Biology 463/563 Ornithology

Dr. David Swanson, Office: CL 180



Metabolism = sum total of biochemical reactions occurring within an organism, serves to provide energy in a usable form (ATP). Energy is obtained from the oxidation of foodstuffs (carbohydrates, fats, and proteins).

Metabolic Rate = rate at which energy (ATP) is produced and used (because very little ATP is stored) from ingested or stored substrates.

- Usually measured (in the laboratory) as oxygen consumption, because direct measurement of heat production (energy use) is very difficult. Oxygen consumption works because the amount of oxygen used to produce a given amount of heat is relatively constant regardless of the substrate being utilized (maximum difference is 10%, usually less because a combination of substrates are being oxidized).

- Metabolic rate can also be estimated (in the field) by two different techniques.

1) Doubly Labeled Water (D2O18, D = deuterium [H3]). Procedure = Inject doubly labeled water

--> Allow to equilibrate with general pool of body water (1 hour) --> Take initial blood sample --> Release bird to environment --> Recapture after about 24-48 hours --> Take final blood sample

  • a) As doubly labeled water equilibrates with general body pool, you get H2O18, D2O, and CO218 within the body.
  • b) Deuterium is lost only via water loss.
  • c) O18 is lost both through water and CO2
  • d) Difference between isotope turnover ratios (after second capture) allows an estimate of CO2-production, which estimates metabolic rate (because CO2 is produced as O2 is consumed during aerobic metabolism).

- Provides a ballpark (+ 10%) estimate of field metabolic rate.

2) Time-Activity Budgets

  • a) Measure cost of certain activities (resting, hopping, flying, etc.) in the lab by oxygen consumption.
  • b) Measure time spent in different activities in the field.
c) Sum costs for times spent in different activities --> provides estimate of metabolic rate.
- Again, gives ballpark estimate of FMR (usually in fairly close agreement (+ 10%) with doubly labeled water measurement, but requires careful measurement of metabolic rates in lab and careful and extended observation in the field.


1) Rate of Heat Loss from body can be described by:

H = C(Tb - Ta)

where H is heat loss (heat loss = heat gain because Tb is constant); C is thermal conductance = a net measure of heat transfer via radiation, conduction, convection, and evaporation (equivalent to the inverse of insulation); Tb is body temperature; and Ta is ambient or environmental temperature.

2) To maintain Tb, birds can respond by:

  • a) modifying conductance, thereby influencing heat loss
    • (i) plumage or postural adjustments
    • (ii) peripheral circulation adjustments

  • b) a change in Tb (modifies gradient for heat exchange with the environment)
  • c) changing heat production to offset changes in heat loss

3) General Metabolic Response to Temperature (for Endotherms)

  • Thermal Neutral Zone (TNZ) - the range of temperatures over which metabolic heat production (H) is unaffected by a change in Ta. Tb is kept constant by passive changes in feather elevation and posture adjustments, also some peripheral vasoconstriction/vasodilation.
  • Basal Metabolic Rate (BMR) - minimum metabolic rate required for maintenance; measured as MR while resting within TNZ at night (i.e. during resting phase) in a postabsorptive state (empty gut).
  • Lower Critical Temperature (LCT) - temperature below which bird must elevate H to maintain constant Tb. Actually, LCT is a gradual transition, rather than a sharply defined point. Insulation is theoretically maximized at LCT. LCT is lower for larger birds, which indicates that larger bids are better insulated, relative to smaller birds.
  • Upper Critical Temperature (UCT) - temperature above which bird must increase H to actively dissipate heat (panting, gular glutter, cutaneous water loss). UCT always occurs at Ta's just below Tb.
  • Summit Metabolism - maximum sustained oxygen consumption in response to cold; not necessarily equal to peak short-term thermogenesis. At Ta below this point, heat loss exceeds heat production, resulting in hypothermia.
  • Slope - of the line below TNZ can be regarded as a measure of thermal conductance.

      1. If the extrapolated line passes through zero metabolism near the Tb of the bird, then insulation is constant (and maximal) below LCT.

      2. If the extrapolation temperature at zero metabolism is different than Tb, this indicates that conductance (and insulation = the reciprocal of conductance) is modified below LCT, presumably by vasomotor changes.

4) BMR in Birds - influenced by many factors

  • a) Body Size - Total metabolic rate (per bird) increases with size, but at a slower rate:

    MR ~ Mass0.75 (not 1:1);

  • - This means that mass-specific MR (per gram of tissue) decreases with increasing body size (i.e., small birds have relatively higher MR than large birds).

  • b) Phylogeny - different taxa may show different metabolic levels (e.g., shorebirds appear to have relatively high BMR).
  • c) Other Effectors
    • (i) Circadian Phase = higher MR during active phase than during resting phase (20% in small birds).
    • (ii) Digestive State - specific dynamic action = energy needed to digest a meal, contributes to higher MR with food in digestive tract than in empty condition.
    • (iii) Environment - BMR may vary with habitat (e.g., tropical birds with lower BMR than those from higher latitudes).
    • (iv) Life History - Food habits may influence BMR (e.g., frugivores typically have lower MR than carnivores).
    • (v) Sex - some species show sexual dimorphism in MR.