Biology 463/563 Ornithology

Dr. David Swanson, Office: CL 180



THERMOREGULATION (continued)

5) COLD ADAPTATION

A) Physical Adjustments

  • (1) Feathers - plumage is generally thicker in winter (and in higher latitudes), but there are not great seasonal changes in insulation, particularly in small birds. Nevertheless the temperature differential between the skin surface and the surface of the feathers may be substantial (up to 40 C in the Black-capped Chickadee).

  • (2) Subcutaneous Fat - forms insulatory layer in only a few birds (e.g., penguins). Most birds store fat only in furcular and abdominal regions.

  • - Birds wintering in temperate-zone or arctic climates (esp. granivorous ground-foragers) generally store more fat in winter than in summer, but these deposits serve as fuel stores and do not contribute greatly to seasonal changes in insulation.

  • (3) Regional Hypothermia and Peripheral Circulation

    • - Regional Hypothermia = regions of the body are maintained at temperatures lower than the body core. (SEE HANDOUT).

    • - Peripheral Vasoconstriction = changes in blood flow so that warm blood is circulated in the muscles and body core and not to the skin and extremities. Allows regional hypothermia.

    • - Countercurrent Heat Exchange = vessels to and from the extremities are closely opposed in a network (rete mirabile) so that heat stays at the body core. This network can be bypassed if heat conservation is not a concern. (SEE PG. 136, GILL).

    • - Changes in peripheral circulation can increase heat loss markedly (by up to 90% through the legs and feet).

    B) Physiological Adjustments

    • (1) Increased Shivering Endurance - occurs in most winter-acclimatized birds (precise mechanism is not known, but may involve changes in carbohydrate and lipid metabolism and/or muscle hypertrophy).

    • (2) Increased Summit Metabolism - occurs in most cold-adapted birds, provides higher total capacity for heat production (although very rarely if ever do birds reach these levels in nature). Generally from 4 - 7 times BMR.

    • (3) Some birds exhibit an increase in BMR, but others don't. Elevated BMR may be indicative of increased costs associated with maintenance of metabolic machinery devoted to heat production.

    • (4) Regulated Hypothermia and Torpor

      • - Most small birds allow Tb to drop 2-3oC at night (less for larger birds).

      • - Some (chickadees, etc.) can allow Tb to drop by 10-12oC below daytime levels. This decrease in Tb allows substantial energy savings by decreasing the gradient for heat loss between the animal and the environment. These birds are still capable of activity and flight at these reduced body temperatures.

      • - Torpor = state of dormancy similar to hibernation, but on a daily basis; usually occurs at night. Tb approaches Ta down to a certain level (minimum = 5-6oC). Tb regulated at this lower level, but always above freezing. (SEE HANDOUT). At very cold temperatures, birds will increase heat production and spontaneously arouse, as torpor becomes too dangerous at these temps. (difficult to generate the amount of heat needed for arousal at very low temperatures). During torpor birds are unresponsive to stimuli.

      • - Occurs in hummingbirds, nightjars (Common Poorwill is the only true hibernator, 2-3 months), swifts, mousebirds, bee-eaters.

      • - Allows great energy savings (e.g., Hummingbirds may decrease oxygen consumption by 75% when allowing Tb to drop to 10oC, saves up to 27% of energy expended to get through the night).

      • - Usually used only in cases of emergency (cold + food deprivation), but has recently been documented in a migratory hummingbird during migration at mild temperatures --> conserves fuel for migration.

    C) Behavioral Adjustments

    • (1) Postural = decrease exposed surface area, tuck legs and bill into feathers, orient perpendicular to sun to intercept maximum solar energy.

    • (2) Microclimate Selection = choose roost sites which protect from the elements (e.g., cavities in trees, thick brush, subnivean space).

    • (3) Huddling = documented for several birds; reduces heat loss by decreasing surface area.

    • (4) Feeding Intensity = show greater foraging intensities at colder times of the year; some birds are also active at lower light levels in winter than in summer.

    • (5) Food Caching = storing food in specific locations. Provides a readily available food source for times when energy expenditures are high. Occurs in Acorn Woodpecker, nuthatches, crows & jays, and chickadees & titmice.


    6) HEAT ADAPTATION - heat stress can result from both exercise and high temperatures.

    • A) Physical Responses

      • (1) Peripheral Circulation Changes - increased blood flow to skin and extremities increases heat loss as Tb is usually higher than Ta. Gulls can lose 80% of metabolic heat produced during flight through their feet.

      • (2) Flatten Feathers - decreases insulative layer and increases heat loss. Slightly lower feather volumes are also present in warm-acclimated or tropical birds.

    • B) Physiological Responses

      • (1) Heat Storage = under heat stress many birds allow Tb to rise by 4-5oC, they then dissipate this stored heat when conditions become more favorable, such as at night or after exercise has ceased. Heat storage reduces the rate of heat gain because the gradient between the body and the environment is decreased (or increased in the case of exercise).

      • - The brain is maintained at near normal Tb during heat stress by the rete mirabile ophthalmicum = vascular network closely associated with the circulatory system of eye. Allows countercurrent heat exchange between veins returning from eye, mouth and nasal cavities (cool) and the arteries going to the brain (warm). (SEE HANDOUT)

      • (2) Active Methods For Promoting EWL and Cooling

      • - Birds lack sweat glands so evaporative water loss must occur via other avenues.

        • a) Panting = rapid shallow breathing; increases evaporative cooling (EC) from nasal, buccal, and upper pharynx regions. Birds use tidal volume of dead space of respiratory tract so that no gas exchange occurs, otherwise they would experience respiratory alkalosis (increased pH of blood) from blowing off too much CO2.

        • b) Gular Flutter = rapid vibrations of gular region (floor of mouth); increases EC from mouth lining and upper respiratory tract.

      • (3) Cutaneous EWL = EC through skin. Important avenue for EC in many birds (can comprise up to 80% of total EWL). Precise mechanism is uncertain, likely involves peripheral vasodilation bringing increased blood flow to skin.

    • C) Behavioral Responses

      • (1) Posture Adjustments = expose increased surface to increase heat loss, orient parallel to sun's rays to minimize surface exposed to solar heating.

      • (2) Microclimate Selection = seek shade, wade if capable, seek windy areas, reduce activity during hot times of the day.

      • (3) Urohydrosis = release liquid excreta to legs for EC (storks, vultures, cormorants, ibises).

      7) ACTIVITY METABOLISM

      • A) Field Metabolic Rate - exceeds BMR by 2-4 times, maybe up to 6X during the breeding and nesting seasons.

      • B) Running - Oxygen consumption increases linearly with speed. Slope of line varies among species, generally lower slopes for larger birds. (SEE HANDOUT).

        • - Maximal oxygen consumption during running exceeds BMR by 14X or more.

      • C) Flying - energetically more efficient than walking or running.

        • (1) Power Output (MR) vs. Speed

        • - Theoretical predictions suggest U-shaped curve with minimum power output at a certain speed (determined by aerodynamic principles: Parasite Power = overcomes drag of body, Induced Power = supports weight of body, Profile Power = overcomes drag of flapping wings).

        • - Speed at minimum power does not equal most economic speed (maximum range per power output). The most economic speed is higher than Vmp and doesn't change much over a range of speeds.

        • - Actual curves don't conform too well to predicted curves in most cases. This may be because wingbeat frequency does not vary in a simple linear fashion with flight speed. Also, it may be that birds simply do not fly at speeds that would show increased power requirements at either end of the theoretical U-shaped curve.

        • - Long-distance flights usually conform closely to predicted most economic speed.

        • (2) Maximal Oxygen Consumption during flight has never been convincingly measured for any bird. Metabolic rates during flight generally range from about 8-14X BMR, but MR up to 25-30X BMR have been reported. This is higher than for cold-induced maximal oxygen consumption (4-7X BMR). Why?

          • a) Muscle mass recruited is less because of isometric shivering.

          • b) Q10 effects

        • (3) Cost of Flight vs. Mass

        • - MR during flight increases with mass similar to BMR (Pi ~ Mass0.73). This results in higher flight MR/g for small birds than for large birds.

        • - Power required for flight is directly (1:1) proportional to body mass. Thus, the intersection between MR and power requirement lines points to a maximum size for flight of about 12 kg. The largest birds capable of long-distance flights are Swans (about 10 kg). Kori Bustards (Africa) weigh about 14-19 kg and are capable of short-distance flights.