Avian Physiology: Metabolism and Adaptation

Avian Physiology Overview

• Birds consume more food and oxygen per weight than other vertebrates.
• Birds can move more rapidly and generate more heat than other vertebrates.
• Avian physiology involves metabolism and excretion, supporting daily activities and environmental adaptation.
• Adaptations related to heat loss, water economy, and heat conservation enable avian survival.
• Homeostasis is achieved via consistent internal environment maintenance.
• Avian physiology includes metabolism, temperature regulation, feeding/digestion, and water economy for physiological balance.
• Aerobic metabolism is sustained by a respiratory and circulatory system. Exchange of heat with the environment for energy conservation happens in the cold, and heat loss happens in the heat.
• Specialized digestive systems gather energy and nutrition.
• Water reserves provide evaporative cooling and electrolyte excretion.
• Further avian physiology features include migration, reproduction, stress response, disease resistance, and hormonal controls.

The High Body Temperature of Birds

• The high body temperature of birds relates to metabolism and endothermy.
• Most birds maintain about 40°C core body temperature, exceeding that of the surrounding air.
• High body temperatures enhance reflexes and enable fast movement.
• Nerve impulse speeds increase 1.8x per 10°C temperature increase, and muscle contractions triple with each 10°C rise.
• Birds have the highest body temperatures and metabolic rates among endothermic vertebrates.
• Endothermy is energetically expensive, so birds consume 20-30x more energy versus similar-sized reptiles.
• High body temperatures risk lethal overheating.
• Birds regulate body temperatures just below temperatures that destroy body proteins, warm amphibians and reptiles strike fast but tire quickly. Increased aerobic metabolism and insulation were major changes in the evolution of reptiles into birds.
• High metabolic demands require rapid oxygen/energy delivery and waste removal; efficient respiratory and circulatory systems balance bird chemistry.

The Respiratory System

• Bird respiratory systems differ from mammals in structure and function.
• Bird lungs are small, compact, and attached to the ribs in the chest.
• Lung tissue is as dense as mammal lungs, occupying half the volume.
• Air flows in one direction in birds as opposed to the in-and-out flow of other vertebrates.
• Birds have interconnected air sacs linked to the lungs and bronchi, which extend through the body cavity and bones.
• Air sacs connect to lungs/bronchi via secondary bronchi and thin walls.
• Birds lack a diaphragm; they inhale/exhale via sternum lowering/raising, which expands/compresses air sacs.
• Wishbone expansions/contractions pump air through the respiratory system during flight.
• Birds inhale air via nostrils at the bill's base, protected by a flap in diving/pollen-sensitive species.
• Each nasal chamber has elaborate folds (conchae) that increase surface area for air flow.
• Conchae cleanse/heat air heading into the respiratory tract and olfactory tubercles sample air chemistry.
• Conchae have nerves and a "rete mirabile" network that controls heat loss.
• Air moves down the trachea, splitting into two bronchi, then subdividing into branches inside the lungs.
• Roughly 1,800 tertiary bronchi interconnect lung tissue and lead to air capillaries that exchange gases with blood capillaries.
• Air flows continuously through lungs and air sacs, needing two inhalation/exhalation cycles for a one-way path.
• Most air passes through the bronchi to the posterior air sacs, moves to lungs, then the oxygen depleted air to the anterior air sacs. This process maximizes fresh air contact with lung surfaces and replaces nearly all lung air per breath.
• Bird gas exchange is more efficient than in mammals because no residual air gets left in the lungs as it does in mammals.
• Alligators may have precursors to avian airflow systems with regions of unidirectional airflow found in their lungs.
• Most birds have nine air sacs which serves to help deliver oxygen and remove body heat during flight and also protect internal organs.
• Theropod dinosaurs had pneumatized vertebrae, like birds, supporting the relationship between the two.

The Circulatory System

• Birds' high metabolic rates require rapid blood circulation for metabolic material transport.
• The circulatory system transports oxygen to body tissues, removes carbon dioxide, and delivers fuel while removing wastes.
• Avian circulatory needs surpass those of reptiles and most mammals.
• Birds and mammals have a double circulatory system with a four-chambered heart.
• Alligators and crocodiles have a three-chambered heart where the two ventricles are not separated.
• Avian four-chambered hearts evolved alike to mammals and completely separated pulmonary/body circulation.
• Oxygenated blood returns to the auricle and ventricle, exits through the aorta, then moves to peripheral arteries. Deoxygenated blood returns to the heart to be pumped back to the lungs.
• Avian hearts weigh 41% more than mammal hearts of similar sized bodies.
• A hummingbird's heart is 2-4% total body mass; few small mammals' hearts exceed 1%.
• The normal resting heart rate ranges from 150-350 bpm, averaging 220 bpm, while heart rates of hummingbirds can exceed 1,200 bpm.
• The performance of a heart can be measured via cardiac output, which is the rate the heart pumps blood into an arterial system.
• Cardiac output is calculated via "heart rate times stroke volume" and it averages 100-200ml of blood per kilogram in birds.
• Major organs receive high percentages of cardiac output.
• Bird hearts pump comparable cardiac outputs to mammals of similar-sized bodies but at slower resting paces.
• Avian ventricles empty more fully than mammal counterparts each contraction, and are made of more muscle fibers which also contain more oxygen dependent organelles called mitochondria. Avian heart-muscles are thinner and more optimized for oxygen and aerobic work.
• Avian heart performance leads to extreme arterial blood pressure, up to 300-400mm of mercury for domestic turkeys.

Metabolism

• Metabolic rate is energy spent over time to maintain body function.
• The rate depends on varying levels of activity, and environmental stressors.
• Complete daily energy expenditure combines all energy uses.
• Basal metabolic rate (BMR) accounts for energy when at rest.
• Birds have high basal metabolic rates when compared to vertebrates.
• Metabolism relates to mass, but isn't a 1:1 ratio
• Large birds have less surface area per unit of volume than small birds, which allow small birds to lose heat faster since they also generate less heat.
• Most of the time, birds spend only a fraction of each day at their BMR. Rather, the rest of their time is spent performing activities that require both energy and oxygen.
• Higher resting versus basal metabolism by 25-80%.
• Swimming Mallards increase metabolism at their most efficient rate by 3.2x and 5.7x in their most swift pace.
• Rheas aerobic metabolism peaked at 36x minimum and birds usually exceed mammal aerobic scope.
• Flight sustains high aerobic metabolism, small birds can fly at operating levels of 10x-25x their BMR whereas small mammals can only maintain 5x-6x.
• Flight metabolism ranges from 2.7x-23x BMR with variation depending on speeds, wing shape, the environment, etc.
• Birds in high altitudes increases in the oxygen affinity of hemoglobin molecules.

Temperature Regulation

• Bird thermal environment relations determine survival.
• Endothermy involves the dynamic between producing/losing internal to external heat. Heat is a metabolism byproduct.
• Heat production expressed in watts or joules per hour.
• Student resting heat equals a 100-watt light bulb.
• Best indicators for heat flux is ambient air temperature in wind-free areas.
• Bird feathers are the best natural insulators, with reduced insulation leading to higher rates of metabolism.
• Frizzled chickens have abnormal feathers which provide little insulation.
• Contour versus down feathers: Contour contributes to insulation, while down provides the primary source, and the amount of plumage impacts insulation.
• Birds utilize feather erector/depressor muscles to adjust positioning and conservation of heat.
• Fluffing increases insulation and air pockets. Covering exposure, such as tucking a bill or holding wings out from the body, is an example of this regulation.
• Pigmentation impacts regulation by absorbing rays and wind impacts plumage.
• Dark Plumage can absorb heat, but may increase convective heat loss, like the black robes of Bedouin tribes in the Sahara.
• Thermal neutral includes the range in what birds expend the least amount of energy.
• Most birds don't have to alter heat production to maintain around 40°C body temperature. Birds may control heat loss by changing feather positions, fluctuating venous blood flow, manipulating blood circulation, or using extremities.
• Shivering/panting regulates temperature by increasing metabolism, both inside and outside thermal neutrality.
• Gambel’s Quail manages desert conditions and avoids heat stress by staying in shade in midday, and nighttime temperatures are within the zone of thermoneutrality, leading to minor thermoregulation costs.
• Critical temperatures connect to habitats, such as lowland titmice with higher upper critical temperatures. Distributions of these and similar NA species may change with global warming.

Responses to Cold and Heat Stress

• Birds respond to cold by tensing muscles, and the temperatures where shivering begins is the lower critical temperature(LCT).
• Pectoralis/leg muscles provide heat through shivering which will thus increase oxygen consumption.
• Birds probably lack the ability for nonshivering thermogenesis that can be found in mammals.
• LCT dependent on average environmental temperature. Snow Buntings maintain body temps below -50°C while Northern Cardinals start shivering at 18°C.
• The LCTs of large birds = lower vs. small birds = higher.
• Seasonal temperature adjustments are called acclimatization; an example is when Goldfinches can maintain body temperatures when subjected to different cold temperatures.
• Birds also find microclimates like holes, evergreen trees, etc. Ptarmigans and grouse burrow the snow to insulate from cold.
• Excessive huddling together can lead to suffocation. Penguins huddling together cut energy rates and weight loss in half in the Antarctic.
• Avian temperature can fluctuate and save energy, which can drop significantly at night.
• The physiological condition is called facultative hypothermia, and is usually tied to internal clocks as well as food deprivation.
• Torpor, or minimal physiological activity, can be found in 43 bird species in 14 families.
• During torpor birds maintain body temps by 6°C at night; those with pronounced hypothermia may reach extremes like hummingbirds and poorwills.
• Upon waking, birds become comatose and are unresponsive to stimuli but maintain body temperature to air temperature.
• Primary challenge is warming up from torpor, must reach coordination temps of 26°C-27°C and need a body temperature of above 34°C. Large and short birds take a length of time to warm up.
• Heat is more difficult to manage in hot environments. Bird metabolisms create high vulnerabilities to heat, more likely to see catastrophic die-offs.
• Birds reduce heat load via avoidance, hyperthermia, and active heat loss via evaporative cooling. More clever examples are domestic pigeons getting trained to operate cooling fans.
• Evaporative cooling occurs above Upper critical temperature (UCT).
• High heat loss is achieved through evaporative cooling; however, these birds may lose 5 times as much water. Some species also defecate onto their own legs to release convective heat loss.
• Gular fluttering regulates upper and lower throat via rapid vibration of muscles. This method is used by seabirds.
• Cutaneous water loss that includes loss of both skin and respiration. Birds don't have sweat glands, and doves are more efficient in this process than in panting.
• Birds use their feet, as well as counter current heat exchange, to keep bodies at certain low air temperatures.

Feeding and Digestion

• Birds must feed frequently to refuel.
• Feeding adaptations are a feature of avian evolution that affect bird movement, capturing food, and the digestive system. Examples are Woodpeckers, hummingbirds, and ducks.
• Gizzard Structures are, in fowl, hard seed-crushing. In birds, they exist as softer bags, Anhinga has softer bags for fish eating, and Hoatzin has tiny pouches.
• Traits separating avian and verts is the lack of teeth, thus functions are reserved to food getting. Food processing via the bill is limited to cracking/tearing and birds reverse sequences of digestion to begin digestion in proventriculus before digestion occurs in the gizzard. Some species also are able to regurgitate food.
• Food enters into the oral cavity and lubricated by mucous glands. Esophagus is able to expand for birds which intake large prey and the esophagus also acts as an organ that produces pigeon milk. Esophagus is also able to inflate for display.
• The crop is an expansion and store to soften food, and have chambers that help break down tougher items.
• Most stomachs are two chambered. shapes and structures of stomachs correspond to their diets. Proventriculus are better developed where fish is consumed.
• Acidity of their walls is pH 0.2-1.2.
• Birds intestinal length averages 8.6x their body size.
• Birds use ceca to aid digestion and further extract useable compounds.
• Birds extract high nutrients/energy via small rapidly processed meals and passing through tract is less than half an hour and from half a day, depending on the material being consumed.
• Avian guts absorb nutrients quickly via being attentive to passive transports. But there are draw backs as sometimes toxins may be absorbed.
• Parrots consume a lot of toxins to deal with that they have an assemblage they use clay.
• Assimilation efficiencies shift with diets, American Robins are able to increase lipids used in fat.
• Fruits give amino acids to assist in digestion.

Energy Balance and Reserves

• Birds manage their energy balances in dynamic ways between intake and expenditure.
• Ideal balance is reached when intake is a mirror image of expenditure but prior during migration birds may eat at a higher rate than metabolism to store energy for trips and in reserves.
• Foraging time includes how much it amounts to to feed depends on expenditure/intake. Foraging times change with what food is available, while limited there's longer foraging activity.
• Adequate foraging times allow reserves or energy-expensive work to take place that benefits survival against microclimates, dominance, property rights, etc. Birds increase efficiency to reduce foraging times.
• Birds rely on lipid reserves with minimal fat to increase flight. Small passerines for temperate zones max out at 10% body mass, Singapore only maintain 5%.
• In general, large bodies can store more fats/faster, a ten gram could get affected in a day if there is no food, larger birds may last 4 days. Penguins do this.
• To manage food shortages there's hoarding. Acorn Woodpeckers build acorn granaries, meat eaters will store. To recover for that requires spatial memory which is processed via an enlarged hippocampal complex within the forebrain.

Excretion and Water Economy

• Daily energy/expenditures is one side and another is water economy which relates in important ways to their specific needs in arid environments/high activities.
• Evaporative water loss is common for high activity/high temps. For example the Towhee can quadruple water use when going from 30° to 40°C.
• Birds replace water from several sources in foods. Meat raptors, nectar, fruits can benefit from this. Birds that eat liquids almost don't visit/require water often. California Quail are relatives who have seed meals that assist.
• Organic compounds make metabolic water. Grams of fat gets kilojoules of energy plus 1.07 g of H2O. Large bodied can take loss/water from this. Certain birds are able to retain what is necessary.
• Loss of water in the air can be reserved via counter current cooling.
• A casual activity in common bodies has drinking fresh water can often get in their way.
• Deserts usually have predators who stay and visit around springs/water holes for birds.
• Diets can consist of standing water for the seed eaters. Dean Fisher noted that there were specific species which had little contact with water, and others who visited which were tied to max temperatures as they'd come in spectacular numbers.
• Water and waste is excreted by the kidneys, intestines and secreted as salt glands.
• Kidneys, reptiles, and mammals have different system anatomies where they get mixed with fecal, add water, absorb water.
• Birds have uric acid. (reptilian) they have more for the protein production to body toxins from turnover.
• To excrete nitrogen like urea it requires rinsing, this is like mammals where waste is high water contents to volume when using salts, for that birds use less but also the volume can fluctuate to the point its more than levels in blood, 3000 versus 300.
• There are some problems. It can be excessive water for some birds via high rates of water from nectar. Water rates do that, however, kidney systems arent to unusual levels from reps.
• High evaporative water loss for birds. High concentration helps selectivity absorb high sugar and keep going through the tract quickly without being processed in kidney.
• Birds concentrate their nitrogenous waste in arid climates with uric acid. They also have external glands. Kidneys themselves do not concentrate high salt/electrolytes, they need extrarenal stuff for water saving issues. Which is seen in salt glands that avoid salt solutions