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How Do Birds Stay Warm During Cold Winter Nights?
17th January 2019
Birds are warm-blooded animals and must maintain their body temperature throughout long winter nights. This requires lots of extra energy to keep warm, so how do they do it?
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.
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.
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.
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.
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.
Putting out good quality bird food is great for garden birds at any time of year. But making sure the bird table and bird feeders are topped up daily during the winter and especially during cold snaps, makes a massive contribution to bird survival rates.