How Do Tiny Birds Survive the Winter?
At this time of year, these dark and cold days and nights might find us grumbling in front of our woodstoves, bundled in blankets, cradling mugs of hot tea, but let’s spend a few minutes considering how the tiny ones outside survive, some weighing less than a teaspoon of sugar or a couple of pennies, and what’s more…with bare feet!
While the short days of winter find many small birds such as rufous hummingbirds, flycatchers, swallows, warblers, and vireos gone from the Coastal Douglas fir forests of the southern Gulf Islands, some of the tiniest ones manage to keep going until the warmer days of spring. If you are out walking the trails during January, you will likely encounter thrushes, finches, and woodpeckers, but have you also noticed the chittering “surround sound” of kinglets and creepers, chattering chickadees, the nasal “neep neep” of the nuthatches, the mellifluous song of the Pacific wren, and the wispy high notes of the less-common bushtit? It amazes me that these little songbirds find the energy to keep themselves going through these coldest days, singing all the while.
The Anna’s hummingbirds, golden-crowned kinglets, and bushtits weigh a diminutive 4-5 grams. Brown creepers weigh only a gram or so more, and the chestnut-backed chickadee, red-breasted nuthatch, and Pacific wren slightly more again at 8-10 grams. While the cost of migration to a tiny bird is very high, staying behind and surviving freezing temperatures is also very challenging. Let’s take a look at their adaptive strategies for keeping warm, finding food, and preventing frostbite in those tiny toes.
Hummingbirds
You may already know that hummingbirds go into a state called torpor, which means their body temperature and heartrates drop drastically at night. They breathe only 6 times/minute in torpor, versus 240 times/minute during the day. Between visits to feeders and winter flower nectar sources, and hunting small insects and spiders, hummingbirds gain around 16% of their body weight each day, which they lose again during a chilly night. Anna’s hummingbirds have only been overwintering in British Columbia since the 1960s. Scientific research has shown that through rapid evolutionary change driven by the availability of hummingbird feeders, their bills have actually changed shape to better access this sugary food source. They also take advantage of the great diversity of plants we humans put in our urban gardens for year-round colour.
Chickadees and Nuthatches
Speaking of feeders, if you are inclined towards setting out seed and suet in the winter, you will be very familiar with our local chickadees and nuthatches. In general, these birds get only about 20% of their food from feeders, but when the ground is frozen, that changes to 80%. Access to water is essential for birds, so putting out a heated bird bath or topping one up with hot water during freezes is very helpful.
In autumn, chickadees and nuthatches cache seeds under the bark of conifers and retrieve them during cold spells. How does a bird with a tiny brain remember where exactly they hid all that food? Researchers have learned that these birds’ brains grow by 30% in fall and winter to help them rise to that particular challenge.
On cold nights, these little birds fluff themselves up to trap warm air next to their bodies and find tree cavities to roost in. Shivering is the body’s way of generating heat when we’re cold, and birds shiver all night, burning huge amounts of energy doing so. An equivalent would be if a 165 lb man spent a freezing night outside and weighed 15 lbs less in the morning! Chickadees can consume 60% of their body weight in food in a day, making them more like eating machines than tiny creatures with bird-like appetites.
Golden-Crowned Kinglets
The very tiny golden-crowned kinglets face additional difficulties in freezing weather, as they do not roost in tree cavities, and while they reach a state of hypothermia by lowering their body temperatures by 12-15 degrees, they do not appear to go into torpor. That these little birds can survive a spell of freezing nights is truly amazing. How do they do it? Calories are crucial, and these birds forage non-stop during the day, picking up insects, caterpillars and spiders from the forest floor and under the bark of trees, and they will occasionally visit suet feeders. At the end of a good foraging day, a golden-crowned kinglet may have eaten the human equivalent of 27 pizzas!
At night these little birds form small groups and huddle closely together for warmth, shivering constantly. Their downy feathers can be fluffed up to an inch thickness, and they grow more downy feathers in the fall to help insulate themselves further. Despite all this, winter mortality rates among kinglets is sadly very high, particularly during long freezing periods. To counter the effects of this, during breeding season kinglets usually lay two clutches with 8-9 eggs in each.
Other Small Birds
Brown creepers and nuthatches both find lots to eat under the bark of conifers, with creepers spiralling upwards using their long spine-tipped tails for support, and nuthatches moving in a downwards fashion – between the two of them, no bark dwelling insect is safe.
Bushtits tend to live and fly in little chattering flocks. They too are virtual eating machines, consuming up to 80% of their body weight each day. They eat insects, spiders, some plant materials, and love suet feeders. They do not go into torpor or into a state of hypothermia at night, but instead huddle into very tight clumps in shrubs and trees.
The Pacific wren (formally named winter wren) is smaller than a chickadee, and its beautiful voice is surprisingly loud for such a diminutive creature. This wren is a woodland bird that inhabits moist forests with a dense understorey. Brush piles, fallen logs, root masses and wood piles all provide essential shelter and food for its survival. Wrens can be spotted moving like little mice along the forest floor, and while generally considered quite solitary, they also form group huddles at night to keep warm.
Heat Exchangers for Small Feet
And exactly why is it that a bird’s little feet don’t freeze during cold weather? The answer is rather fascinating. Their foot temperature always stays low, which is especially crucial during cold weather to avoid losing valuable body heat and wasting precious calories. The key is a heat exchange mechanism whereby the arteries and veins in the feet are intertwined. In this way, the warm blood in the arteries is cooled by the veins to reduce heat loss to the environment, and through heat exchange the cool blood of the veins is warmed before entering the body, thereby preventing the bird’s core temperature from cooling. Thus, frostbite in tiny toes and fatal hypothermia are kept at bay.
What You Can Do
So now that we’ve looked at how these little feathered wonders try to cope during the winter, how can we help them?
Firstly, by leaving natural habitats intact for them to forage for food and find shelter; by creating brush piles away from our homes rather than burning debris, and if planting a garden, by adding year-round foods such as fall berries and flowers. Leave snags standing and rotten logs on the ground to provide shelter and grubs, and if possible provide a year-round water source. There are free DIY plans available online for building roost boxes (a bit different from nesting boxes, but those are good too). I recently purchased a heated hummingbird feeder and it has made feeding those little gems during long cold spells much easier. If you choose to put seed out during times when the ground is frozen, offer a variety of seeds, nuts and suet to attract a variety of birds. To avoid encouraging rats, place trays under feeders, sweep up fallen seeds, and bring feeders inside when it gets dark. Another option is locate feeders on a pole away from structures, or use a baffle or cone to prevent rodents from accessing the free food.
So next time you are taking a frosty walk on our forest trails, enjoy the birdsong and be amazed by the tiny little feathered ones who spend their winters here with us.
References
As always, a debt of gratitude to the Cornell Lab who provide much useful information on all things bird-related.
Many thanks to Mike Yip of Nanoose Bay, BC, who generously provided some of his beautiful photos for this article. For more of Mike’s work, see his books Birds of Vancouver Island (4 Volumes).
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