BIO 554/754 Ornithology

Avian Energy Balance  & Thermoregulation  

Birds have high basal metabolic rates & so use energy at high rates. Among birds, songbirds (passerines) tend to have higher basal metabolic rates than nonpasserines. And, of course, the smallest birds, hummingbirds, have the highest basal metabolic rates of all birds. In general, basal metabolic rate (or BMR) is related to mass, with larger birds expending less energy per unit weight than smaller birds.  

Species Mass (gms) Kcal/kg/day

Trumpeter Swan 8900 47Brown Pelican 3500 75Common Raven 850 108American Kestrel 110 157White-crowned Sparrow

27 324

House Wren 11 589Rufous Hummingbird 3.5 1600

Comparison to humans?

Double logarithmic relationships between BMR and body mass for 21 avian herbivores from the literature (black circles) together with data obtained for the Rufous-tailed Plantcutter

(Phytotoma rara; short video clip; black triangle). The line represents the regression obtained with these values, with the equation: BMR = 4.95 mb

-0.286 (Rezende et al. 2001).

Log10 mass-specific basal rate of metabolism as a function of log10 body mass in a barbet, toucans, a hornbill,

fruit pigeons (including Ducula pacifica), and flying foxes (pteropodids; Dobsonia moluccensis & Pteropus vampyrus). Also shown are the standard curves for nonpasserines (Aschoff and Pohl 1970 ),

all birds (Reynolds and Lee 1996 ), and all mammals (Figure from: McNab 2001).

As with metabolic rates, birds tend to have higher body temperatures than mammals. In general, body temperatures of birds range from about 38 - 42 degrees C. Body temperatures of large flightless birds (e.g., ostrich & emu) and some aquatic birds (e.g., penguins) are on the lower end of this range (& within the range of mammals).

Source: http://fig.cox.miami.edu/~schultz/fall97/13res.html

 

The Life of Birds by David Attenborough - The Limits of Endurance

Relationship between metabolic rate & size, food habits, & altitude --  McNab (2003) reported that 99% of the variation in metabolic rate among different species of birds of paradise (N = 13) was based on three factors: body weight, food habits and the altitude at which the birds live. New Guinea is home to most birds of paradise. The birds get their name from the unique circumstances surrounding their discovery by Europeans in the 1500s. When the first preserved specimens reached Spain, their feet had been removed. The oddity of these seemingly footless birds, combined with their unique colors and shapes, prompted the Spanish to conclude they spent their whole lives aloft - in the metaphorical paradise of the sky - without ever alighting on Earth.     McNab's experiments involved placing the birds in sealed chambers, pumping in air and measuring how much oxygen they consumed - to determine their basal metabolic rates.  As with all animals, the biggest factor in determining metabolism was body size. The bigger the bird, the more energy it used, with body size alone accounting for 91% of the variation in metabolism between species. Food habits and altitude also influenced metabolism. Species that eat only fruit had lower metabolism than those that ate fruit and insects, and birds living at altitudes below about 3,500 feet had lower metabolisms than those at higher elevations.  Together, the three factors accounted for 99% of the variation in metabolic rates among different species, a "really unusual" result, McNab said.         McNab said it isn't clear why it takes less energy to keep fruit-

A female brown sicklebill bird of paradise

rests on the hand of University of Floridazoologist Brian McNab in an enclosure

atthe Papua New Guinea University of 

Technology. 

(www.napa.ufl.edu/2003news/paradisebirds.htm)

eating, lower altitude-living birds of paradise alive. One possible explanation, he said, may be that fruits are seasonal so there may be some periods when they are unavailable, and birds that use less energy are better equipped to survive such periods of scarcity. Lower altitudes also tend to be warmer, so lower-altitude birds would not need as much energy to remain warm and active, he said. 

Birds of Paradise  

Basal metabolic rate of birds is associated with temperature and precipitation, not primary productivity (White et al. 2006) -- Basal metabolic rate (BMR) represents a significant component of animal energy budgets, and is correlated with a range of ecological, physiological and life-history variables, as well as phylogeny. However, even after accounting for the effects of body mass, considerable interspecific variation remains, and understanding the causes and consequences of this variation is key to understanding how animals function in the wild and the limits that are set on their physiological performance. A classic adaptive explanation for variation in BMR derives from the observation that species from hot arid environments have lower BMRs than species from non-arid environments. This is postulated to result from the need to reduce (i) the rates of endogenous heat production in hot environments where evaporative water loss is restricted by water scarcity, and (ii) the food requirements and energy expenditure in environments where resources are sparse and widely distributed. Complementary to this, the high BMRs of species from temperate and polar latitudes are associated with high maximal rates of thermogenesis and increased cold tolerance.

A link between low environmental productivity and animal metabolism in arid environments is intuitively appealing. However, a simple dichotomous distinction between hot- and non-arid environments conceals a suite of biotic and abiotic characteristics, any or all of which could account for the observation of lower BMRs with increasing aridity. Characteristics of hot arid environments include low primary productivity, low and temporally unpredictable precipitation, extreme ambient temperatures and high potential evapotranspiration, each of which has been shown to be related to BMR. For example, among birds, BMRs of falconiform species from hot climates are lower than those from colder climates, and the BMR of larks (Alaudidae) is negatively correlated with a continuous measure of aridity that incorporates ambient temperature and precipitation. Among mammals, BMR is positively correlated with net primary productivity (NPP) and rainfall, and negatively correlated with ambient temperature and rainfall variability.

The BMRs of birds from the wet tropics are also thought to be lower than those of temperate species, but since wet tropical (luxuriant) environments are characterized by both high temperatures and high environmental productivities, a specific causal relationship between BMR, temperature and productivity has not been established. Previous studies have favoured single cause analyses, such as the classic arid-mesic dichotomy, and therefore failed to separate these variables as predictors. Other studies failed to include habitat productivity as a potential predictor of BMR, or they considered only a limited range of ambient temperature or environmental productivity. It is therefore unclear which component of the 'arid-luxuriant' dimension (precipitation, ambient temperature, productivity or some combination) best accounts for broad-scale variation in metabolic rate. White et al. (2006) collated BMR measurements for 92 populations representing 90 wild-caught species and examined the relationships between BMR and NPP (annual net primary productivity), Ta (extreme ambient temperatures), annual temperature range (Tr), and precipitation and intra-annual coefficient of variation of precipitation (PCV). Their analysis of the relationship between avian BMR and environmental variables revealed that BMR was negatively associated with Ta and Tr, and positively associated with PCV. Thus, the relatively low BMR of birds inhabiting hot arid environments arises as a consequence of generally high temperatures with seasonal extremes. In hot environments, low BMR presumably reduces endogenous heat load, and the low BMR of birds inhabiting luxuriant environments also arises as a consequence of the associated high temperatures.

In male Leach's Storm-Petrels, the relationship between residual BMR and (a) the lifetime hatching success of males whose BMRs were measured in 2000 (open circles) or 2001 (filled circles) (p = .65), untransformed lifetime hatching success data are presented to facilitate interpretation; (b) Julian hatching date in 2001 (P = 0.001); and (c) chick wing growth rate in 2001 (P = 0.003). Residual BMR was calculated using a GLM incorporating body mass, year, breeding age, sex, Julian date on which BMR was measured, and time of day when measured.

Basal metabolic rate and reproductive performance in storm-petrels -- Despite evidence that some individuals achieve both superior reproductive performance and high survivorship, the factors underlying variation in individual quality are not well understood. The compensation and increased-intake hypotheses predict that basal metabolic rate (BMR) influences reproductive performance; if so, variation in BMR may be related to differences in individual quality. Blackmer et al. (2005) evaluated whether BMR measured during the incubation period provides a proximate explanation for variation in individual quality by measuring the BMRs and reproductive performance of Leach's Storm-Petrels (Oceanodroma leucorhoa) breeding on Kent Island, New Brunswick, Canada, during 2000 and 2001. They statistically controlled for internal (body mass, breeding age, sex) and external (year, date, time of day) effects on BMR, and found that males with relatively low BMRs hatched their eggs earlier in the season and that their chicks' wing growth rates were faster compared to males with relatively high BMRs. Conversely, BMR was not related to egg volume, hatching date, or chick growth rate for females or to lifetime (23 years) hatching success for either sex. Thus, for males but not for females, our results support the compensation hypothesis. This hypothesis predicts that animals with low BMRs will achieve better reproductive performance than animals with high BMRs because they have lower self-maintenance costs and therefore can apportion more energy to reproduction. These results provide evidence that intraspecific variation in reproductive performance is related to BMR and suggest that BMR may influence individual quality in males.

Thermoregulation

To maintain body temperature, the physiological & metabolic reactions that produce heat must be balanced against those that radiate or conduct it away. Except within a range of

ambient temperatures called the thermoneutral zone, maintaining a constant body temperature makes a steady demand either on the biochemical processes of heat production and/or the physical mechanisms for heat loss.

When ambient temperatures fall below the thermoneutral zone (lower critical temperature or LCT), heat production must increase and/or heat loss must decrease. Above the other end of the thermoneutral zone (upper critical temperature or UCT), heat loss must increase.

Thermoneutral zones and basal metabolic rates may vary with season (e.g., West 1972):

During the winter, Willow Ptarmigans produce a denser coat of feathers and actually lower     basal metabolic rates, which reduces the gradient between the internal body temperature and

          the external temperature. This is the same as keeping your house at a cooler temperature to           reduce your heating bill. Both responses reduce the amount of energy necessary to stay warm.

(Source: Ricklefs 1993, p. 189, Fig. 10.11;  http://ecology.botany.ufl.edu/ecologyf02/homeostasis.html)

Seasonal acclimatization by American Goldfinches -- Liknes et al. (2002) evaluated seasonal changes in cold tolerance, basal metabolic rate (BMR), and summit metabolic rate (maximum rate of

Seasonal variation in summit metabolism and  cold tolerance for American Goldfinches. Bars 

represent the mean ± SD summit metabolism achieved 

metabolism in response to cold exposure) for American Goldfinches (Carduelis tristis) from South Dakota to determine if goldfinches differ in pattern of metabolic acclimatization from other species of small birds. Goldfinches were captured in winter (Jan–Feb), spring (April), and summer (June–August) and tested on the day of capture. Cold exposure tests involved subjecting individual birds to a decreasing series of temperature. The temperature eliciting hypothermia was designated the cold limit (Tcl). Whole-animal metabolic rates were analyzed. Winter goldfinches demonstrated significantly higher BMR

by acclimatized American Goldfinches in winter 

(Jan–Feb), spring, and summer (June–August).  Mean ± SD temperature at cold-limit (Tcl), the  bath temperature at which goldfinches became 

hypothermic, is represented by circles connected 

by a line. Note the positive relationship between

summit metabolism and cold tolerance in this  species. 

(46%) and summit metabolic rates (31%) and significantly lower Tcl (- 9.5°C vs. 1.3°C) than their summer counterparts. Spring goldfinches also showed significantly higher summit metabolic rates (21%) and significantly lower Tcl (- 5.3°C) than summer birds. Winter birds had higher BMR (23%) and summit metabolic rates (8%) than spring birds. In winter birds, Tcl was also significantly lower than in spring birds. These data support the view that prominent winter increases in summit metabolic rates and BMR are components of winter acclimatization in American Goldfinches in South Dakota and that seasonal changes in

metabolism in goldfinches are similar to those for other small temperate-wintering birds.

Birds are larger in the temperate zones than in the tropics. The gradient is quantitatively stronger in winter than in summer. In summer, phylogenetic and adaptive

responses of birds contribute equally to the gradient. In winter, the gradient in North America is much stronger than that expected by taxonomic turnover, and responses of

species independent of their family membership drive the overall pattern. The results of this analysis by Ramirez et al. (2007) confirmed Bergmann's rule in New World birds and indicate that winter temperatures ultimately drive the pattern, exerting selection pressures

on birds to become larger (i.e., with relatively less surface area). However, in summer,

the movement of migratory species into the temperate zone weakens the gradient and generates a pattern more congruent with that expected from the taxonomic composition

of the fauna.

Energy transfer between birds and their environment is influenced not just by changes in ambient temperature, but also by changes in factors like wind velocity.

Metabolic heat production of a House Sparrow exposed to no wind and a wind speed of 2 meters/second.

Infrared Thermography image of a House Sparrow exposed to 15°C and no wind (left) vs. that of a sparrow exposed to 15°C and a wind

speed of 2 meters/second (right).  Red colors represent higher temperatures grading into cooler temperatures with blue colors.

(Source: http://www.woodrow.org/teachers/esi/2001/Princeton/Project/zerba/researchpage2.htm)

Heat production:

The primary means for increasing heat production for birds is shivering. The large flight muscles (pectoralis) as well as the leg muscles play an important

role in generating heat by shivering

Different shivering threshold temperatures in different muscles -- Carey et al. (1989) observed that the thermal thresholds for the onset of shivering in the leg muscles of winter acclimatized adult House Finches (Carpodacus mexicanus) were substantially below the thresholds for the onset of shivering in the pectoralis muscle. Shivering began in the pectoralis at an ambient temperature of 20°C, while in the gastrocnemius, tibialis and peroneus muscles the corresponding temperatures were -5, -11, and -14°C, respectively.  (Photo source: http://audubon.wku.edu/daviess/hofinch.html).

Heat loss:

birds have no sweat glands so evaporation, one way to lose heat, can occur only via respiratory system:

o gular flutter rapid fluttering of the gular area observed in many birds, including pelicans, cormorants, turkey

vultures, roadrunners, quail, & goatsuckers (nighthawks & poor-wills)

o panting

Click on the photo to view a short video of an young Burrowing Owl panting.

other means of regulating heat loss include:

o Plumage number of feathers varies seasonally, with more during the winter

than during the summer position of feathers controlled by dermal muscles

lower temperatures > muscles contract  ('goose bumps') to 'erect' feathers > erect feathers create more air space > more, warm air trapped in the plumage acts as an effective layer of insulation

Range of solar heat

loads acquired by the skin for black and

white pigeon

plumages as a

function of wind speed.

Upper boundary

for plumages of either

color represents heat loads

for completely depressed plumages.

Lower boundaries represent heat loads for fully erected

plumages. Solar heat gain is the fraction of incident

solar radiation

that is absorbed by

the plumage and that

generates heat.

Clearly, darker coats (e.g. those with higher absorptivities) acquire greater heat loads from

solar radiation than do lighter

plumages. In dark

plumages, especially

at low wind speeds, ptilo-

erection results in

huge decreases in solar heat

loads to the skin. In contrast,

ptilo-erection results in

only small decreases in solar heat

loads to the skin in lighter

plumages at low wind

speeds (Wolf and Walsberg

2000).

o Posture - at low temperatures, birds can withdraw feet into plumage to reduce heat loss tuck head & neck under wing to reduce heat loss

Sleeping Black-tailed Godwit Source: http://www.cpcayless.fsbusiness.co.uk/answersold.htm

See Temperature Regulation and Behavior

Legs & Feet o heat loss is limited (but not eliminated; see figure below) in cold weather

because of a counter-current mechanism that 'saves' heat o can serve as heat 'radiators' during hot weather:

increased blood flow storks & vultures defecate on legs to increase heat loss by

evaporation Bill

o heat loss from bills may be common (but more study is needed)

Side view of a Toco Toucan's bill showing visible blood vessels. Scale bar, 1 cm.

Heat exchange from the toucan bill -- Toco Toucans (Ramphastos toco), the largest member of the toucan family, have the largest beak relative to body size of all birds. This exaggerated feature has received various interpretations, from serving as a sexual ornament to being a refined adaptation for feeding. However, it is also a significant surface area for heat exchange. Tattersall et al. (2009) found a remarkable capacity for Toco Toucans to regulate heat distribution by modifying blood flow, using the bill as a transient thermal radiator. Heat loss from the bill is highly variable, and, depending on air speed and ambient temperature, could account for as little as 25% (minimum) to as much as 400% (maximum) of resting heat production in adult toucans, the largest reported for an animal. This capacity for heat loss might become a liability at low temperatures. However, toucans and toucanets are well known for tucking their bills beneath their wings and orienting their tail feathers rostrally during sleep; this posture increases insulation of the bill and mitigates heat loss incurred during sleep.Thus, the toucan's bill is, relative to its size, one of the largest thermal windows in the animal kingdom, rivaling elephants’ ears in its ability to radiate body heat. These results demonstrate that the constraints of heat exchange and the bill’s potential use as a thermoregulatory organ should be considered in understanding the distribution, ecology, and behavior of toucans. Furthermore, given the rapid radiation of bill structures and diversity of beak morphologies of birds, thermal constraints from bill heat loss may prove to be a common feature among many birds.

  Toucans can use their bill to keep cool.

Average apportionment of total evaporation in Inca Doves at 42°C. Values in parentheses indicate average rates of evaporative heat loss.

Cloacal evaporation and thermoregulation -- Hoffman et al. (2007) presented the first experimental evidence that a bird is capable of evaporating enough water from the cloaca to be important for thermoregulation. They measured rates of evaporation occurring from the mouth, the skin, and the cloaca of Inca Doves (Columbina inca) and Eurasian Quail ( Coturnix coturnix). Inca Doves showed no significant increase in cutaneous evaporation in response to curtailment of buccopharyngeal evaporation. Cloacal evaporation in doves

was negligible at ambient temperatures of 30°, 35° and 40°C. However, at 42°C, the apportionment of total evaporation in doves was 53.4% cutaneous, 25.4%

buccopharyngeal and 21.2% cloacal, with cloacal evaporation shedding, on average, 150 mW of heat. In contrast, the evaporative apportionment in quail at 32°C (the highest

ambient temperature tolerated by this species) was 58.2% cutaneous, 35.4% buccopharyngeal, and 6.4% cloacal. These results suggest that, for some birds, cloacal

evaporation can be controlled and could serve as an important emergency tactic for thermoregulation at high ambient temperatures.

Birds living in cold environments must conserve body heat to avoid hypothermia. However, blood flowing from the body core to the periphery (like the legs & feet) carries heat can be readily lost through the skin. To prevent such loss, brids have a countercurrent heat exchanger -  blood vessels in the legs (arteries going in & veins coming out) in close proximity that allow heat to be recaptured and saved. The principle of countercurrent heat exchange is so effective and ingenious that it has also been adapted in human engineering projects to avoid energy waste, e.g., by ensuring good ventilation of buildings while avoiding the loss of heat to the environment on a cold winter's day. 

In a countercurrent exchanger, flow in two adjacent tubes (like blood vessels) is in opposite directions. Imagine these are blood vessels in a bird's leg: the artery on top & the vein on the bottom. The artery is bringing warm blood into the legs. As the arrows indicate, heat from the blood in the artery is transferred to the blood in the vein (but, of

course, oxygen & nutrients continue on to supply the cells in the feet). As a result of this heat 'exchange', blood in the bird's feet is relatively cool & little heat is lost. So,

even a duck standing on ice loses little heat from its feet.

Source: http://ecology.botany.ufl.edu/ecologyf02/homeostasis.html

At and below 0 degrees C, intermittent pulses of increased blood flow to extremities are used to prevent freezing and tissue damage, reflected in increased heat loss from the feet

and an increased metabolic rate (Kilgore and Schmidt-Nielsen 1975). [Source: http://www.sfu.ca/biology/courses/bisc445/lectures/regulation_heat_gain.html]

o Behavior

communal roosts seeking favorable microclimates, e.g., Willow Ptarmigan may

burrow in the snow for up to 21 hrs a day during extremely cold weather (Andreev 1991)

huddling

A wintering group of about 2500 Emperor Penguins (From: Gilbert et al. 2006).

Although huddling has been shown to be the key by which Emperor Penguins (Aptenodytes forsteri) save energy and sustain their breeding fast during the Antarctic winter, the intricacies of this social behavior have been poorly studied. Gilbert et al.

(2006) recorded abiotic variables with data loggers glued to the feathers of eight individually marked Emperor Penguins to investigate their thermoregulatory behavior

and to estimate their “huddling time budget” throughout the breeding season (pairing and incubation period). Contrary to the classic view, huddling episodes were discontinuous

and of short and variable duration, lasting 1.6 ± 1.7 (SD) hours on average. Despite heterogeneous huddling groups, birds had equal access to the warmth of the huddles.

Throughout the breeding season, males huddled for 38 ± 18% (SD) of their time, which raised the ambient temperature that birds were exposed to above 0 °C (at average external temperatures of − 17 °C). As a consequence of tight huddles, ambient temperatures were

above 20 °C during 13 ± 12% (SD) of their huddling time. Ambient temperatures increased up to 37.5 °C, close to birds' body temperature. This complex social behavior therefore enables all breeders to get a regular and equal access to an environment that allows them to save energy and successfully incubate their eggs during the Antarctic

winter.

Emperor Penguins  

Thermoregulatory status of the three bird categories (isolated, loosely grouped, free-ranging), and associated processes of energetic benefits.

Values are mean rectal temperatures ± SD (Gilbert et al. 2008).

Emperor Penguins (Aptenodytes forsteri) are the only birds that breed in the middle of the Antarctic winter. Males and females fast for 45 days during the pairing period, and males fast another 70 days to assume the incubation task, during which their body temperature has to be constant and high in order to maintain their egg at 35°C. Gilbert et al. (2008) examined the energetic benefits accrued from huddling and estimated the respective

contributions of wind protection, exposure to mild ambient temperatures, reduction in cold-exposed body surfaces and body temperature adjustments in these energy savings.

The metabolic rate of `loosely grouped' birds (restrained in small groups of 5–10 individuals that are unable to huddle effectively) is reduced by 39% compared to

metabolic rate of `isolated' birds, with 32% of these energetic benefits due to wind protection. In addition, metabolic rate of `free-ranging' Emperor Penguins, i.e. able to

move freely and to huddle, is on average 21% lower than that of `loosely grouped' birds. Exposure to mild ambient temperatures within the groups and reduction in cold-exposed body surfaces while huddling, though overestimated, would represent a 38% metabolic

reduction. About two thirds of metabolic lowering is attributable to the reduction in cold-exposed body surfaces and one third to the mild microclimate created within the groups.

Moreover, body temperature adjustments contribute to these energetic benefits: maintaining body temperatures 1°C lower would represent a 7–17% reduction in energy expenditure. These processes, linked together, explain how huddling Emperor Penguins save energy and maintain a constant body temperature, ensuring a successful incubation

in the middle of the austral winter.

Stopovers exhaust migrating songbirds - Wikelski et al. (2003) found that migrating thrushes use more energy during stopovers than on the wing. It may seem counterintuitive, but "birds have to prepare for their long flights by accumulating fat and preparing fuel", says Henk Visser, one of the investigators . Finding food during rest breaks and sitting through the cold night can use lots of energy. The discovery may prompt enthusiasts to build bird 'service stations' along migratory routes, says Visser. These reserves might enable similar, but endangered, species to rest, feed and gear up for the next leg of their journey. Every spring, millions of Catharus thrushes migrate from Panama to Canada - a journey of 4,800 km that takes around 40 days. They have regular breaks and fly for just a few hours at night. Over the entire journey, the energy used by a single bird is equivalent to that in half a kilo of worms. Less then 30% of this is used on flight, the team found. The group took blood samples from 6 radio-tagged thrushes before and after a 7-hour migratory night flight to calculate how much energy the birds used. As the birds headed north, the team gave chase in a fleet of cars and a light airplane. "This was the easy part," says Visser. The hard bit was catching them at the other end, he recalls. - Helen R. Pilcher, Nature Science Update

Veery (Catharus fuscescens)

Photo by Dan Sudia

Avian hypothermia:

some birds, particularly small birds that depend on insects or nectar, allow their body temperature to fall from normal levels

Avian facultative hypothermic responses (McKechnie and Lovegrove 2002) -- Facultative hypothermic responses have been reported in species from 29 families (see Figure to the right) representing 11 orders). However, the capacity for hypothermia in the majority of the 138 avian families remains unknown. The capacity for moderate hypothermia, during which body temperature (Tb) is reduced by <20°C, appears widespread. However, more pronounced hypothermia during which Tb is reduced by 20°C or more has been reported in only the Trochilidae, Apodidae, and Caprimulgidae (Fig. 1 ). The passerine capacity for hypothermia is limited, and Tb reduction of >10°C has been reported in only 7 of 28 species for which measurements of minimum body temperature exist. The lowest body temperatures in passerines were recorded in the Hirundinidae and Nectariniidae.

The capacity for facultative hypothermia shows considerable variation within the Strigiformes. Body temperature reduction of more than 6–8°C has not been observed in owls (Strigidae and Tytonidae). In contrast,

Phylogenetic distribution of avian facultative hypothermic responses. The phylogeny is based on Sibley and Ahlquist (1990) . The extent of

body temperature reduction (delta Tb) is given as the difference between normothermic rest-phase body temperature (Tnorm) and minimum hypothermic body temperature (Tmin), i.e., delta Tb = Tnorm - Tmin. An asterisk indicates that Tmin was recorded in

chicks, not adult birds (From: McKechnie and Lovegrove 2002). 

reduction in body temperature of more than 10°C appears to be widespread in the Caprimulgidae. The Caprimulgidae also include the only known avian hibernator, the Common Poorwill.

The limited data (ca. 1% of extant species) are insufficient to objectively infer general patterns in the phylogenetic distribution of the avian capacity for hypothermia. At best, the available data suggest that the capacity for pronounced hypothermia (i.e., torpor) increases with the relative age of taxa. A pattern of more pronounced hypothermia in phylogenetically older taxa is consistent with current ideas regarding the evolution of heterothermy (daily torpor and hibernation). Some authors have argued that heterothermy may be phylogenetically primitive, although it frequently constitutes a functionally advanced adaptation associated with small body size and unpredictable food supplies. 

hypothermia is generally triggered by reduced availability of food or depletion of stored energy reserves or both

two major categories of hypothermia in birds:o nocturnal hypothermia:

body temperature may decline 8 - 10 degrees (no lower than about 30 degrees C)

observed in doves & pigeons, turkey vultures, & several passerines, including chickadees

conserves energy to help a bird survive periods of reduced food availability or, in some cases, to facilitate the process of premigratory 'fattening'

Change in body temperature (Tb) and O2 consumption (VO2, measured in mL O2 g–1 h–1) with time for an individual Malachite Sunbird at two

ambient temperatures. Small dot = VO2; square = Tb 

Nocturnal heterothermy & torpor in the Malachite Sunbird -- Downs and Brown (2002) examined thermoregulation of Malachite Sunbirds and found daily fluctuations in body temperature (Tb) & oxygen consumption (VO2).  Surgically implanted minimitters were used to measure Tb continuously. As ambient temperature decreased, VO2  during the rest phase did not increase to maintain Tb. No birds remained normothermic during the night. At 5°C, Malachite Sunbirds became torpid with a decrease of 15°C in Tb. Birds increased Tb to active-phase levels with the onset of light. Malachite Sunbirds conserved energy nocturnally by reducing metabolic rate and Tb. This plasticity in Tb shows that daily variations in Tb of homeotherms are biologically important. Further, heterothermy (particularly nocturnal hypothermia and torpor) in small birds would be important in an unpredictable environment where food resources fluctuate to prevent an energy deficit.

Dominant birds stay leaner than their subordinates -- Legend says that the early bird gets the worm, but research suggests that the bird that dines just before going to bed has the real advantage. Pravosudov et al. (1999) found that socially dominant birds in three species (Carolina Chickadee, Tufted Titmouse, and White-breasted Nuthatch) are generally leaner than subordinate peers, probably because they can eat when they want. Dominant birds stay lean during the day, then pack on the fat just before a chilly winter night. Staying lean helps birds stay more maneuverable during attacks by predators. Lean birds also have more time to watch for predators, rather than looking for food. "Natural selection would like to keep a bird as thin as possible, but also have enough fat to get through the night," said Thomas Grubb, a co-author of the study. By eating late, dominant birds can reduce their risk of predation without increasing their risk of starvation." Subordinate birds have a less predictable food supply during the day because they must look for food in places that dominant birds wouldn't normally bother with," according to Grubb. Also, dominants can displace subordinates from food. "It seems that subordinate birds must carry more fat during the day as an 'insurance policy,' making them more vulnerable to capture by predators," he said. A bird can gain as much as 10% of its total body mass each day in fat. Gaining fat before nightfall can help birds survive in winter because they often go into hypothermia as a survival mechanism. Birds can lower their body temperatures by as much as 42 degrees F. During the night, birds tend to lose their accumulated body fat. "Extra fat at roosting time means a bird needs to go less far into hypothermia at night because it has more energy for metabolism," Grubb said. "Hypothermia is thought to be a cost, because it makes a bird less aware of its surroundings, therefore increasing its vulnerability to

 

Tufted Titmouse

White-breasted Nuthatch

nocturnal predators."

Photos by David Roemer

 

o torpor : greater drop in body temperature (to below 10 degrees C in some

cases) & substantial drop in respiratory rate (to as low as 1 or 2 breaths/min compared to rates as high as 300-400 breaths per minute in some hummingbirds) & heart rate (to as low as 30 beats per minute compared to typical rates greater than 500 per minute in hummingbirds)

documented in goatsuckers (e.g., common poorwill), hummingbirds, sunbirds, & swifts

permits birds to reduce rates of energy consumption by as much as 10 - 60%

Source: http://ecology.botany.ufl.edu/ecologyf02/homeostasis.html

Big Birds Can Keep Their Cool Too - Researchers

have found the biggest example yet of a bird that cools down to conserve energy (Kortner et al. 2001). The Australian Tawny Frogmouth (Podargus strigoides), which weighs in at half a kilogram, uses ‘torpor’ — as a controlled reduction of body temperature is properly called — as a survival strategy in winter, even though it is almost ten times larger than any other bird known to do so. Fritz Geiser and colleagues of the University of New England, Armidale, Australia, fitted seven frogmouths with external temperature-sensitive transmitters and three of those with internal temperature-sensitive transmitters, and tracked the birds’ temperatures every 10 minutes for 9 months. During winter, Geiser’s team was surprised to find, these big birds regularly enter torpor. “This enables the bird to remain resident in its territory throughout the year.” Avian torpor, they predict, is much more common than is currently believed(check this short video of a Tawny Frogmouth).

  Tawny Frogmouths

Useful links:

Dominant Birds Stay Leaner Than Their Subordinates

Feast of the Albatross

Metabolism

Metabolism and Thermoregulation

Spread-Wing Postures

Temperature Regulation and Behavior

The costs of being cool: a dynamic model of nocturnal hypothermia by small food-caching birds in winter

The Physiological Adaptations of the Emperor Penguin

Thermogenic mechanisms during the development of endothermy in juvenile birds

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