Warm-blooded animals use a lot of energy to keep their core temperatures stable, and this energy comes mostly from food. The higher the metabolic rate of that animal, i.e., the quicker the food is processed, the more energy is produced = more heat.
This causes your heart rate to increase, and you get hotter.
As any heat lost from the animal is proportional to surface area, and any heat produced is proportional to mass, the smaller the warm-blooded animal, the quicker they lose heat – it is easier to stay warm if you’re a bigger beastie. This means that there is a point at which a warm-blooded animal would be too small to survive, as it would lose more heat than it could ever produce.
If there is no food, then the body will start to burn off any fat reserves to stay warm enough. Birds are not known for their heftiness, and your average songbird, small and quick, usually has around two days’ worth of energy after eating, at best, which is why during winter any supplementary feeding can literally mean the difference between life and death. But how often do we consider the elements at the other end of the scale?
Heat stress is when you can’t rid your body of excess heat. Unfortunately, this causes your heart rate to increase, and you get hotter. If you are unable to take any precautions or take yourself away from the external heat source, you will eventually die.
Being smaller animals than most, birds tend to use a lot more energy to perform normal basic functions like breathing, which means that they have a higher metabolic rate than most animals. This means that their core temperature usually sits somewhere between 37.7-43.5°C / 99-110°F.
Animals can help their bodies stay cool by resting and keeping their respiratory rates low. An average human has a rate of between 12 and 16 breaths per minute at rest. Your average songbird such as the house sparrow usually has a resting respiratory rate of around 50 breaths per minute. Give that tiny furnace some blazing external heat, and you’re going to run into problems quite soon.
On a reasonably hot day, say around 25°C / 77°F, this increases slightly to 60/min, but if the air temperature goes above 27°C the bird starts to experience heat stress, and the rate can soar to almost the same rate as when they are flying, around 160/min, and temperatures above 40°C will exceed that rate: without even moving anywhere. It’s quite obvious that this kind of increased energy output is just not viable.
Thermoregulation is a process where warmer blood cools and then circulates throughout the body, lowering the animal’s overall body temperature. When we get hot, those of us not afflicted with anhidrosis (a condition that prevents you from sweating normally) will start to leak at the seams.
Some birds, most notably vultures, will also unashamedly urinate on their legs.
An average human has around 3 million sweat glands, each producing moisture that gives a cooling effect when it evaporates from the surface of our skin. Breezes can help you feel this relief, but the mechanism behind sweating is that in order to convert this liquid to gas, you need heat, which just so happens to be right there in your skin. The heat from your blood moves outwards and evaporates the sweat and your cooler blood then travels off to the rest of your body and helps to bring your core temperature down.
Birds don’t sweat. And we don’t mean they don’t get nervous or worry, we mean they have no physical mechanism – no glands and pores – through which to sweat. As they can’t sweat, they need to lose this excess heat through other ways.
Staying still and resting in the hottest part of the day and finding shade is the usual resort. If they have to be sat in the sun’s glare, for example if they are incubating eggs, then birds with lighter coloured plumage may turn their lightest parts toward it, so more heat is reflected away from their bodies.
Any slight shift in posture or orientation with the sun can make a huge difference. Herring gulls, like many gulls, nest out in the open and are at great risk from overheating, and so they rotate 180 degrees to constantly face the sun on hot days. By presenting their heads, neck and breasts to the heat source, which are smaller surface areas compared to their backs and wings, and covered in white plumage, they reduce the effects of direct radiation and reflect much of the heat back.
Birds of course can also keep cool by going for a swim. The Galapagos penguin can spend 90% of a hot day in the water doing just that. Often, birds then fluff up their feathers after a swim and open their wings to catch the breeze, helping them cool off even more, as can be seen with cormorants.
Birds can also lose heat through any parts of their body that are unfeathered, such as the gape of their mouths, their feet, and underneath their wings if there are any exposed areas. This type of heat loss with the air is called cutaneous cooling.
Some tropical birds like toucans have huge bills that are graced with a rich blood supply. On hot days, the birds increase the blood flow to their bills to release heat and that helps reduce their inner temperatures.
Most birds don’t visibly pant like dogs, but they do open their beaks and use the same principle of passing air over the moist surface of the tongue and inner mouth parts to cool off. Many birds also flutter the throat area, called gular fluttering: this prompts heat loss from the throat membranes.
Bones in the throat flex to allow a surge of blood to the area, and a combination of rapid, open-mouth breathing and quick vibration of the membranes causes evaporation. Some birds, most notably vultures, will also unashamedly urinate on their legs to take advantage of evaporative cooling: the white paste of the urine and faeces also helps reflect the sunlight.
The heat is on
Climate change is a phrase that used to evoke End of Days style images: volcanoes erupting, seas boiling, hundreds of miles of scorched earth. Whilst that may yet be still to come, we now know that the effects of this issue are more subtle and some are already here.
As the planet grows warmer, the effects of heat stress on any organism trying to survive outside the temperature envelope they evolved in is now showing. Researchers have been logging world temperatures for decades and the period between 2015-2019 was the warmest 5-year period with record temperatures being felt across the globe, and the frequency, intensity and duration of these “heat waves” is only expected to increase. Heat stress is a creeping killer that many of us are now learning to battle with.
You can help the birds you are most likely to encounter deal with this phenomenon and keep cool by taking a couple of simple steps. One of the best ways to cool down is to avoid the heat altogether.
Many birds will forage in the early morning, evening or throughout the night so they can conserve energy and rest during the hottest part of the day, so offer them a perfect hideaway and shade for their rest with bushes, shrubs and trees. If you’re planning on any pruning or cutting back, try and wait until temperatures cool off, later in the year.
Heat stress is a creeping killer.
By providing a shallow bird bath, you give them the chance to have a dip and a drink if they need it. Position it so that it’s protected from the majority of the afternoon sun and don’t fill it any more than a couple of inches deep. Be aware of course that the heat of the day will evaporate that water, so keep an eye out and top up when necessary, and clean it out every day if you can.
Understanding how birds need to regulate their temperature on hot days can help you put things in place that will attract them to your home habitat, and then both you and the birds reap the benefits, all summer long.
Did you find this interesting? Here are more cool facts about how birds survive winter. Read more!