H s = H m + H r + H k + H c + H e

1. Hs = Hm+ Hr + Hk+ Hc+ He Hs is heat stored in body; Thermoregulation is balancing right side of equation so Hs = 0

2. 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)

3. 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

4. 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

5. -Hk = C (Tb – Ta) Tb constant, Ta out of bird’s control, can control C (resistance heat flow = insulation provided by feathers)

8. 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

10. 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)

11. 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

12. 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

13. Thermoneutral Zone (68-95F) Within the thermoneutral zone, birds can maintain body temperature without expending energy

14. 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)

15. 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)

18. 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

19. 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

20. 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

22. 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

23. 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