Birds, like humans, are endothermic (warm-blooded) animals. Like us, their body heat is generated from within and must be kept within a certain range for survival. However, because most birds are fairly small, they have a larger body surface relative to their size, meaning they lose heat faster than we do. So how do they keep warm?
Photo by Blalonde (via Wikimedia Commons). Licensed under the Creative Commons Attribution-Share Alike 4.0 International licence.
As the seasons turn and the temperatures drop, we’re able to wrap up warm in thick knits or stay indoors with the benefit of central heating to keep us cosy. But wild birds, who live their entire lives outside, have no such luxuries.
Some UK birds, unwilling to tolerate the cold, simply migrate to warmer regions of the planet until spring rolls back around. But many others—including finches, robins and sparrows—stay here and stick it out. In fact, due to climate change, some wild birds that used to migrate to the southern hemisphere every year (such as swallows) have started wintering in Britain.
So what are their strategies for dealing with the winter chill in the UK?
Layering on the fat and feathers
Eat to heat
As temperatures cool, birds will increase their energy intake to build up their fat reserves. However, while mammals can build up an excess of fat stores in preparation for a long winter, birds are limited in how much fat they can hold on their bodies.
This is because birds also need to be able to fly so that they can escape predators. To balance their need for warmth with their need for flight, birds will often spend each day building up just enough fat to get them through a few nights at most, or sometimes even just one night at a time.
Birds who don’t manage to build up sufficient fat stores through the day are at risk of perishing overnight. Help your local wild birds prepare for the twilight frost by keeping your bird feeder stocked with energy-dense seed mixes and fat balls.
Fluffy feathers
Just as mammals can thicken up their coats for winter, so too can birds add extra feathers for insulation. These feathers are added next to their body, underneath their exterior feathers. Also known as down feathers, this extra layer is softer and finer than their tougher outer and flight feathers.
The fluffiness of down feathers makes them excellent for trapping plenty of warm air under the stiffer exterior feathers. You might sometimes see birds fluffing up their exterior feathers—this is their strategy for trapping even more warm air amongst their down feathers.
Practical preening
A bird’s strong exterior feathers also play an important role in trapping the warm air held by the down feathers and keeping it from escaping. Birds will preen these tough outer feathers, distributing oil along them to keep them clean, well-lubricated and waterproof in order to protect the soft down underneath.
Specialised scales
Birds lack feathers on their legs and feet, but make up for this with special scales that help minimise heat loss from these areas. They’ll also sometimes tuck one leg up into their feathers or crouch down to cover both legs, protecting them from the chill.
Bird behaviours to get cosy
Huddling and flocking
Like emperor penguins surviving the darkest months in the Antarctic, many birds that winter in the UK will huddle up together for warmth. Sparrows, wrens, swallows, tits and finches are just some of the bird species you may see bundling together on tree branches when temperatures drop, sharing their body heat for survival.
In a similar vein, birds will flock close together to reduce individual exposure to the cold air.
Twilight torpor
At night, birds will sometimes enter a state known as torpor to conserve energy.
Torpor is similar to hibernation. In torpor, birds will temporarily drop their body temperature to a lower level that’s easier to maintain than their ideal body temperature, allowing them to decrease their metabolism and conserve fat stores.
Nesting to escape the night frost
Night-time nests
Birds endeavour to find the warmest spots available to avoid freezing overnight. Preferred hideaways include tree cavities, thick vegetation and roof spaces. These nooks allow birds to get out of the cold, plus they provide protection from hungry predators.
Unfortunately, not all birds survive the winter freeze, due in part to a lack of sufficient shelter. Adding some simple nesting pouches or a stylish nesting box to your garden can make a huge difference and help these exposed little birds make it through the harsh winter months.
Nesting Boxes Nesting Wool
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