to moving air.

How does breathing affect warmth in winter?

In cold weather, the nasal passages of many birds recover some heat and moisture from outgoing air before it leaves the body. Combined with fluffed feathers and lowered body temperature at night, this reduces heat loss. See how birds stay warm in winter.

How fast do birds breathe?

Bird breathing rate depends mainly on body size and activity. Small birds breathe much faster than large ones, so a resting songbird takes many more breaths per minute than a swan. In flight, rate and depth rise sharply, and in many species breaths are linked to wingbeats. Exact numbers vary too widely to quote one figure for all birds.

Breathing rate depends mainly on body size and activity. As a general rule, small birds breathe much faster than large ones, just as a mouse breathes faster than an elephant. A resting songbird takes many more breaths per minute than a swan or an ostrich, which breathe slowly and deeply. Exact figures vary widely with species, temperature and stress, so be cautious with any single number quoted for “birds” in general.

Activity changes everything. In flight, breathing rate and depth rise sharply to meet the demands of the flight muscles. In many species studied, breathing is linked to the wingbeat cycle, for example one breath per one or several wingbeats, although the pattern differs between species and flight styles. The mechanical link makes sense: the flight muscles and breastbone move with every wingbeat, and the same structures pump the air sacs.

At rest and in sleep, breathing slows. On cold nights, some small birds lower their body temperature and metabolic rate, and breathing slows further to save energy.

How do diving birds hold their breath?

Diving birds hold their breath by storing extra oxygen in blood and muscle, slowing the heart during dives and managing the air in their plumage and air sacs. Emperor Penguins are the champions: the Australian Antarctic Division has recorded a dive to 565 meters and two dives that lasted 22 minutes, while typical dives last three to six minutes.

Diving birds face the opposite problem from high-flying geese: they must go without breathing at all. Penguins, loons, cormorants, auks and diving ducks share several adaptations:

• Oxygen stored in blood and muscle. Diving birds tend to have high levels of hemoglobin and myoglobin, the proteins that hold oxygen in blood and muscle. Their dark red flight or swimming muscles are a sign of rich myoglobin.
• Slowed heart rate. During dives, many birds show a dive response, slowing the heart and redirecting blood toward the brain and heart.
• Managed air volume. Some divers exhale before or during dives, or compress their plumage to reduce buoyancy from trapped air.

Emperor Penguins are the deep-diving champions among birds, with a recorded dive to 565 meters and dives lasting 22 minutes, according to the Australian Antarctic Division. Most backyard birds never need such adaptations, but ducks on a local pond show a milder version every time they dive for food.

Can birds get respiratory infections?

Yes, and several are relevant to people who feed birds. The air sacs and lungs can be infected by fungi such as Aspergillus, which grows in damp, moldy seed, by viruses such as avian influenza, and by Mycoplasma bacteria, one strain of which causes the crusty eye disease seen in House Finches. Clean feeders and dry seed lower the risk.

• Aspergillosis is a fungal infection caused by Aspergillus molds that grow in damp, moldy seed, bedding or hay. Spores lodge in the air sacs and lungs, where they can cause serious disease. Wet, clumped seed in feeders is a risk.
• Avian influenza is a viral disease that can affect the respiratory system, among others. Wild waterfowl, poultry and some raptors and scavengers are most affected, but feeder birds can be too. See our guide on bird flu and backyard feeders for current advice.
• Mycoplasma bacteria cause respiratory disease in poultry, and one strain causes the swollen, crusty eyes seen in House Finches at feeders in North America.

Clean feeders, fresh seed and dry storage reduce these risks.

Do birds breathe with lungs?

Yes. Birds breathe with two lungs, but they work differently from ours. Bird lungs are small, dense and fixed against the ribs and spine, so they barely change size. The air sacs do the pumping, and the lungs are where oxygen actually passes into the blood, inside tiny tubes called parabronchi and the air capillaries branching off them.

This division of labor is the key difference from mammals. In people, the lungs both pump air and exchange gas, inflating and deflating with every breath. In birds, the air sacs act as bellows and the lungs act purely as the gas exchanger. That is also why the myth that bird lungs inflate like balloons is wrong: it is the air sacs that inflate.

Do birds breathe through their mouth?

Birds normally breathe through their nostrils, called nares, which sit at the base of the upper bill. They can also breathe through the open bill, for example when panting to cool down. In a resting bird, though, open-mouth breathing is a warning sign: avian veterinarians list it with tail bobbing as a sign of breathing difficulty.

Air entering the nares passes through the nasal cavity, where it is warmed, moistened and filtered, before reaching the trachea. If a backyard bird sits fluffed up with its bill open on a mild day, see our guide to sick birds at the feeder.

Myths and facts about how birds breathe

What does this mean for backyard birders and bird owners?

Understanding bird breathing has direct, practical uses. Because birds pump air with their ribs and breastbone, holding one tightly around the chest can stop it breathing. Tail bobbing and open-mouth breathing are signs of breathing trouble, and efficient lungs make birds especially vulnerable to fumes, sprays and moldy seed.

1. Never squeeze a bird’s body. Because birds need to move their ribs and breastbone to breathe, holding a bird tightly around the chest can suffocate it. If you must move an injured bird, lift it gently in a towel and place it in a ventilated box. Our guides on what to do with an injured bird and finding a baby bird cover safe handling.
2. Watch for labored breathing. A bird at your feeder breathing with its tail bobbing heavily, gaping or puffed up for long periods may be sick. See sick birds at the feeder for what to do.
3. Be careful with fumes indoors. Pet birds are vulnerable to fumes from overheated nonstick cookware, aerosol sprays, scented candles and smoke. Avian vets widely advise keeping birds well away from kitchens.
4. Keep feeders clean. Moldy seed can harbor fungi such as Aspergillus, which can infect a bird’s air sacs and lungs. Store seed dry and clean feeders regularly.
5. Avoid pesticides near feeders. Airborne sprays can harm birds quickly because of how efficiently they breathe.

Quick facts

• Birds have small, rigid lungs and usually nine air sacs.
• Air flows in one direction through the bird lung.
• A single breath takes two full breathing cycles to move through the system.
• Birds have no diaphragm and breathe by moving the ribs and breastbone.
• Cross-current exchange helps birds extract oxygen efficiently.
• Bar-headed Geese have been tracked flying at about 6,000 meters over the Himalayas.
• Alligators also have one-way airflow in their lungs.
• Efficient breathing makes birds sensitive to airborne toxins, the reason canaries were used in coal mines.

Sources

• Farmer and Sanders (2010), Unidirectional airflow in the lungs of alligators, Science
• Hawkes et al. (2011), The trans-Himalayan flights of bar-headed geese, PNAS
• O’Connor and Claessens (2005), Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs, Nature
• Cornell Lab of Ornithology, All About Birds
• dvm360: Avian respiratory medicine (Proceedings)
• US Geological Survey: The trans-Himalayan flights of bar-headed geese
• Guinness World Records: Highest flying birds
• Australian Antarctic Division: How deep can emperor penguins dive?