Hs = Hm Hr Hk Hc He

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Hs Hm Hr Hk Hc He

• Hs is heat stored in body Thermoregulation is
  balancing right side of equation so Hs 0

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Hr Radiation

• Bird bodies radiate, always some loss (-Hr)
• Can gain by absorbing sunlight (Hr)
• Basking behavior behavioral thermoregulation
• Better absorption if plumage dark (melanin
  pigments in feathers)

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He Evaporation

• Heat lost when water evaporates at moist surfaces
  (-He)
• Water conservation adaptations reduce evaporative
  heat loss
• High body temperature reduces evaporative heat
  loss

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Hk Conductance

• Diffusion of heat
• Heat flows from warmer bird bodies to cooler
  environment (-Hk)
• Rate of loss depends on difference between body
  temperature and air temperature

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-Hk C (Tb Ta)

• Tb constant, Ta out of birds control, can
  control C (resistance heat flow insulation
  provided by feathers)

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Insulation

• Erect feathers to increase insulation for
  short-term regulation
• Wider layer warm air trapped next to body
  increases resistance to heat flow
• Can increase density of insulating feathers for
  seasonal regulation
• Heat lost through uninsulated bill, legs

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Mechanisms to reduce heat loss from bill and legs

• Tuck under feathers to insulate
• Reduce blood flow to legs
• Countercurrent blood flow to legs
• Cold veinous blood returning from legs removes
  heat from warm arterial blood going into legs
  (veins proximate to arteries)

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Hc Convection

• Heat lost to moving medium (wind chill versus air
  temperature)
• Operates similar to Hk, same effects of
  insulation, Ta Tb (-Hc)
• Greater in birds with dark plumage because of
  removal of Hr from feathers

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Hm Metabolism

• Heat production at BMR, more with activity (Hm)
• In cold conditions birds regulate (balance
  equation) by adjusting Hm
• Birds rely on heat produced by shivering (not
  burning of fat) in cold conditions
• Pectoralis muscle primary source

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Thermoneutral Zone (68-95F)

• Within the thermoneutral zone, birds can maintain
  body temperature without expending energy

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Adjustments if too cold within thermonetural zone

• Increase insulation
• Reduce exposure bill, legs
• Reduce blood flow legs
• Move out of wind, move into sun (seek a favorable
  microclimate)

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Adjustments if too hot within thermonetural zone
(activity causes overheating)

• Reduce insulation
• Expose bill, legs
• Increase blood flow legs
• Bypass countercurrent flow in legs
• Stand in wind, avoid sun
• Sweat through skin (no sweat glands)

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Mechanisms above upper critical temperature
(expend energy)

• Panting increases He respiratory system,
  especially upper portion (inefficient)
• Gular fluttering increases He in upper
  esophagus, pharynx (more efficient)
• Storks and New World vultures excrete legs

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Overheating

• Active regulatory mechanisms require water, are
  not very effective
• Birds may undergo hyperthermia (Hs), regulate at
  higher temperature
• Do on routine basis when overheat due flight
• Limited by proximity to lethal temperature
• Very high air temperatures are problematic

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Mechanisms below lower critical temperature
(expend energy)

• LCT is lower in larger birds (retain heat
  better), harsher climates (adaptation)
• Rely on shivering, large birds handle better
• May undergo hypothermia (-Hs), regulate at lower
  temperature

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Hypothermia

• Tb Ta less, -Hk less, saves energy
• Many birds engage in shallow hypothermia (5-10F
  lower)
• Chickadees, hummingbirds engage in torpor (deeper
  hypothermia) nightly
• Chickadees drop as much as 20F
• Hummingbirds drop as much as 35-55F, to 40F
• Poorwills enter torpor for days to weeks

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The Economics of Torpor

• Save energy, but vulnerable due immobility
• Energy savings must balance cost reheating
• Larger birds lose heat more slowly, reheat more
  slowly, thus torpor pays at longer periods in
  larger animals
• Short-term torpor pays for smaller animals, but
  they have less capacity for longer torpor