Lecture 15

The Insatiable Appetite – Costs and Benefits of the High Metabolic Rates of Birds

   Next lectures will cover a bit of physiology – how do you keep an engine running at such high RPMs? -- and then seque into the annual cycle of birds. Because resources are not available at constant levels throughout the year (daylength changes, winter, dry & wet seasons in the tropics, etc.), and because they have such insatiable appetites, birds have "scheduling conflicts."

 “The Primary Directive”:
Reproduce and survive to reproduce again.
Survival requires:
            Eat (and don’t be eaten)
Eating requires:
            Being where there’s food - Migration
            Having feathers - Molting

But first, Metabolic rates:

 I.           Physiology = metabolism & excretion

              High body temperature - ca. 40^oC (104^oF)

                          maintained by production of metabolic heat = endothermy

                          permits use of cold habitats/climates

                          provides power and endurance needed for flight

                          requires advanced systems of

                                      heat conservation

                                      heat loss

                                      water economy (evaporative cooling and excretion)

                                      respiration & circulation

                                      waste removal (especially N and CO₂)

              Specifics -

                          nerve impulses travel faster

                          muscles are stronger

                          but birds need 20-30 x as much energy as similar sized reptile

                          on fine line where proteins are being denatured

              Respiratiory system

                          components

                                      nostrils (nares) & nasal chamber (rete mirabile)

                                      trachea (syrinx)

                                      lungs

                                      air sacs (display in Frigatebirds)

                          The trick - flow-through lungs

                                      mammals and herps have dead-end, always dead air

                                      birds replace virtually all air in lungs with each breath

                                      air intake by sternum

                                                  lower sternum, fill posterior air sacs

                                                  raise sternum, force air into lungs

              Circulatory system

                          what does it do?

                                      transport O₂ and glucose & fatty acids (fuel)

                                      removes CO₂ and other metabolic waste products

              The heart

                          four-chambered (like mammals)

                                      reptiles have 3-chambered-but alligators have 4-chambered

                                      oxygenated blood from lungs goes to right atrium/ventricle

                                      systemic blood from system goes to left atrium/ventricle to lungs

                                     50-100% larger than comparable mammals

                          output

                                      large proportion goes to legs

                                                 more than pectoral muscles! - heat loss?

                                                 legs & brain get 10-20% of cardiac output

                                                 heat loss and conservation through rete

                                      resting rate 150-300 bpm

                                                 lower than some mammals

                                                 output higher

                                                             heart larger

                                                             stroke more efficient (ventricle empties more)

               Metabolism

                          High rates

                          much heat produced

              Temperature regulation

                          TNZ (Thermo-neutral Zone) ca. 18-35C (64-90F)

                          LCZ (lower critical zone)

                                     shiver

                                     can’t metabolize brown adipose tissue as mammals do

                                     seek microclimate

                                                 evergreens

                                                 burrow in snow

                                                 cavities (often huddle in groups)

                         UCZ - (upper critical zone)

                                     gular flutter

                                     panting - evaporative water loss

                                     controlled hyperthermia

                                                 reduces heat loss by decreasing gradient btn body and envirnm. which saves water (less lost in evaporative cooling

II.         Water Economy

              Loss through evaporative cooling - especially important in arid regions

                          Brown Towhee evaporative H₂0 loss quadruples when ambient temp goes from 30 to 40^oC

              Replaced how?

                         Drinking

            Metabolic water: 6O_{2 +} C₆H₁₂O₆ -> 6CO₂ +6H₂O  (high metabolic rate of birds results in more metabolic water than comparable mammals)

             food - fruit, prey - Sooty Falcons nest where temp in shade is 49^OC (120^oF)

              Reduce loss

                          by countercurrent mech. in nasal chambers

                          excrete uric acid instead of urea

                           molocule of uric acid has 2xs the N as one of urea insoluble, flushed out with less H₂0 - mammals use 20  times the H₂0 that birds do to excrete the same amount of N

                          kidneys different - short loops of Henle, so excretion of electrolytes (salts) is not as efficient.

                          salt glands

                                      located above eyes, drain into nasal passage

                                      widespread in non-passerines that have salty diets (mostly marine) - examples?

                                      absent in passerines - not clear why.

                                      enzymatically run (therefore energetically expensive)

                                                may increase resting metabolism by 7%  

Avian Physiology – Continued & details
Air sacs
            9 in most birds (12 in some):
                        cervical – 2
                        thoracic – 4
                        abdominal –2
                        interclavicular – 1
                                    extends into bones (humerus & sternum)
                                    contacts syrinx, essential in sound production
            heat loss, and cushion internal organs

Furcula – spring-like action helps with movement of sternum & hence air flow.
Retes – nasal cavities have a “temporal” countercurrent as well as a vesicular one.

(From Gill - Ornithology)

Activity Metabolism
 
            BMR is rare—almost any activity (slight movements, digestion, etc.) raises                            metabolic rate above BMR
            Small birds 10-25 times BMR for hours (mammals only 5-10)
            Flight is very expensive, but efficient – compare to mammals
                        Higher per unit time, but you go farther per unit time
                        Varies with wing shape, flight type—examples?
            Egg production, etc.

Insulation & Heat Loss
 
H = (Tb – Ta)/I

(Tb is body temperature, Ta is ambient)
 
How can a bird manipulate this equation?
 
Behavior (short & long term) & (longer term) molting