Individuals visited a feeder in Portland, OR, during a 4-d period in Dec when ambient temperature were above -4.4°C for only 1 h and dropped to -11.1°C (Taylor and Kamp 1985). Wind chill at times dropped the effective temperature much lower (-35°C). Birds were present at the feeder only from about 1200 to 1630 h. The final intake of sugar solution at the end of the day was estimated as a maximum of 28.46 kJ. The observations indicate survival under very low temperatures, but except for observations of 1 male that sat in the shelter of a rhododendron between feeding bouts, nothing is known about the 19-h period when the birds were out of sight.

Like many other hummingbirds, Anna's Hummingbird may respond to energy shortage by dropping its body temperature. At ambient temperatures down to about 12°C, healthy, well-fed Anna's Hummingbirds may regulate their body temperatures between 36 and 40°C when resting in the dark' in contrast to temperatures above 40°C in active birds (Lasiewski 1964). Birds that have inadequate energy reserves, have lost insulating plumage, or have vascularized growing feathers may become torpid. Torpor reduces the energy requirement during that period, enabling the bird to survive with reduced food intake. Bartholomew et al. (1957) described 2 captive birds induced into torpor. The torpid birds were inactive, feathers fluffed, eyes closed, bill pointed upward; 90 min were required for an immature, injured bird to enter torpor at an ambient temperature of 2°C; body temperature dropped from 36.4 to about 17°C. A fasting adult male entered torpor at 21-23°C. Oxygen consumption and respiratory rate also dropped; at a body temperature of 9-12°C, breathing was suspended for up to 5 min. The lowest body temperature recorded was 8.8°C at an ambient temperature of 8.2°C. Arousal at an ambient temperature of 21-23°C took only minutes; body temperature rose at a rate of 1-1.5°C/min. Lasiewski (1963) obtained a value of 0.0034 kJ/g/h (0.94 W/kg) for a torpid bird at an ambient temperature of 15°C. Powers and Nagy (1988) found torpid birds to be generating heat at rates from 0.004 to 0.016 kJ/g/h (1.11-4.44W/ kg). Incubating or brooding females do not usually enter torpor, but Vleck (1981) noted torpor in an incubating female for 4.5 h following rain and ambient temperatures to 11°C. Breathing rate at 20°C is 265/ min but 6.3/min in torpid birds (data for Allen's and Anna's hummingbirds combined; Withers 1977).

During cold temperatures, Anna's Hummingbirds gradually gain weight during the day, as they convert sugar to fat (Beuchat et al. 1979, Powers 1991). The stored fat and a full crop of nectar, combined with a period of torpor, enable them to balance their energy expenditures.

Between ambient temperatures of 25 and 35°C, Anna's Hummingbirds have an intramuscular body temperature of 42.9°C +/- 0.75 (SD); at an ambient temperature of 40°C, body temperature rises to 44.3°C +/- 0.60 (Powers 1992). The thermoneutral zone (TNZ) is approximately between 33 and 37°C during the daytime; metabolic rate in the TNZ is about 0.126 kJ/g/h (35 W/kg). Cooling is by evaporative water loss when ambient temperature is higher than body temperature. At 40°C the evaporative water loss is 37.1 +/- 11.7 (SD) mg H2O/g/h, the highest measured for any endothermic vertebrate (Powers 1992). Yet at 40°C the bird is generating heat through metabolism at a rate greater than evaporation is cooling. The response is an elevation of body temperature to increase heat loss through convection, conduction, and radiation. High humidity reduces the efficiency of evaporative cooling, imposing further stress on birds. Anna's Hummingbirds keep tarsi and toes fully extended when feeding above 32°C (to dissipate heat?), but keep them concealed in feathers of the lower abdomen below 24°C (Udvardy 1983).
 

28.46kJ = 6.9 dietary Calories = ~2g or 1/2tsp of white sugar
 
 
 

Anna's Hummingbirds can adjust their body temperature without entering
torpor to conserve energy.
 
 

Anna's are unlikely to enter torpor unless stressed by low body fat, loss
of body feathers or are actively molting.
 
 

They enter torpor slowly.  The body temperature drops from 40°C (104°F)
to between 9 and 12°C (48-54°F) and breathing all but stops.  Birds come
out of torpor rapidly when conditions warrant.
 
 
 
 

Temperature regulation. Thermal environment at highest breeding latitudes of this species is not appreciably cold (see Habitat. climate), so no special metabolic demands are imposed in its northern distribution. Mean resting metabolic rate of 0.55 mlO₂/min at an ambient temperature (T_a) = 30° C (n = 10 Rufous,7 Broad-tailed, similar body mass, no statistically significant distinction, data pooled). From 30°C to 0°C, metabolism increases linearly to maintain body temperature at 39°C (Bucher and Chappell 1989): mlO2/min = 1.330.024 T_a. The slope of this metabolic increase is equivalent to heat loss coefficient ("thermal conductance") of 8 milliwatts power/°C. Resting metabolic rate at 20°C is about 2.5 times the minimal metabolic rate of Rufous exposed to 34-39° (Lasiewski 1963). In 102 bouts (Rufous and Allen's) hovering at a feeder, foot extension was inversely correlated with T_a, 100% fully extended above 33°C, 75% fully retracted into abdominal plumage at 15°-20°C, 40% fully retracted at T_a 21°-26°C, presumably facilitating heat loss or conservation as needed (Udvardy 1983).

Hypothermic Torpor. This species regulates energy loss in contrast with Calypte anna, which regulates energy gain (Beuchat et al. 1979). Year-round, birds are able to conserve energy via a drop in tissue temperature, which slows tissue metabolic rates, with decreases in oxygen consumption (1.1 to 0.8 ml/min) and ventilation frequency (242 to 14 breaths/min), latter mostly by a non-ventilatory pause during inspiration (Bucher and Chappell 1992). In torpor, minimum body temperature (T_b) of about 12°C is regulated when T_a goes below 10°C, requiring a metabolic rate about 7% of the rate that maintains the body at 39°C. Threshold (minimum body mass or energy reserve level) for torpor entry decreases gradually during the night in parallel with rate of energy depletion and actual need to extend endurance. This minimizes time spent in torpor, onset of which varies according to remaining energy reserves, while timing of arousal is fixed relative to daybreak (Hiebert 1990, 1992). Torpor allows this species to: (1) survive an energy emergency, (2) shift energy balance so as to facilitate fat accumulation for "fall" (southbound) migration, and (3) respond to food restriction or anticipation of shortage (Hainsworth et al. 1977, Carpenter and Hixon 1988, Hiebert 1991).

Rate of diurnal body mass gain, determined from captured birds, is progressively higher through Jan. If gains represent fat, the accumulation of 3.3 kJ/d in early Jan is insufficient to support nocturnal regulation of high body temperature, so torpor would be necessary. By mid-Jan, the projected accumulation of 6.8 kJ approaches adequacy. By month's end, torpor would no longer be needed to survive the night, as 11.2 kJ/d.
 
 
 
 

Rufous Hummingbirds hold their body temperatures at a constant level
and adjust their metabolic rate to maintain body temperature.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Body temperature held at about 12°C during torpor.  Sleeping birds enter 
torpor only as necessary based on energy reserves during the night.  
Arousal is triggered by daylight.

Torpor allows this species to: 
  (1) survive an energy emergency
  (2) shift energy balance so as to facilitate fat accumulation for "fall" 
                    (southbound) migration 
  (3) respond to food restriction or anticipation of shortage
 
 
 
 

The fatter a bird gets the less necessary torpor becomes.