Table of Contents 1.

It’s no secret that feathers are fantastic as insulation, after all, we use them ourselves in some of our high-end winter coats and those of us without allergies snuggle beneath feather duvets. But how do our winged garden friends manage to regulate their body temperature when the outdoor temperature in the UK can fluctuate between -10˚c or less in a particularly hard winter, and 35˚c or worryingly more each summer?

The secret lies (mostly) in their feathers. Let’s focus on the winter months since we’re now facing those shorter days and colder temperatures.

Most birds, that are resident in Britain, undergo a complete moult in late summer or early autumn, at the end of the breeding season. This replaces worn and damaged feathers which have lasted through the rigours of displaying and then the demands of raising young, which often takes a physical toll on the parent bird. It also provides them with extra layers of soft, downy feathers that allow them to trap an insulating layer of warm air against their bodies, acting in the same way as thermal underwear. When the weather is especially chilly, they can often be seen with these feathers fluffed up to increase the air pockets and reduce heat loss, and it’s this practice that makes them look rather spherical. So, the origin of the round Robin on your Christmas cards is the feathery equivalent of a thermal vest!

Water birds such as ducks and gulls also have specially adapted legs and feet to minimise the heat lost to cold water or ice and keep their core body temperature stable.

Some birds use this opportunity to moult into a different plumage too, opting for a duller colouration than their breeding colours. Waders such as Ruff are an excellent example of this. This offers them more camouflage to hide from potential predators which, in turn, means that they will expend less energy in evading those predators. In the colder months, food is a vital resource and, since the days are shorter, there is less time in which to forage, so any reduction in energy expenditure is valuable. Many smaller birds like Blue tits and Goldcrests may need to consume as much as a whopping 30% of their bodyweight daily, just to survive the night in cold weather, and can spend almost every daylight minute doing so.

Of course, while we like to think of the birds we see in our gardens as “ours”, even they may move if the weather is severe. Blackbirds are a good example of this. They may not move far, and you may not notice their absence, as their presence may be replaced by that of others from further north, for example. The long-distance migrations of our seasonal visitors are not the only movements recorded, but these smaller-scale shifts in populations are harder to detect without regular monitoring and may often go unnoticed. In fact, despite its diminutive size, the least likely bird to migrate is our aforementioned round Robin. These are one of the few birds to sing all winter, those creamy trills a declaration of their territory and a warning to others to keep out of it. They are perfectly adapted, with those layers of fluff making them so round, to weather the winter storms, and are deserving of their place on the front of our Christmas cards!