As you pull the curtains closed and turn out the light on a chilly winter’s evening, spare a thought for our feathered friends huddled in the dark outside. How do birds survive out there when the mercury drops?
It seems miraculous that a wren, a bundle of feather and bone barely as heavy as a pound coin, should make it through the night, and not just once, but a hundred times, until the welcome warmth of spring.
The clue is on your bed. Duvet is the French word for ‘down’, as in eiderdown, the wonder material with which many quilts were once stuffed. An eider duck’s down feathers provide the phenomenal insulation necessary to survive winter on Scotland’s coast, and other even harsher environments overseas.
During the breeding season, the female duck twice plucks this super-soft fluff from her own breast as a snug lining for her nest. Thirty eider nests yield around 450g of down. A trade study famously found that an eiderdown sleeping bag offers sufficient warmth at –35°C, while synthetic fill is effective only to –7°C.
What are down feathers?
Eiders aren’t the only species with down – all birds possess it. What’s special about these small, whitish, grey or brown feathers is their complex fluffiness. Put many together and you maximise the internal surface area, which is the best possible structure for capturing pockets of air, an effect known as loft.
“The more static air a material can hold, the better it is at keeping the hot side hot and the cold side cold,” writes ornithologist Thor Hanson in his brilliant book Feathers: The Evolution of a Natural Miracle. “Down feathers are the most naturally insulative material on Earth.”
Britain’s heaviest bird, the mute swan, has more than 25,000 feathers in its plumage, while a songbird such as a robin or blackbird typically has 2,000 to 4,000. Of these, the vast majority are down feathers, or have a fleecy area near their base, sometimes called the ‘afterfeather’.
We never see these parts of a bird unless they are revealed during preening, or it has fallen victim to a predator, leaving a tell-tale explosion of fluff on the ground as if someone has just had a pillow fight. Though hidden, these feathers give a bird its vitality and heft.
By preening and ruffling their plumage, birds can change their shape or size at will. On cold winter nights, they take this ability to extremes. Small species, such as our 8cm–12cm wren, puff up until near-spherical and barely recognisable, resembling the pom-poms on woolly hats. Only the tips of their bills and legs poke out. Comical maybe, but preferable to falling off your perch.
Birds don’t rely purely on feathers for warmth in winter. Other survival strategies include shivering and huddling in groups, pressed together for warmth, with their heads pushed into the middle of the scrum.
Wrens often roost in empty nest boxes and under eaves: the British record is 61 birds sharing a single box. Robins are feisty little creatures that see red, yet overcome their natural aggression to spend the night in a communal huddle.
Treecreepers – mouse-like birds that creep up tree trunks for a living – squeeze into cracks in the bark for the night, and have learned how to scoop out treecreeper-sized hollows in the spongy bark of ornamental giant redwoods.
Wildfowl and shorebirds must tough it out on marshes and mudflats, whatever the weather. Size may be on their side – big animals lose heat less rapidly than small ones – yet they still need a few tricks to beat the winter chills. So they stand on one leg, with the other tucked in, and bury their heads and bills in the soft plumage behind their necks.
How do birds look after their feathers?
To birds, feather care is a matter of life or death. It’s why they spend so much time rearranging their feathers until they lie perfectly, frequently after bathing or rolling in earth (particles of dirt clean feathers as efficiently as water). You may also see birds reaching round to pick up waterproof oil from a gland near the base of their tail, then wiping it through their plumage. To deal with wear and tear, most feathers are moulted and replaced every year, sometimes more than once.
Of course, feathers are not only about warmth. These wonders of nature (formed from keratin, the protein in your nails and hair) provide bright colours for display, patterns for camouflage and an aerodynamic body-covering for flight. Light, strong, flexible – feathers have enabled birds to conquer the skies like no other creature.
“From the standpoint of evolution, feathers are a runaway hit,” Hanson writes. “Nothing competes for sheer diversity of form and function.” He has a point. Delicate feather fringes that muffle sound allow owls to swoop in silence.
Snipe create an eerie hum with modified outer tail feathers that vibrate in mid-air – their version of a courtship song – while plummeting towards moor and mire. Kingfishers dazzle with their neon plumage, yet rely on a trick of the light: the plumage is actually brown, but appears blue due to the feathers’ structure.
And which British bird has the most amazing feathers of all? The jay, perhaps, with feathers barred in electric-blue and black? No; my vote goes to the waxwing, a gorgeous berry-guzzling winter visitor from the Arctic. Half a dozen of its wing feathers look as if they’ve been dipped in red sealing wax. Why? No one knows.
