Welcome to the third and final instalment of the: Keeping Cool in the Delightfully Warm Summer Series. Previously we explored how Mammals and Reptiles have overcome the challenges of the warm weather associated with summer in the beautiful African Lowveld.
What has been a continuous theme with Mammals and Reptiles, is that organisms both big and small are constantly trying to balance their core body temperatures – Birds are no different.
Birds do not sweat and instead have to rely on other ways to stay cool. Either by using metabolic heat exchange or using the surrounding environment, they have to endure tough conditions to avoid overheating, which when reaching certain levels, can lead to illness or even death. Birds are no different to mammals in that they produce their own metabolic heat and have adaptations and methods to counteract their ‘warm-blooded’ heat regulation system.
Drinking and Submerging in Water
Essentially, when it’s warmer, birds drink more water because water acts as a natural cooling agent and by staying well hydrated, birds can use water for evaporative cooling. This is easy for birds associated with water such as water birds and waders.
A Black-winged Stilt is reflected in the shallow water. These waders will walk around the edge of a waterhole probing the water for any aquatic insects, tadpoles or small fish.
However, those that live further afield such as Raptors may be seen standing in water to cool the bare scaly skin, bellies and vents which cool the blood effectively.
A Tawny Eagle descended to the water’s edge to have a drink. It is seen standing in the water, which also helps with thermoregulation.
Other birds with small feet and short legs that cannot make a quick getaway from predators at the edge of the water, prefer not to stand in the water but will plunge and dive from a perch to completely soak themselves. This is seen with the Kingfishers, Bee-eaters and Drongos which allows for an efficient option for cooling down while minimising the risk.
A Malachite Kingfisher plunges into the Sand River on a hot day.
The African Fish Eagle is cooling down and drying its wings after a plunge dive on a hot day. By drooping its wings and facing the breeze. The feathers are slightly ruffled, allowing for the circulation of air against the skin.
Countercurrent heat exchange
Many birds have a physiological thermoregulation system known as countercurrent heat exchangers, which are circulatory adaptations that allow heat to be transferred from blood vessels containing warmer blood to those containing cooler blood. To see how this works, let’s look at an example of a White Stork.
The artery that runs down the leg carries warm blood from the body. The artery is positioned right alongside a vein that carries cold blood up from the foot. The descending, warm blood passes much of its heat to the ascending cold blood by conduction. This means that less heat will be lost in the foot due to the reduced temperature difference between the cooled blood and the surroundings. The blood moving back into the body’s core will be relatively warm, keeping the core from getting cold. This ensures the bird’s body temperature is well-balanced on a hot day.
Image credit: modified from Counter current exchange in birds by Ekann, CC.
Gular Fluttering
Possibly the most important adaptation that the bird kingdom has utilised. When temperatures rise to extreme levels and the core body temperature reaches a critical level, an involuntary mechanism kicks in, known as Gular fluttering. Characterised by rapid vibrations of the throat skin, the bird will open its beak to allow for more effective airflow which allows air to pass rapidly over the moist mouth membrane and the tongue which results in the evaporative cooling of the blood in the brain.
A Red-billed Hornbill opens its beak to allow cool air to flow over the membrane in its mouth, allowing for rapid cooling.
This is an effective cooling mechanism but comes at the huge cost of expeditiously losing water and energy. However, if a bird cannot prevent its body temperature from rising to the upper lethal temperature (ULT) then the brain will lose the ability to control metabolism and hyperthermia will set in resulting in rapid death.
Urohydrosis
Vultures and Storks may appear, from time to time, to have a layer of ‘whitewashing’ covering their legs. When the weather is warm, they defecate on their legs and use faeces as an agent to keep cool. The initial evaporation begins the cooling process. The faeces is usually a whitish colour and in doing so continue to help reflect the solar radiation from the sun which then maintains the temperature of the bird.
The whitewashed markings on this White Storks legs is caused by the use of faeces to keep cool. This process is known as Urohydrosis.
Overheating of Eggs and Chicks
Various bird species use the warmer summer months with abundant food supply to breed efficiently. The hotter conditions can lead to overheating of eggs and chicks, especially for birds that nest in exposed sites, such as high up in trees with little cover, or exposed on the open ground. The temperature of eggs can be regulated through incubation, however, it is more difficult to protect chicks from the harsh sun once they have hatched. In order to provide shade, certain birds will ‘mantle’ over the chicks with their wings spread out.
A Grey Heron spreads its wings in order to protect the young chicks lying down in the nest from the harsh rays of the sun
Occasionally, a pair of storks will bring stomach-fulls of water to the nest, which they regurgitate over their young as a cooling shower and in extreme cases, some herons cool their chicks in a less hygienic way by defecating over them in a relieving but undignified way.
There you have it, the Keeping Cool during the Delightfully Warm Summer series has come to an end. I hope you enjoyed reading about how animals and birds are able to balance their body temperatures out in the wild and the amazing ways in which they are able to do so. Maybe we could use some of these tricks, but certainly not all of them.
Nick Tennick
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