how do birds maintain homeostasis

The annual Great Backyard Bird Count came during the coldest days of the year when temperatures dropped below freezing and areas of the Valley experienced power outages. Despite the hardship, residents stocked their bird feeders, bundled themselves in layers of clothing and blankets and sat at their windows counting birds.

The GBBC is an inter-organizational effort of the Cornell Lab of Ornithology, National Audubon Society and Birds Canada. It is a four day event each February that began in 1998. Birds Canada joined the project in 2009; the event became global in 2013.

Many birds migrate to the Valley’s warmer winter climates and both resident and visiting birds have ways to survive unseasonable bouts of severe weather.

Perching birds fluff their feathers to trap heat and slow metabolism. They shiver, creating additional heat from circulation and muscle movement. Birds of a species find a wind break and huddle together to share warmth. At night, birds gather together in thick shrubs, or squeeze together on tree branches that block the wind.

Birds of all sizes alternately stand on one leg and tuck the other leg under their belly to keep it warm. Small birds, shore birds, water birds and ducks hunker down, covering their legs and feet with their warm bodies. They tuck their head under their scapular feathers and conserve heat by breathing air warmed by their body. Birds have oil producing glands that allow them to preen a coating of waterproof onto their feathers to avoid their downy under coats getting wet.

Some birds enter a state of torpor, a short-term condition where a bird’s body temperature, heart, respiration and metabolism are lower; they require less food during this temporary state.

“In cold-climate species, counter-current blood flow is the textbook example of one adaptation birds have to maintain homeostasis in severe cold,” said Quinta Mazatlán’s Urban Ecologist John Brush.

Ducks, gulls and wading birds have a built-in heat-exchange where the arteries with hot blood running to the feet pass right next to the cold blood running in the veins back to the body. The hot blood in the arteries passes heat to the cold blood in the veins before the blood reaches the feet. Heat is returned to the body and the process results in cold blood in the feet; cold feet lose very little heat to a cold ground.

During the recent below-freezing weather, about 500 black-bellied whistling ducks, a warm-climate duck, chose to spend the days not on the banks of a resaca where vegetation would seemingly protect them, but rather in pods bobbing upon the choppy water.

Brush said that we could venture that black-bellied whistling ducks also have that counter-current adaptation but cannot say for sure without proof from studies addressing that directly. “Ducks have very good water proofing and a dense layer of downy feathers,” he said. “Water off a duck’s back is an idiom for a reason, and it seems safe to say these birds have well-insulated coats. Water is a good temperature regulator. It could be that the water actually was a bit warmer than the surrounding air and potentially could have been beneficial for their thermoregulation.”

Temperature Regulation and Behavior: Birds can thrive in an amazing variety of environments, including tropical, temperate, and polar regions, thanks to their capacity to maintain a high and stable body temperature. This achievement is not without cost, however. The “expenditure” of producing heat through metabolism must be offset by consuming enough energy to balance the amount used, and there must be systems in place to release extra heat when needed. Birds increase their metabolic rate when the outside temperature drops in order to keep their body temperature from dropping as well. On the other hand, when the temperature outside rises too high, birds must use water to evaporatively cool themselves (just like humans do when we perspire) and prevent themselves from dying from heat exhaustion. Since they lack sweat glands, birds must pant or, in the case of nonpasserines, vibrate their upper throat and thin palate rapidly (“gular flutter”) in order to lose heat through their respiratory system. Birds regulate their rates of heat gain and loss using a range of morphological and behavioral features to reduce the energy cost of temperature regulation, or “thermoregulation.” Surfaces of the body without feathers or insulation are crucial locations for heat exchange with the surroundings. Therefore, certain heat-stressed birds—like Black Vultures—excrete onto their bare legs in an effort to increase heat loss through evaporation. The legs are a potential source of heat loss in the winter due to their lack of insulation. Many birds’ legs have arteries and veins that are in contact with one another, acting as a countercurrent heat exchange system to minimize heat loss. While the venous blood in the bird’s foot is relatively cool, the arterial blood exits the bird at body temperature from its core (trunk). Heat transfers through conductance from the heated arteries into the chilly veins as the cool blood flows back toward the core. As a result, venous blood that reaches the core has already warmed while arterial blood that reaches the feet is still cold. Furthermore, a bird can lessen heat loss by decreasing blood flow to its feet in cold weather by narrowing the blood vessels in its feet. Therefore, even though a duck or gull standing on ice may have a core temperature of 104 degrees Fahrenheit, its feet may only be marginally warmer than freezing. Another important factor in lowering the quantity of heat lost from unfeathered surfaces is behavior. A duck or gull on ice minimizes the amount of exposed unfeathered leg surface area by standing on one leg and tucking the other among its breast feathers; by sitting down and covering both legs, the amount of heat loss from the limbs is reduced. When it’s chilly outside, finches such as juncos and sparrows that forage on the ground often pause their food-gathering to cover their feet and legs with their breast feathers. Shorebirds are frequently observed resting with their beaks nestled among their feathers on chilly or windy days. They occasionally also stand or sit on one leg. Of course, birds can also thicken their “coat” by fluffing out their feathers, which will increase their effectiveness as insulation. Behavioral thermoregulation can also be achieved in other ways. Heat gain and loss can be adjusted by small adjustments to posture or orientation toward or away from the sun. For example, gulls frequently experience overheating issues because they typically nest in open environments with little to no available shade. On hot, windless days, nesting Herring Gulls will turn 180 degrees to face the sun continuously. By reducing the surface area exposed to direct solar radiation and enabling the gulls to display only their most reflective plumage (white head, neck, and breast) to direct sunlight, this effectively reduces the amount of radiative heat gain. Gulls avoid overheating by standing—better yet, by standing in water—thanks to their unfeathered legs and feet. Until they are a few days old, gull chicks are incapable of controlling their body temperature and suffer from severe overheating. Chicks in many shadeless colonies, like California Gull colonies, protect themselves from heat stress by standing in the shade that their parents provide. To prevent heat loss, shorebirds retract their legs and tuck their heads under their wings. Left to right:American Oystercatchers, Semipalmated Sandpiper, Sanderling, Western Sandpiper. In colder climates, birds can passively raise their body temperature by utilizing ambient heat instead of increasing their metabolic rate. For example, anhingas do this by spreading their wings when they sunning, whereas many small- and medium-sized species—particularly passerines—assume this posture when they squat or sit with their wings drooping and their feathers slightly erected. Typically, the bird is facing so that its back is completely exposed to the sun’s rays. It’s interesting to note that some grebes and Greater Roadrunners that sunbathe in this manner have black-pigmented back skin or back feathers, which likely help with heat absorption. Birds have developed the capacity to regulate their body temperature to a slightly lower degree when they are not moving or when they are deprived of food. This “controlled hypothermia” helps numerous species, from passerines to raptors, save a substantial amount of energy. For instance, Red-tailed Hawks lower their nocturnal body temperature by 5 to 7 degrees Fahrenheit during the winter when they are food-deprived. When extremely bad weather strikes, hummingbirds, swifts, and poorwills go into a state of torpor where their body temperature can drop by as much as fifty degrees Fahrenheit for several hours at night or even for days. When birds go into a state of torpor, they save a lot of energy, but because they can’t react fast, they are more vulnerable to predators. Arousal from torpor also entails a significant metabolic cost that must be immediately offset by food intake. However, new research suggests that birds may be far more common than previously thought to be able to briefly enter shallow torpor. Keep thermoregulatory needs in mind when you are watching birds. Indeed, one of the easiest things to see and understand about bird behavior is how shorebirds and gulls regulate their body temperature. SEE: Metabolism; Spread-Wing Postures; Feet. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.

The National Audubon Society, Birds Canada, and Cornell Lab of Ornithology collaborated to create the GBBC. Started in 1998, it is a four-day event held in February each year. Birds Canada joined the project in 2009, and in 2013 it went worldwide.

Some birds go into a temporary state known as torpor, during which their body temperature, heart rate, respiration rate, and metabolism all decrease. During this time, they need less food.

According to Quinta Mazatlán’s Urban Ecologist John Brush, “counter-current blood flow is the textbook example of one adaptation birds have to maintain homeostasis in severe cold.”

Brush suggested that we could speculate that black-bellied whistling ducks have that counter-current adaptation as well, but we can’t be certain of that without direct evidence from studies. He said, “Ducks have a dense layer of downy feathers and very good water proofing.” There’s a reason the saying “water off a duck’s back” exists, and it seems reasonable to assume that these birds have warm coats. Water is a good temperature regulator. It’s possible that the water’s slight temperature differential from the ambient air helped them regulate their body temperature. ”.

Birds that perch fluff their feathers to retain heat and slow down their metabolism. They shiver, creating additional heat from circulation and muscle movement. When a species of birds spots a wind break, they gather to share warmth. Birds congregate in dense shrubs or huddle together on wind-blocking tree branches at night.