Temperature Regulation in Birds
Birds are endotherms, which means they maintain a near-constant body temperature by generating heat within the body. Unlike reptiles, which may become inactive for entire seasons, birds must regulate their body temperature using various physiological and behavioral means to remain active every day.
Despite being endotherms (warm-blooded), avian body temperatures range pretty considerably, with averages between about 97°F (36°C) and 113°F (45°C). They can maintain these temperatures using internal processes alone if they stay within a certain range of temperatures known as their thermoneutral zone. Above or below these crucial limits, some interesting behavioral strategies come into play.
In this guide, we’ll take a closer look at avian thermoregulation and how birds survive in such a wide variety of habitats.
The Science of Bird Body Heat
Sources of Heat
Birds gain heat from their environment or generate heat internally through their metabolism, which relies on a steady supply of fuel from their diet and oxygen from their respiratory system.
Within their comfortable ambient temperature range (thermoneutral zone), birds generate the warmth they need through their minimum or basal metabolic rate (BMR), which varies between species.
However, their metabolic rate may vary depending on conditions like unfavorable temperatures or during intense muscular exercise like sustained flight.
Heat Conservation
Bird feathers trap air, creating excellent insulation against the cold. Down is particularly effective for trapping heat, so birds from cold climates tend to have a denser covering of these soft feathers. Exposure to water cools the body much faster than air, so birds like Penguins also have a layer of subcutaneous fat that aids insulation.
Like us, birds can generate heat by shivering, which involves rapid contraction and relaxation of skeletal muscles. The flight muscles of the chest (pectoralis and supracoracoideus) are the most important since they are large and situated near the heart. While highly effective for generating heat, shivering is also very energy-expensive.
Bird feathers trap air, creating excellent insulation against the cold. Down is particularly effective for trapping heat, so birds from cold climates tend to have a denser covering of these soft feathers. Common Eider
Behavioral Strategies for Heat Management
Sunbathing and Shading
Birds use their metabolism and other physiological processes to manage heat within the body, but external factors are just as crucial. Radiation from the sun is an important source of heat that birds may use to warm themselves or avoid to cool off.
Birds will seek shade to cool off or sunbathe to warm themselves. In the shade, birds avoid direct solar radiation but also minimize heat gain through their feet and from the heated surfaces around them.
Postural Adjustments
Birds can gain or lose significant amounts of heat through exposure to the sun or wind. Moving to a sheltered spot is one way of controlling body temperature, but something as simple as body posture also plays an important role.
For example, a nesting Gull facing into the sun minimizes the surface area exposed to direct radiation. In contrast, a sunbathing Cormorant may lower or spread its wings to maximize its exposure to warm sunlight.
Great Cormorant. A sunbathing Cormorant may lower or spread its wings to maximize its exposure to warm sunlight
Circulatory Adaptations for Thermoregulation
Counter-Current Heat Exchange
The avian vascular system plays an essential role in thermoregulation by distributing heat around the body. Some birds cannot avoid exposure to cold environments and surfaces, which is especially troublesome for species that expose their unfeathered feet and legs to cold water, ice, and snow.
Bird’s legs and feet contain relatively few blood vessels, but many species have a way to warm the cool, deoxygenated blood in their limbs before it returns to the body.
Veins returning cool blood run alongside arteries, bringing warm, oxygenated blood from the heart. The two currents equalize their temperatures by cooling blood before it enters the feet and warming blood before it enters the body. This clever system is known as a counter-current heat exchange.
Blood Flow Regulation
In most birds, blood flowing through the legs is not insulated by feathers, allowing greater heat exchange with surrounding air or water. Birds can use this situation to their advantage by controlling the volume of blood reaching their extremities.
By increasing blood flow through the legs, birds can help heat dissipate, while decreasing blood flow to the lower limbs reduces heat loss in a cold environment.
Evaporative Cooling in Birds
Panting and Gular Flutter
When water evaporates from a bird’s skin or mouth, it attracts energy to break the bonds of the liquid. This physical change draws heat out of a bird’s body to reduce its temperature.
The inner surfaces of a bird’s mouth and throat are wet, so they often open their bills to pant and cool off in hot weather. Birds like Owls, Chickens, and Pelicans use a more specialized technique called the gular flutter, which ‘flutters’ their throat to create airflow through the oral cavity.
Bathing
Evaporation is very effective for cooling, but birds don’t sweat like we do, so they cannot wet their skin unless they visit a water source to bathe.
Some stand in water to cool their legs and belly, while others splash in the shallows or even dive in from above before cooling off on a nearby perch.
Pelicans use a more specialized technique called the gular flutter, which ‘flutters’ their throat to create airflow through the oral cavity
Thermoregulation Across Habitats
Arctic and Antarctic Birds
Many birds survive cold climates by simply avoiding them for much of the year. For example, most birds that occur in the Arctic are migrants who head south for the winter. However, some brave birds remain in high latitudes throughout the year, and these species have evolved special mechanisms to survive the cold.
Polar birds have many adaptations for keeping warm, including dense down feathers, subcutaneous fat layers, and even feathered feet. These physical adaptations are often combined with certain behaviors and survival techniques.
Emperor Penguins, for example, survive extreme conditions by huddling together. On the opposite end of the world, Rock Ptarmigans endure cold winter nights by burying themselves in the snow.
Desert Birds
Keeping cool in the desert is a serious challenge, yet birds live in some of the hottest places on the planet. Foraging in the shade or avoiding activity during the heat of the day are helpful strategies, but evaporative cooling is the most effective method when temperatures really soar.
Metabolism creates heat, and lowering body temperature requires water, which is a problem for birds that live in arid environments. Interestingly, desert-adapted birds tend to have significantly lower basal metabolic rates, creating less heat.
Fortunately, most birds are highly mobile, so they can usually fly to the nearest water source for a cool drink. Where standing water is unavailable, these birds may rely on water in their food source and an efficient excretory system that produces little liquid waste. Even away from the water, birds can cool themselves by panting or urohidrosis (defecating/urinating on their legs).
Emperor Penguins survive extreme conditions by huddling together
Cactus Wren. Keeping cool in the desert is a serious challenge, yet birds live in some of the hottest places on the planet
Temperature and Life Stages
Chick Temperature Control
Birds must maintain their body temperature within a certain range to avoid dangerous hypothermia and hyperthermia. Adult birds use the physiological processes and behaviors already mentioned, but developing embryos and young chicks cannot regulate their own body temperature.
Eggs and hatchlings rely on their parents for thermoregulation, which is usually achieved by incubation and brooding. During this process, one or both adults will ‘sit’ on the developing egg or chick to transfer body heat.
Many birds develop an area of naked, vascularized skin on the breast (called a brood patch) to avoid the insulating properties of feathers. However, excessive heat is also dangerous for eggs and chicks, so adult birds from hot climates may shade their eggs or even wet them to cool them off.
Temperature Challenges During Molt
Birds rely on their plumage to insulate their bodies, but they must periodically molt to grow replacement feathers. With fewer feathers, birds lose more heat, so they may increase their basal metabolic rate to compensate.
Impact of Climate Change on Thermoregulation
Changing Temperatures
Over millions of years, birds have adapted to survive in the climatic conditions within their natural range. Temperatures have always fluctuated, but accelerated climate change may alter conditions faster than some species can adapt.
Metabolic rates, vascular systems, and thermoregulatory behaviors have all evolved to match specific lifestyles and habitats, but birds that live in hot climates already inhabit an extreme environment, putting them in increased danger from rising temperatures.
Adaptation and Survival
The world is getting hotter due to global climate change, and birds are not immune to the rising temperatures. Birds are changing their migratory patterns, molting patterns, and distribution ranges in response to warmer weather.
As changes in climate alter natural habitats, birds are forced to adapt or seek suitable conditions elsewhere, causing declines due to factors like competition for limited resources.
Greater Roadrunner. Over millions of years, birds have adapted to survive in the climatic conditions within their natural range
Summary
We’ve all experienced the discomfort of stepping outside on a blistering hot or freezing cold day. Yet birds are able to manage their body temperature everywhere from the polar ice caps to the hottest deserts.
To survive in these varied conditions, birds rely on their insulating feathers, high metabolic rates, and other physiological adaptations like shivering and counter-current heat exchange. Often, these involuntary processes aren’t enough, which explains why we see such a variety of thermoregulatory behaviors like panting, bathing, and sheltering.
As the climate of our planet changes, understanding avian thermoregulation is becoming increasingly important for modeling changes in bird populations and distributions, and for planning conservation strategies to protect species at risk.