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How Do Birds Keep Warm & Survive the Winter?
18th January 2021
Are birds warm blooded?
Like humans, birds are warm blooded or endothermic, which means their bodies generate heat and maintain a constant temperature even in cold weather. While many of the dinosaurs from which they are descended were cold-blooded, birds as we know them are believed to have evolved into warm-blooded creatures many millions of years ago.
Unlike other animals they cannot afford to lay down extra fat as this would hamper their ability to fly and put them at risk of predation. So only small amounts of fat are stored, perhaps just enough to keep them warm for a single night. In fact, small birds can lose over 10% of their total body weight during a single winter night.
Likewise, they cannot grow a thick winter coat (as many mammals do) but they do add extra, light downy feathers. These appear during the autumn moult and remain under the main body and flight feathers providing extra insulation (so good in fact, we still use them in pillows and duvets today) without adding excessive weight. At night, they can then fluff themselves up and tuck their head under their wings for extra protection.
Where do birds go in the winter?
A final trick they employ is to find shelter: thick vegetation, a nest box, a hole in a tree, often huddling together for warmth. Houses and other heated property is a favourite and any gap, nook or cavity will be occupied, resulting in many a household being woken in the night by the sound of birds scratching around in the eaves.
Millions of birds migrate from the UK to warmer climes each winter, some flying thousands of miles as far as sub-Saharan Africa or even Latin America in a journey that can take up to a month. Among the common UK garden birds that migrate south in the winter are swallows, martins, nightingales, yellow wagtails, cuckoos, swifts, ospreys and terns. Many types of swans and geese also migrate to different parts of Europe during the winter.
Best Bird Food for Winter
Ultimately all of these tactics rely on finding enough food to provide the energy required to keep body temperature up during the night, whatever the weather. This is particularly hard for them because daylight foraging is reduced to 8 hours or less, compared to over 16 hours during the summer. BTO research has shown that small birds must spend over 85% of daylight hours just foraging for food to be able to consume enough calories to survive the long night.
No wonder gardens with bird feeders are so busy during winter days. Large flocks of birds will travel surprising distances to attend gardens where they know the feeders will be full. When you see a blue tit or a blackbird, it will be one of dozens attending and sharing the food. Tagged studies have shown more than 30 different blackbirds and 35 different blue tits, (among many other species), visit a single garden in a day.
Putting out good quality natural bird food is great for garden birds at any time of year. Peanut granules and sunflower hearts have great nutritional value for the cold winter months. But making sure the bird table and bird feeders are topped up daily during the winter and especially during cold snaps is most important and makes a massive contribution to bird survival rates.
During winter otherwise territorial and aggressive birds such as robins become tolerant of others, driven by the need to feed. Robins and other small garden birds need to eat almost their own weight in food to be able to survive a 16-hour winter night, leaving no time or spare energy to waste on fighting.
Where do birds sleep in the winter?
Birds have an incredible ability to find warm spots in which to sleep during the coldest nights. Some sleep in the gap between loose bark and tree trunks, using both natural cavities and ones they have carved out themselves. Others might use hedgerows, thick vegetation, vines and creepers on buildings or available roof spaces to keep warm.
Birds to spot in the winter in your garden
Some of the most commonly sighted garden birds to keep an eye out for in winter include:
Prolonged periods of cold weather can be catastrophic for garden birds. Consecutive frozen nights deplete their reserves, weakening them night after night until eventually, exhausted and cold they die. It is not unusual after a particularly hard frost to find corpses of birds frozen solid, still perched on the branch where they perished. Garden bird population counts regularly show that their numbers can plummet by 50% or more during cold winters.