Physiology

High Body Temperature

Birds maintain body temperatures around 40oC (human body temp is 37oC).  Higher temps mean faster nerve impulses (enhancing reflexes and reaction time) and increases in the speed and strength of muscle contraction.  Endurance is also enhanced, which allows some birds to fly for long periods of time.

The down-side of maintaining high body temp is that it requires a lot of energy, and the bird risks overheating.  (Temps above 46oC are fatal.)  The demands of high body temps requires efficient respiratory and circulatory systems to deliver the amounts of oxygen and nutrients required - and to remove wastes.

 

Respiratory System 

Bird lungs...

  • tend to be smaller and denser than mammal lungs.
  • allow air through in one direction only (instead of in and out like us)
  • are moved by the sternum, as birds don't have diaphragms

Bird respiration works like this...

  1. The bird inhales through the nostrils in the bill.  (Diving birds and flower-feeders have an operculum [flap] to keep water and pollen out.)
  2. Air passes into the nasal cavity, which has folds, called conchae.  The conchae warm and clean the air.  One section (olfactory tubercle) provides a sense of smell.  A network of nerves and blood vessels helps regulate heat loss.
  3. Air moves down the trachea (windpipe) and into the bronchi, which carry air into the lungs (by way of the posterior air sac).
  4. Inside the lung, the bronchi split into secondary bronchi, which then split into tertiary bronchi (parabronchi).
  5. The parabronchi lead into tiny passageways called air capillaries.  These intertwine with blood capillaries for gas exchange.

Breathing is a four-step process...

  1. The bird inhales - air goes through the primary bronchi to the posterior air sacs.
  2. The bird exhales - air is moved into the lungs, gases are exchanged.
  3. The bird inhales - air is moved from the lungs into the anterior air sacs.
  4. The bird exhales - air is pushed out of the anterior air sacs, up the bronchi, trachea, and out of the bird.

The benefit of this set-up is that there is no residual air left in the lungs during breathing, as there is with mammals.  It also maximizes the amount of surface area - inside the lung - that comes in contact with fresh, oxygen-rich air.  This allows the bird to move more air and absorb more oxygen from the air - so gas exchange is more efficient.

Most birds have 9 air sacs, although it varies from 6 (weavers) to 12 (shorebirds and storks).  Air sacs have thin walls, and extend throughout the body cavity and into the leg and wingbones.  They connect directly to primary and secondary bronchi.  They serve the following functions:

  • Allow a one-way air flow through the bird.
  • Deliver the large amounts of O2 needed to support the bird's metabolism.
  • Remove excess (potentially lethal) body heat during flight.
  • Act as cushions to protect delicate organs during flight.
  • Air pressure from one air sac (interclavicular) is necessary for vocalizing.

During flight, movements of the wishbone (due to flapping) complement the movements of the sternum (which cause breathing - inhaling and exhaling), so that wingbeats reinforce breathing (instead of interfering with it).

Breathing rates vary with size and activity.  Smaller birds breathe faster than larger birds.  Birds breathe faster when active than when resting.

 

Circulatory System 

Bird bodies require large amounts of oxygen and fuel, and generate large amounts of waste.  As a result, birds must have a very efficient circulatory system to transport these materials.

A bird's heart has four chambers, like a mammal's heart.  Bird hearts tend to be larger (up to 41%) than mammal hearts and pump more blood per heartbeat than mammals.  This is because the ventricles are more muscular, empty more completely on contraction, and fill more completely between contractions.

Bird hearts tend to beat slower than mammals while at rest; faster than mammals while active.  Medium sized birds average 220 beats per minute while resting.  Small hummingbirds can have rates as high as 1200 beats per minute.

The heart muscle fibers (cells) are thinner in birds, which allows faster gas exchange and speeds up aerobic respiration.  This gives birds a higher capacity for aerobic work and greater endurance than mammals.

The cost of having such a high-performance heart is high blood pressure.  In domestic turkeys, which are raised on high-fat diets, this often leads to death by rupture of the aorta.

 

We skipped the section on Metabolism.

 

Temperature Regulation 

Endothermy is the ability of an animal to maintain a stable internal temperature.  (We might call this being "warm-blooded".)  Heat is produced as a byproduct of chemical reactions in the body.  Rate of heat produced or lost is measured in watts (or joules per hour).

Bird plumage is one of the best natural, lightweight insulations.  While contour feathers help insulate, most insulation is provided by the down feathers.  Thus, tropical birds have much less down than arctic birds.

Since thicker plumage means more insulation, some birds molt in late summer/early fall so as to have a new coat for winter.  (This replaces the old, worn coat.)  Tropical birds and migratory birds have less need to do this.

Ways in which birds conserve heat include...

  • Fluffing feathers to create pockets of warmed air within the plumage.
  • Tucking the bill under the feathers.
  • Tucking the feet into the feathers of the belly.
  • The Greater Roadrunner raises its back feathers and orients itself so that morning sunlight heats the dark-colored skin underneath.
  • Roosting in places that are sheltered from the wind, such as in holes or burrows.

Ways in which birds enhance heat loss include...

  • Holding the wings out away from the body. 
  • Raising the back feathers to expose bare skin.
  • Panting.
  • Exposing the legs.
  • Wetting the abdomenal feathers periodically.
  • Black feathers concentrate solar heat near the surface, where a slight breeze can carry it away - this prevents the heat from penetrating.  Desert ravens do this, as do the Bedouin tribes of the Sahara - who use black tents and wear black robes.

Model of Endothermy

The model of endothermy states that the thermoneutral zone is the range of temperatures in which a bird does not need to expend extra energy on producing heat or losing heat.  In this range, the amount of oxygen consumed does not change with variations in temperature.

Below the thermoneutral zone, birds use more energy on shivering to raise body temperature.  Above the thermoneutral zone, they use more energy on panting, evaporative cooling, and the fact that cells use more energy at higher temperatures.

 

Responses to Cold Stress

When a bird gets cold, it starts to shiver.  Shivering tenses the muscles, increases oxygen consumption, and generates body heat.  Most of this heat is generated by the flight muscles (pectoralis) and, in some birds, the legs.

Birds that live in different environments have different thermoneutral zones because they are adapted to their particular environment.  As a general rule, smaller birds lose heat faster than larger birds.  This is due to different area-to-volume ratios.

Acclimatization is a set of natural adjustments to seasonal changes in temperature.

Microclimates are small areas where weather/temperature conditions are different from the general climate.  Birds choose microclimates that reduce their heat loss - such as nest holes or in the shelter of evergreen trees.  Some birds, such as grouse and ptarmigan, burrow into the snow for insulation from the cold winter air.

Another strategy for keeping warm is huddling together.  For example, approximately 100 Pygmy Nuthatches were ounce found in a single tree cavity; Inca Doves often sit on top of each other in bird "pyramids"; and male Emperor Penguins huddling together as they incubate their eggs actually gain an extra three weeks of wait time before they need to go to sea to feed.

Hypothermia and Torpor

Most birds' body temps vary a few degrees during the day and drop a bit at night, as a way to save energy.  This drop in body temp below normal is called facultative hypothermia.  Many birds can drop their core body temp by 6oC or slightly more on cold nights - they become mildly hypothermic.

Some birds can drop their body temps a large amount and enter a state of torpor (pronounced hypothermia).  Hummingbirds can drop their body temp to between 8o and 20oC, and the Common Poorwill can drop its temp to 4.3oC.  Birds in a state of torpor do not respond to most stimulus and cannot maintain normal activity, but does regulate its new, lower body temp and oxygen consumption.

Most birds cannot enter a state of torpor.  Only six bird families are known to have this ability:

  • Todies
  • Mousebirds
  • Hummingbirds
  • Swifts
  • Nightjars
  • Pigeons

Facultative hypothermia saves energy supplies when food is scarce or when the bird (such as geese and hummingbirds) need to build up fat reserves for migration.  The cost of this is in warming up the next day.  A hummingbird takes up to an hour to arouse from torpor.  A kestrel would take 12 hours - so torpor is not practical for larger birds.  (The exception is the Common Poorwill, which "hibernates" for 2 to 3 months.)

 

Responses to Heat Stress

The three main ways in which birds respond to too much heat are...

  • Avoidance behaviors, such as resting at midday, finding shade, bathing, and soaring in cooler layers of air.
  • Controlled hyperthermia, which involves raising the body temperature 4o to 6oC above normal.  This lowers the amount of heat absorbed from the environment.  If the body temp rises above air temp, the bird loses heat without needing to use evaporative cooling (thus, saving water).
  • Evaporative cooling, which is the most effective method of losing heat but also is a major source of water loss.  Evaporative cooling can be accomplished by...
    • Panting (evap. from upper respiratory tract)
    • Gular fluttering (rapid vibration of muscles and bones in the throat, this speeds up the rate of evaporation from the mouth/upper throat and supplements panting)
    • Cutaneous water loss (water evaporated directly through the skin - not sweating: birds don't have sweat glands)
    • Defecating directly onto legs to enhance evaporative cooling (storks and New World vultures)

Bergmann's Rule states that body size tends to increase as the temperature of the local climate drops.  In other words:

  • Hot, humid climates favor smaller birds (more surface area compared to volume, so heat can be lost easily)
  • Cool, dry climates favor larger birds (less surface area compared to volume, so heat can be stored easily).

Countercurrent Exchanges

Countercurrent exchange is what happens when warm blood travelling away from the core of the body is used to heat cooler blood returning from the extremities.

At the base of a bird's leg (near the body), veins and arteries intertwine in a network of blood vessels.  Heat from the arteries (which carry blood to the feet) is transferred to the veins (which carry blood back up from the feet).  This can reduce heat loss by 90%.

When the bird is overheated, it can bypass the network and bring warm blood directly to the feet.  In this case, most of the excess heat can be lost through the legs and feet.

Countercurrent heat exchange also takes place in the head, which most birds maintain about 1oC cooler than their bodies.  The wattles of fowl are designed for this, as the birds regulate blood flow into and out of them.

Flight produces enough heat to cause lethal increases in body temperature.  As a result, some birds will not fly at temps above 35oC.  However, flight also increases convective cooling.

 

Digestive System

Birds burn through a lot of energy, so they need to feed frequently to support their active lifestyles.  The structures of their digestive tracts are designed to support this.

Structures such as the tongue and the gizzard vary, depending on the bird's diet.  Since the diet may change with the seasons, the digestive tract may also change size and structure with the seasons (especially in migratory species).

For mammals, digestions starts in the mouth with chewing and the action of saliva.  Birds have no teeth to chew, have few taste buds, and make little saliva.  Instead, the bill may be used for opening seeds or tearing prey, and the esophagus secretes mucus for lubrication.

The path of food through the digestive system is as follows.

  1. Food is taken in through the mouth.
  2. The esophagus carries food from the mouth to the stomach.  Some birds (like fish-eaters) can expand it as needed.  In pigeons, it produces "pigeon milk" - a nutritious fluid for the young.  Many species can inflate it for display or sound resonance.
  3. The crop is an expanded section of the esophagus that stores and softens food, and controls the rate of food passing through.  Crops vary in size and structure by species, depending on the diet.  In some birds, it allows them to swallow a lot of food quickly, then fly to a safer place for digestion.
  4. The proventriculus is the first part of the stomach.  It secretes digestive juices (including stomach acid) to begin digestion.  Some species, such as petrels, uses it to store oily by-products of digestion that are later regurgitated to feed young or as a defense mechanism.
  5. The gizzard is the second half of the stomach.  It is muscular and grinds the food.  The structure of the gizzard varies by species.
  6. The intestine absorbs nutrients into the blood.  Birds use active transport to take in sugars and amino acids, but other nutrients are absorbed by passive transport along with fluids.  This is quick and uses little energy, but toxins can also be absorbed along with the nutrients.
  7. The ceca (sing. cecum) are side pockets off the end of the small intestine that aid in digestion of tough plant material, absorb nutrients, aid in absorbing water, produce antibodies to fight disease, and help convert uric acid into amino acids.
  8. The cloaca is the final holding area for wastes until they are excreted through the vent.

The entire process, from taking in food to letting out waste, may take anywhere from half an hour (fruit and berries) to half a day.

Because bird intestines absorb most nutrients passively with fluids, toxins in fruit and seeds may also be absorbed.  (The process is indiscriminant - there is little or no selection of materials.)  Some birds, such as parrots eat clay grit as an antidote to toxins in the foods they eat.  The negatively charged clay particles bind the the positively charged toxin molecules in the bird's stomach.

Fruits and berries can be digested quickly (as little as 20 min) because they contain "predigested" nutrients: amino acids instead of proteins, simple sugars instead of complex carbohydrates.  There are two main categories of fruits (and fruit-eating birds):

  • Carbohydrate-rich/Lipid-poor - these provide energy, but not a balanced diet (think of them as "junk food").  Birds may need to eat other types of foods along with these berries to get a balanced diet.
  • Lipid-rich/Carbohydrate-poor - these take longer to digest and require different enzymes than the other type of fruit.

Many songbirds cannot digest sucrose - a common, complex sugar.  Eating too much can cause sickness and diarrhea, because the birds cannot break it down and absorb it.  (Hummingbirds, on the other hand, feed on nectar that is high in sucrose - and utilize 90 to 95% of the energy in nectar.)

Waxes are the most difficult food to digest.  Seabirds (petrels and auklets) can digest the waxes in crustaceans - although this requires the food to be cycled back and forth between the small intestine, gizzard, and proventriculus.  Yellow-Rumped Warblers and Tree Swallows eat wax-coated bayberries during times when insects are not available.  Honeyguides eat the wax from bee hives - or from candles in Christian missions in Africa.

 

Energy Balance & Energy Reserves

Energy balance is the relationship between energy intake (feeding) and energy expenditure (activity and life processes).  If intake and expenditure are equal, then the bird does not gain or lose weight - its body mass is stable.  Fat reserves may be increased before winter or before migration.

Foraging time is the amount of time each day that a bird must spend feeding.  This depends on the bird's energy requirements and its rate of energy intake.  For example, Sunbirds spend less time foraging when flowers are producing more nectar.  When nectar production drops off, foraging time increases.  (Humans can decrease bird foraging time by providing food and water at feeders and birdbaths.)

Most birds have only minimal fat reserves, because more fat means higher flight costs and less chance of escaping predators.  Larger birds can go without food (fast) for longer than smaller birds.  (For example, a warbler may only survive one day without food in the wintertime, but a kestrel can survive for five.)  Some birds cache (store) food for future use.  (For example, Acorn Woodpeckers hoard acorns and shrikes impale prey on thorns for later.)

 

Water Economy

Conserving water is just as important as balancing energy expenditures.  Bird's high body temperatures and activity levels cause water to be lost rapidly, especially in dry environments.

There are several source from which birds can replenish their lost body water. 

  • Water present in food.  Birds that eat nectar, fruit, or meat (including insects) can get most or all of their daily water needs met by the water present in their food.
  • Metabolic water is water that is formed when organic molecules are broken down by chemical reactions in the body.  This supplements ingested water sources.  Because of their high metabolism, birds produce more metabolic water than other vertebrates.
  • Countercurrent cooling in the nasal passages and the respiratory pathways can reclaim water that would otherwise be exhaled with each breath.
  • Drinking free water from streams, water holes, dew, snow, etc. is common.  However, visits to water sources (especially in desert environments) can mean increased danger from predators.

Excretory System

Metabolism creates nitrogenous wastes - which can build up and become toxic if they are not removed.  In all vertebrates, kidneys are the organs that filter these wastes out of the blood so that they can be removed from the body.

Bird kidneys are different from mammal kidneys.  Mammal kidneys concentrate nitrogenous waste into a form called urea.  This uses a lot of water to flush the urea out of the body.  Birds are able to form nitrogenous wastes into uric acid, which is more concentrated.  (Uric acid is made up of white crystals, which gives bird droppings their color.)  The comparison of urea to uric acid is...

  • 370 mL of nitrogenous waste, as urea, requires a mammal to excrete 20mL of water.
  • 370 mL of nitrogenous waste, as uric acid, requires a bird to excrete 0.5 - 1.0mL of water.

The waste from a bird's kidneys passes through the ureter into its intestine, where more water can be reabsorbed from the body.

Birds can concentrate uric acid in the cloaca up to 3000 times the acid level in their blood.  Kangaroo rats - the best mammals at conserving water - can only concentrate urea up to 30 times the acid level in their blood.

Hummingbirds, because of their liquid diet, have the problem of taking in too much water.  To combat this, they must excrete a volume of water that is more massive than their bodies each day.  They do this by absorbing sugar and a little water in their intestines, then allowing most of the water they've taken in to pass right back out of the body.

While birds are good at excreting nitrogenous wastes, their kidneys are not efficient at removing excess salt.  Some birds have developed nasal salt glands  to remove the excess salt.  These glands are located in the nasal cavity and filter salt directly out of the blood.  Salt is concentrated in solution (up to 5% salt - compared to 3% for sea water), which then runs down a duct, out the nostril, and along a groove to the tip of the bill where it drips off.  (Some birds, like Storm Petrels, sneeze or blow it out.)

 

Site on "Bird Digestion: Food and Feeding Habits" with video.  Click here.

 

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