How Do Birds Stay Warm on a Cold Winter’s Night?

Winter is a challenging time for backyard birds such as cardinals, woodpeckers, nuthatches, chickadees, titmice and wrens.

But roosting in tree cavities, bird boxes and an assortment of makeshift shelters can help these birds stay warmer at night and give them an energetic edge.

Finding a snug place to sleep on a cold winter’s night is particularly important to Carolina wrens. They belong to a primarily tropical genus of wrens and seem to have limited capacity to deal with really cold weather and snow.

Carolina and other wren species nest in natural places such as upturned roots, tree stumps, vine tangles and tree cavities, but they also nest in a whimsical array of human artifacts. Roost sites and nest sites are often one and the same.

Birds of North America cites some favored nesting spots of the wren:

• Glove compartments of abandoned cars and inside garages.
• Old shoes and shelves.
• Mailboxes and tin cans.
• And an equally whimsical array of settings are listed as the wren’s roosting places:
• Bird nesting boxes and abandoned hornet’s nests.
• An old coat pocket and a hanging fern.
• Garages and barns.
• Squirrel nests and old bird nests.
• And at least one ceramic pig planter.

It seems that any place that can support a messy domed nest of twigs leaves, root fibers and moss is fair game for a wren.

I can add retractable awnings to the list. Every year, a pair of Carolina wrens builds its nest in an awning over my back deck – despite the mechanical activity of the awning and all the commotion below it. And in winter the pair uses the nest for roosting as well, particularly when the temperatures are below freezing.

I can be reasonably certain these are the same wrens in summer and winter because Carolina wrens don’t migrate, and pairs stay on the same territory throughout the year.

It’s been documented that the northern boundary of their range ebbs south and flows northward depending on how cold or mild the winters are over a series of years. When sustained below-zero temperatures and snow descend on the landscape, the wrens simply perish.

Near the northern edge of their range, there are clear benefits of backyard bird feeding to Carolina wrens. A study in Michigan found that bird feeding may even be supporting their northward expansion.

The research showed that in winter, wrens disappeared from areas with no bird feeders and remained common in areas that had bird feeders.   It isn’t necessarily temperature that is driving wrens southward when the snow falls. It is their inability to find food during snowy conditions.

During the day, birds must eat enough to build up the fat reserves needed to keep them alive through long and cold winter nights.

Roosting in protected places gives the wrens (and other birds) a better chance of weathering the elements and conserving the hard-won energy reserves they gained during the day.

Studies of birds roosting in nests have shown that such domiciles confer critical energy savings. The same goes for roosting in tree cavities and bird boxes.

It’s not just wrens that are availing themselves of shelter when it gets cold. Woodpeckers and nuthatches nearly always use cavities to roost regardless of the weather, but chickadees and titmice resort to using cavities most often when it is very cold.

A study of the benefits of roosting in tree cavities on a cold night found that not all cavities are equal in their comfort levels. The bottom line of the research: cavities in bigger trees that are alive (not dead) hold the heat of the day longer and stay warmer at night.

In the coldest times, some species will huddle together to stay warm. In his book Birds Asleep, Alexander Skutch shares this report on the tendency of winter wrens to huddle together on cold nights:

Nine slept in an old nest of a thrush,

Ten squeezed into a coconut shell and

Forty-six into a nesting box.

One observer noted over one hundred pygmy nuthatches entering the same dead pine trunk on a cold night. Bluebirds also roost communally in tree cavities and nesting boxes during frigid spells.

In milder weather, these same species tend to roost alone or in family groups. One individual winter wren was observed alternating between roosting alone and with other birds as the temperature oscillated from mild to frigid.

While our backyard birds might be just fine on their own, we can help birds by offering places for them to roost in winter.   The easiest way to do this is to have a variety of bird boxes available during winter. If you are really creative, you can try to expand the list of wacky places where wrens will nest and roost. Poke around your yard for potential roost sites and watch and listen for birds going to roost just after sunset on winter days.