Bird Physiology and Respiratory System

High Body Temperature

• Birds have many physiological adaptations, including metabolism, temperature regulation, feeding/digestion, and water economy.
• Endothermy maintains body temperature through metabolic heat.
• Birds maintain high body temperatures (~40°C or above).
• Physiological processes increase with temperature: Nerve impulses travel faster, and muscle contractions are quicker with increased temperature.
• Birds have the highest body temperatures of all endothermic vertebrates.
• Energetically expensive: Birds require 20-30 times more energy than cold-blooded reptiles.
• High activity levels and endurance open new ecological opportunities.

Respiratory System

• Bird respiratory systems differ structurally and functionally from other animals.
• Bird lungs are small, compact, and shaped along the ribs.
• They weigh half the volume of a mammal with the same body weight.
• Air sacs extend throughout the body cavity, cervical vertebrae, and major wing bones.
• Secondary bronchi connect lungs and bronchi through accessory channels.
• Contraction of sternum and ribs compress, and expansion/contraction of furcula helps pump air.
• Most birds have 9 air sacs (with variations between 6-12), which help dissipate internal heat.
• Theropod dinosaurs exhibit pneumatized vertebrae, consistent with the presence of air sacs (anterior and posterior).
• Inhalations occur through nares (nostrils), some birds have an operculum to protect nostrils.
• Conchae increase the surface area in the nasal cavity, which is important for birds like the Northern Fulmar and Turkey Vulture.
• Olfactory tubercles sample air in conchae, which helps with olfactory functions.

Circulatory System

• High metabolic rates require rapid blood circulation to deliver oxygen to body tissues and remove carbon dioxide.
• Blood circulates fuel (like glucose) and removes toxic waste.

Basal Metabolism

• Metabolic rate (BMR) is the amount of energy expended to maintain the functions of the body and varies with activity levels.
• All birds have a high basal metabolic rate compared to most vertebrates.
• BMRs are related to body mass, but not in a 1:1 relationship.
• Large bird heat loss rates are slower than smaller birds' rates due to a lower surface area to volume ratio.

Active Metabolism

• Birds usually must expend energy above their basal metabolic rate even while resting.
• Metabolic rate increases with exertion.
• Birds in flight can have a metabolic rate 10-25 times their BMR.
• Many animals switch to lactic-acid (anaerobic) production with maximum exertion.
• Birds have a higher aerobic scope than mammals, related to their high-altitude adaptations.

Temperature Regulation

• Bird feathers are efficient lightweight insulations.

• Contour feathers create insulation.

• Down feathers provide most of the insulation.

• Insulation increases with plumage amount, but abnormal plumage results in lowered insulation.

• Birds adjust the position of their feathers to regulate heat loss and conservation.

• Using wind-sheltered sites reduces heat loss.

Responses to Cold Stress

• Lower critical temperature (LCT) is the temperature at which shivering begins.
• Shivering produces heat and increases oxygen consumption.
• Birds lack brown adipose tissue, so they don't have nonshivering thermogenesis.
• Adaptations include colder environmental tolerance levels, ranging from -50 °C in snow buntings to 18°C in Northern Cardinals.
• Selecting warmer microclimates, as well as huddling, are other methods for cold tolerance.
• Facultative hypothermia is a physiological condition where body temperature drops below normal.
• Torpor can reduce body temperature at night (daily or prolonged torpor).
• Some birds can have hypothermia to 4-8°C.

Responses to Heat Stress

• High metabolism in birds makes them vulnerable to heat stress.
• Avoidance behaviors, controlled hyperthermia, and active heat loss through evaporative cooling are methods of reducing heat loads.
• Evaporative cooling is very efficient at dissipating heat, but it leads to significant water loss.
• Body size variations occur based on environmental conditions. Larger birds are better suited for cold/dry conditions.

Feeding and Digestion

• Birds frequently feed to accommodate their high metabolic needs.
• Bird tongues are adapted based on their feeding function.
• Some birds have a crop for storing and softening food.
• Their digestive systems include a proventriculus for chemical digestion followed by a gizzard for physical digestion.

Energy Balance and Reserves

• Energy balance is a dynamic relation between energy intake and expenditure.
• Constant weight is maintained when these are balanced
• Foraging time is dependent on energy requirements and intake rate.
• Birds have minimal fat reserves in the 5-10% range (lipid).
• Larger birds can store more fat, and migratory birds build fat reserves in preparation for their trip.

Water Economy and Excretion

• Birds balance water loss in high-activity conditions, like deserts.
• Evaporative water loss increases with higher temperatures, but it decreases with larger body size.
• Birds obtain water from food, drinking sources, and metabolic processes.
• Seed-eating birds get most of their water from food.

Excretion of Waste

• Excrete waste as uric acid to conserve water.
• Concentration of uric acid is very high in the cloaca.
• Hummingbirds and nectar-feeding birds have comparatively higher water intake through their diets compared to other birds.
• Salt glands in seabirds help to efficiently excrete excess salts from drinking seawater.