No sweat to keep cool
ONE of the effects of climate change is extremes of weather – extra heat in summer, and colder in winter. Birds are very hot-blooded creatures, with a body temperature averaging 40°C (104°F). In extremely hot weather they have to try to lose heat, but unlike mammals they don’t have sweat glands. So how do they keep cool?
One simple way is to stay in the shade, and this is what many birds do during the hottest part of the day. Remaining inactive conserves energy, but moving around is not only tiring, it also generates heat.
Birds have a rapid respiration rate which enables them to lose heat effectively through breathing. On a hot day it is not uncommon to see a bird with its beak open, panting.
The breathing rate of a House Sparrow rises from 57 breaths per minute at 30°C to 160 per minute at 43°C.
Some birds are able to vibrate the skin of the throat and the floor of the mouth, called gular fluttering, which is most easily seen in herons and cormorants. A pelican’s pouch is also very effective at losing heat.
Water can help birds to stay cool as they can get their feathers and skin wet. If there is a breeze, evaporation will cool them down. This is why it is important to keep bird baths topped up during the hottest weather. Many waterbirds, gulls and waders will stand in water when it is hot to allow heat to transfer from their legs into the cooler water. Heat loss into water is four times higher than that into air.
Storks (Marabou and White), as well as New World vultures (such as Turkey and Black) and condors, are known to excrete their liquid droppings onto their legs, allowing evaporation to help cool down the skin of the legs and the blood vessels beneath. The dried droppings are white which also helps to reflect the sun, further keeping the legs cooler. This behaviour is known as urohidrosis.
Vultures and birds of prey use thermals to soar. Rising hot air gives them lift, and they tend to soar the highest on the hottest days, with one benefit being the cooler air found at higher altitudes.
It has been known for some time that birds with especially large bills, such as toucans and hornbills, can use them as radiators to help them cool down. Blood vessels which go to the bill surface can be dilated so the hot blood at the surface cools down. This method is particularly useful as it doesn’t dehydrate the birds as other means might.
Recent research on various sparrow species in North America has shown that coastal subspecies in dry, arid conditions tend to have larger bills and this helps them to lose heat. Darwin’s finches on the Galápagos, which have species with varied bill sizes, also use this method of keeping cool.
Some birds have bare skin patches, like the face of a Magellanic Penguin, and dilated blood vessels making the face bright pink is a common sight on sunny days. Turkeys can expand their wattles to radiate heat and other species radiate through the skin in their legs and feet.
Plumage colour can also play a part. Some birds make use of their feather colour by turning the lightest parts of their bodies towards the sun. On very hot, windless days nesting European Herring Gulls have been observed facing directly into the sun which both minimises the amount of the body exposed to the heat and keeps the white part of their bodies in the sun, reflecting some of the heat. The incubating birds follow the sun as it crosses the sky, rotating 180 degrees throughout the day.
Research on 96 birds in Spain showed that darker species tend not to occupy environments with a high ambient temperature. In Australia birds in hot, dry areas have plumage which can reflect near-infrared light, helping them to avoid overheating. Only the upperparts do this, as this is where the sunlight falls, and smaller birds are more reflective as they tend to dehydrate more quickly.