Physiology of Birds

Physiology of Birds

  • Overall Activity
    • Active throughout the year, including extreme environments.
  • Thermoregulation
    • Birds are endotherms, maintaining high body temperatures for survival efficiency.
    • Systems in place to:
    • Conserve heat
    • Facilitate heat loss
    • Conserve water

Body Temperature of Birds

  • Average Body Temperature: Approximately 40°C (104°F)
  • Advantages of High Body Temperature:
    • Increased rates of physiological processes.
    • Nerve impulse transmission speed increases 1.8 times for every 10°C rise in temperature.
    • Muscle strength and speed of contractions triple with every 10°C rise.

Disadvantages of High Body Temperatures

  • Energetic Costs
    • Energy consumption is 20-30 times that of similar-sized reptiles.
    • Overheating risk:
    • Above 46°C, cell proteins degrade faster than they can be replenished.
    • High oxygen and energy delivery required to sustain physiology.
  • Adaptations: Powerful respiratory and circulatory systems to cope with energy demands.

Avian Respiratory System

  • Structure and Function:
    • Unique compared to mammals, employing air sacs.
    • Nearly complete air replacement with each breath, maximizing efficiency.
    • Absence of diaphragm; ventilation occurs through contraction of sternum and ribs.
  • Breathing Rates:
    • Decrease as bird size increases:
    • 2g hummingbird: ~143 breaths/minute.
    • 10 Kg turkey: ~7 breaths/minute.
    • Flight increases rates by 12 to 20 times resting state.

Avian Lungs and Air Sacs

  • Lung Characteristics:
    • Small, compact, spongy structures.
    • Occupy half the volume of mammalian lungs despite similar weight.
    • Gas exchange occurs in air capillaries, which fork into primary, secondary, and tertiary bronchi.
  • Air Sacs:
    • Thin-walled, interconnected with primary/secondary bronchi.
    • Allow continuous air flow through lungs, crucial for efficient respiration.
    • Typically 6-12 air sacs, 9 is most common.

Avian Circulatory System

  • High Demand:
  • Birds possess a double circulatory system.
  • Large hearts (2-4% body mass in hummingbirds), 50%-100% larger than comparable mammals.
  • Cardiac Performance:
    • Resting heart rate is about half of similarly sized mammals.
    • Average for medium-sized birds: ~220 bpm.
    • Increased stroke volume allows for similar cardiac output to mammals.
    • Avian ventricles are more muscular, thinner, and mitochondria-rich for effective oxygen transfer.

Metabolic Rates in Birds

  • Basal Metabolism:
    • High basal metabolic rate (BMR) across bird species.
    • Passerines exhibit highest BMR among vertebrates.
    • BMR increases with mass but not linearly; roughly a 0.72:1 increase in BMR:mass.
  • Activity Metabolism:
    • Birds can operate at levels 10-25 times their BMR compared to small mammals (5-6 times their BMR).

Thermal Relations with the Environment

  • Heat Production Equation:
    • H=Tb−TaIH = \frac{Tb - Ta}{I}
    • Where:
    • HH = heat production,
    • TbTb = body temperature,
    • TaTa = ambient temperature,
    • II = insulation coefficient.
    • Heat loss correlates with the temperature difference; insulation (feathers) can be adjusted via molting and feather positioning.

Regulation of Body Temperature

  • Thermal Neutral Zone:
    • Range of ambient temperatures where metabolism remains constant.
  • Critical Temperatures:
    • Upper Critical Temperature (UCT): Above this temperature, metabolism must increase to maintain heat.
    • Lower Critical Temperature (LCT): Below this temperature, metabolism must also increase.
    • Larger birds often have lower LCTs.

Responses to Cold and Heat Stress

  • Cold Stress:
    • Mechanisms include shivering, utilizing microclimates, and communal roosting.
    • Hypothermia and torpor are strategies to conserve energy during low temperatures.
  • Heat Stress:
    • Behavioral adaptations include seeking shade, bathing, and gular fluttering for evaporative cooling.

Water Economy

  • High Water Loss Rates:
    • Increased temperatures lead to quadrupedic water loss; birds replace water through diverse food sources (nectar, fruit, insects).
    • Some can rely on metabolic water produced during metabolism.
    • Birds have specialized nasal passages that reduce water loss during respiration.

Excretory Systems

  • Waste Management:
    • Nitrogenous wastes result from protein metabolism; primarily excreted as uric acid which conserves water.
    • Seabirds possess salt glands for excreting excess salt, essential for those consuming saltwater.
    • Excretion requires energy, which can increase metabolic rates by up to 7%.