Birds and heat by Kate Crowley
By Kate Crowley
In the past I have written about the ways birds survive our deadly cold winters, but I haven’t written about the reverse. How do they make it through extreme heat waves? We know how humans suffer when the temperature nears 100F and the serious health crisis that can arise because of overexposure to the heat. It is probably fairly obvious to everyone that birds are much less active as the sun gets higher into the sky, even on days when the temperature is not hovering in the 90s. The best times of day to see birds are soon after dawn and before dusk. This can be hard for those of us who are not ‘early birds’ ourselves, but the longer you wait the fewer you’re going to see.
Even if they remain hidden and inactive during the middle of the day, birds still need to have ways of adapting to the hot temperatures. You may be surprised to learn that the same feathers that keep them so warm through the long cold winters actually help them cope with the heat too.
To begin with, birds start out with a higher metabolic rate than mammals. One example is the Golden-crowned Kinglet, which is an inch smaller than a Black Capped Chickadee. The Kinglet has a body temperature of 111F. That is a temperature that leaves little room for increase before the body begins to shut down. So birds face two challenges in hot summer weather – both their internal temperature and the external.
It turns out that the complex layers of feathers on a bird act as a heat shield. Birds can also orient themselves so that less of their bodies are exposed to the direct sunlight. They can raise and ruffle their back feathers, thereby creating more air space, aka insulating properties.
When we look at dogs on a hot day, we notice that their mouths are open and their tongues are lolling out. With a body covered in hair, this is their main way of dispersing heat – that and through the pads on their feet. Cats do the same. Birds also pant with beaks open, though you are less likely see their tongues hanging out. And they have another advantage when it comes to panting. Unlike mammals that inhale and then immediately exhale, birds have a complex system of nine or more air sacs to supplement their lungs. It is only after the bird exhales a second time that the original inhalation of air goes back out through their mouth or nostrils. In the meantime it has been pushed through those sacs which surround the internal organs and extend inside their hollow leg and wing bones. This process extends the surface area available for internal evaporation, cooling rapidly and efficiently the muscles that are generating the heat.
Since they have bare, scaly legs, birds can also lose heat by shunting more of their blood to these extremities. Gulls and herons can shunt blood flow to their legs and feet by twentyfold. Some birds dangle their feet during flight, as a means of cooling. And sometimes you will see a parent bird stand up when it is on the nest, both to cool itself and to not ‘overcook’ the eggs. Some birds, such as vultures and storks will actually squirt liquid excrement onto their legs – this is known as ‘urohydrosis’, as a means of cooling themselves through evaporation. There are even blood vessels in the bill that can release body heat. That is why some tropical birds, like toucans have developed such oversize bills. These then are the means of adjusting to the external heat source.
Flying is a highly muscular activity and the flight muscles of birds make up as much as a third of their body weight and can produce as much as 90% of the energy they use as excess heat. From a perched position to flight increases body heat as much as ten or twenty times. Most of us will never see this, but there are bare patches of skin on a bird’s body. They are called apteria. The feathers on their body can be moved and adjusted by their muscles in such a way as to cover or uncover these bare patches. They can also release water through these bare patches as well, though they do not have sweat glands. Each of these would allow for wind and evaporation to cool the body.
I watch the hummingbirds that continue to visit the feeders in the heat of the day and wonder how they do it. They must take regular breaks in the shade of the trees. I know the hummers will leave us and spend the winter in other places with equally hot temperatures, so I guess they are truly adapted to summer conditions unlike me, who longs for the cooling temperatures of fall and ….. Yes, winter.