Cold Adaptation in Birds Types of Adjustments Physical = affects heat exchange between bird and environment Physiological = adjustments associated with energy provision and metabolism Behavioral = certain behaviors can help save energy

Physical Adjustments Feathers (Insulation) Plumage generally thicker in winter (and at higher latitudes), but plumage replaced at molt and shows wear thereafter Not great seasonal differences, particularly for small birds Temperature differential between skin and outside may be substantial (up to 40°C in Black-capped Chickadee)

Physical Adjustments Subcutaneous Fat Forms insulatory layer in only a few birds (e.g., penguins, grouse) Most store fat only in furcular and abdominal regions Most small birds (esp. ground-foragers) store more fat in winter Serve as fuel store and don’t contribute much to insulation

Physical Adjustments Regional Hypothermia and Peripheral Circulation Regions of body maintained at lower temperatures than body core Peripheral vasoconstriction shunts blood away from body surface Countercurrent heat exchange = closely opposed vessels allow heat exchange to keep warm blood near core (see figure). Can bypass if heat conservation not a concern. Changes in peripheral circulation can  heat loss markedly (by up to 90% through legs and feet)

Countercurrent Heat Exchange P. 136, Gill back

Physiological Adjustments Increased Shivering Endurance - occurs in most winter acclimatized small birds. Precise mechanisms unknown, but may involve changes in carbohydrate and lipid metabolism and/or muscle hypertrophy. Increased Summit Metabolism - occurs in most cold adapted small birds. Provides higher total capacity for heat production (4-7 X BMR), although rarely if ever do birds reach Msum in nature.

Cold tolerance and Msum appear to be linked in small birds. American Goldfinch 1 2 3 4 5 6 7 -14 -12 -10 -8 -6 -4 -2 2 4 6 Tcl (C) O2 Consumption (mlO2 min-1) Winter April Summer

Birds with high Msum also showed high levels of shivering endurance 60 Junco: R 2 = 0.54 (P < 0.001) 40 20 Time hypo Residuals -20 -40 -0.08 -0.04 0.00 0.04 0.08 log M Residuals sum Birds with high Msum also showed high levels of shivering endurance

Physiological Adjustments Some birds show winter increases in BMR, but others don’t. Elevated BMR may indicate cost associated with maintaining enhanced metabolic machinery devoted to thermogenesis, rather than being directly adaptive.

2 Summer Winter 1 BMR (ml O2 min -1) BCCH WBNU DOWO

Physiological Adjustments Regulated Hypothermia and Torpor Small birds: Tb drops 2-3°C at night Some birds (e.g., chickadees) allow Tb to drop 10-12°C below daytime levels, yet remain responsive to external stimuli (= Regulated Hypothermia) Torpor = state of dormancy, usually occurs on a daily basis in birds; Tb may drop to temperatures near ambient (down to 5°C in some birds). Regulated at lower Tb.

Euthermic MR Torpid Tb Ambient Temperature

Physiological Adjustments During torpor - unresponsive to stimuli. At very cold temperatures, birds will increase heat production and spontaneously arouse, as torpor becomes too dangerous at these temps. (difficult to generate the amount of heat needed for arousal at very low temperatures). Allows great energy savings because metabolic rate is much reduced. May reduce overnight energy expenditures by 1/3.

Physiological Adjustments Example: Hummingbirds may decrease MR by 75% when allowing Tb to drop to 10oC; saves up to 27% of energy needed to get through the night. Torpor occurs in hummingbirds, nightjars, swifts, mousebirds, bee-eaters. Poorwill is only true hibernator (2-3 months dormant). Formerly thought to be used only in cases of emergency (low food availability coupled with low temperatures), but may be more common (e.g., saving fat for migration).

Behavioral Adjustments Posture - decrease exposed surface area; tuck legs and bills into feathers, orient perpendicular to sun to receive maximum solar radiation Microclimate Selection - choose roost sites that protect from the elements (cavities in trees, thick brush, subnivean space). Can reduce energy expenditure by up to 50% compared to open areas.

Behavioral Adjustments Huddling - documented for several bird species. Reduces heat loss by decreasing surface area of each individual bird exposed to air (see figure). Feeding Intensity - show greater feeding intensities during colder times of year; arctic birds active at lower light levels in winter than in summer.

Behavioral Thermoregulation in Starlings back

Behavioral Adjustments Food Caching - storing food in specific locations. Provides a readily available food source for times when energy expenditures are high Birds that cache generally store less fat than those that don’t (decreases flight costs) Occurs in Acorn Woodpecker, nuthatches, crows and jays, chickadees and titmice