The final week of December was a cold one for us in the Pacific Northwest as what we call a nor’easter blew through. The temperature never got above freezing, and dropped to 8° in some areas, highly unusual here in western Washington where the ocean moderates our temperatures. At my home the temperature was in the low teens for several days, and with the north wind we had wind chills well below zero Fahrenheit.
For us inside our warm houses it was an inconvenience but for the birds outside it was a challenge just to survive. Birds must maintain an internal body temperature of at least 104°F, which allows them to maintain a high level of activity all day, all year — birds are faster, stronger, and have more endurance than similarly sized mammals. Flight requires it.
So, birds have a variety of adaptations — physical and behavioral — that help them maintain body warmth in cold conditions (Endothermy and Thermoregulation
). The most visually obvious adaptation is how they can morph from a sleek aerodynamic shape into a spherical fluffball.
Fluffing up is a conscious decision a bird can make, contracting special muscles in their skin. Technically, raising a feather is called ptiloerection. Even though birds look totally covered with feathers, their skin is not actually all that densely spaced with them. By mass, only about 6% of a bird is feathers, and that’s all types, not just the down and contour feathers that primarily insulate them. Hummingbirds have about 1000 feathers total. Considering how critical feathers are for flight, protection, thermoregulation, waterproofing, and appearance, a bird’s arrangement and control of them is pretty important. (The Integumentary Morphology of Modern Birds—An Overview)
Another adaptation when it comes to insulation is that birds have more feathers in winter; they grow a full set after summer molt and don’t start shedding them until spring.
A little bird’s round shape in freezing weather doesn’t seem to slow it down much, although I noted that the birds who did the most flying, like finches and woodpeckers, were somewhat more aerodynamic in shape than birds who primarily foraged on the ground, like the sparrows, at least while they were on the move. A study with juncos found the birds that were more fluffed and/or in heat-conserving postures took longer to take flight — as much as 50% longer — which would be dangerous around predators. There’s a trade off between warmth and quickness. (Heat-conserving postures hinder escape: A thermoregulation-predation trade-off in wintering birds)
Note how my local hummers and finches were more fluffed up when just perched, as compared to when they are flying to and from feeders.
The birds primarily visiting the feeder were more streamlined, zipping back and forth between there and the staging trees.
Video clip of a busy day. The plum tree’s main function in my yard is as a staging spot for feeder birds, winter and summer.
The ground-feeding birds were eating the same black oil sunflower seed (which I spread in places not too snowy) but they weren’t flying around as much. Moreover, the nearby shrubs gave them cover, out of the wind. In general they were fluffier than the birds flying around in the open, coming and going to the hanging feeders.
The thrushes were very fluffed up. They flew around but not fast or far, spending most of their time either perched on branches resting or plucking the berries. I don’t think they like the viburnum and cotoneaster berries all that much, compared to other kinds (like raspberries in summer) but this is what’s around right now. I have lots of shrubs around my yard: berries for birds in winter, and flowers in summer for pollinators.
Being small makes you quick and nimble, a useful advantage in escaping predators, but there’s a downside: it’s harder to maintain body warmth than for a larger animal. Small creatures have a higher surface area relative to their volume, more area to lose heat across into the freezing air. Minimizing surface area is why you tuck your arms close to your body when you’re out in the cold rather than waving them around. There’s only so much tucking a small bird can do, especially when they’re packing in calories for heat, so insulation becomes crucial. One metastudy looked at insulation in small vs large birds and found that while the mass of feathers for all birds increases proportionally with their body mass, the number does not: small birds have more feathers relative to their body mass, and they are lighter. More small feathers allows smaller birds to get puffier. Have you ever seen a spherical eagle or raven? (The allometry of number of feathers in birds changes seasonally)
(As a related sidebar, marine birds have a greater tendency to lose body heat than terrestrial birds since water conducts heat 23 times faster than air: that’s why there are no tiny marine birds. Or tiny mammals for that matter. No marine kinglets. Or mice. Except for the Water Shrew who eats more than its own weight everyday to generate enough heat to survive. If you’re too small, no amount of insulation can prevent heat loss across your proportionally large surface. To improve their insulation, diving birds have denser feathering than other aquatic birds, and birds living in colder climates do also. How feathered are birds? Environment predicts both the mass and density of body feathers)
Once the weather warmed up, the birds de-fluffed noticeably. There was still snow on the ground but when the temp rose into the 30s their appearance and behavior moderated. They weren’t eating nonstop as they’d been. We weren’t refilling the feeders as often.
Some side by side comparisons, Fox Sparrow and American Robin, before and after the big freeze, a few days apart:
I didn’t go out to the beach during the worst of the freeze but as the cold snap lessened we made a visit, and saw the small birds there with the same fluffing. Shorebirds are about as small as “aquatic” birds get.
Take a look at the flock of dunlins. They are so focused on foraging that one bumps into another. The bumpee does an odd thing: it fluffs its breast feathers. Is that a way of telling off the bumper? Or did it need to resettle its feathers? The bumper also fluffs its own breast feathers immediately after. Clearly the fluffing is intentional, whatever the reason.
Meanwhile, the Trumpeter swans, being huge birds, have a much smaller surface area to volume ratio, that is, not nearly as much surface over which to lose heat compared to their inside. They are more able to wait out the freeze, curled up with their head and feet tucked into their plumage, which further reduces their surface area.
While we’re on the topic of fluffing, I should note there are other reasons birds puff up which shouldn’t be confused with that primary goal of keeping warm in cold weather. In hot weather, lifting wings and feathers exposes their skin to a breeze or cool water. When birds are sick they often sit listlessly and fluff up. This can be due to not grooming or an attempt to cool off, since sick birds are often febrile (high temp).
Birds will also fluff up to look bigger, to impress other animals, either in defense — I’m scary! — or in breeding displays to attract a mate — Look how alluring I am!
During grooming birds will fluff up, to shake off water and dirt, and to get access to feathers underneath.
And finally, birds will fluff up their feathers just for fun. One of my very favorite bird videos is this one with a dancing parrot. Every time I watch it I laugh, as much for the hilarious reaction of his parrot buddy as for his dancing chops. Enjoy.
Any fluffiness going on in your part of the country?
The Dawn Chorus is open for your birdy reports of the week.