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How birds cope with extreme cold

  • Feathers serve many purposes, including trapping body heat in air pockets formed when feathers are fluffed—one of many adaptations that help birds survive periods of extreme cold. Like humans in down parkas, plump is the profile. Photo by Francie Von Mertens


Wednesday, January 10, 2018

Wildlife adapting to extreme winter cold seems a fitting topic these days.

For birds, most species migrate in response to winter. Migration has many perils, however, and about 20 percent of bird species stay put to face the challenges of winter instead of the challenges of migration.

Food, or its absence, spurs migration more than winter's cold.

Obviously flycatchers have to migrate while most seedeaters stay put.

Birds that are "sedentary," the term for non-migrants, have evolved a number of winter survival tricks.

To give a sense of the sophistication of some adaptations, black-capped chickadee brain neurons are refreshed each fall, all the better for chickadees to remember where they cached food for the winter.

Chickadees in wintry latitudes have a larger hippocampus (the brain's memory center) with more neurons than those in milder climates where ice and snow don't cover food resources.

Studies have determined that chickadees can't store enough calories to get them through the longest, coldest night—a situation that triggers hypothermia of a sort.

Chickadees enter a temporary torpor. Body temperature falls up to 15 degrees from a normal 108 degrees, and calorie expenditure can drop a potentially life-saving 25%.

For all winter birds, overnight roosting sites out of the wind conserves critical energy.

Ideal spot for a chickadee: a tree cavity not much larger than it is.

The photo of a blue jay shown here is my attempt at a photo of a bird with fluffed feathers. Similar to down comforters and parkas, a bird's inner downy feathers trap body heat. When not foraging—a fulltime job for most small birds during the bitterest cold—birds hunker down and fluff their feathers to trap body heat in the air pockets formed.

Feet disappear in the downy warmth, further minimizing heat loss.

As added assist, birds of northern climes have significantly more feathers in winter than summer.

The white-throated sparrow feeding with juncos just beyond my desk window has some 2,500 contour or body feathers compared to summer's 1,500.

Alaska's resident chickadees have a lot more winter feathers than Pennsylvania's sedentary chickadees.

I wonder if chickadees transported from Alaska to Pennsylvania would develop extra feathers as winter approaches.

I doubt it.

Shifting light levels trigger most seasonal adaptations, and light levels towards the Arctic Circle really, really shift.

As another key to winter survival, birds of the far north (redpolls, crossbills and other winter finches) have crops that store food that gets digested overnight, keeping a small bird's furnace burning through nighttime dark that can last up to 18 hours.

Redpolls, like ruffed grouse, also are known to burrow under snow to minimize loss of body heat—similar to a chickadee's snug roosting cavity.

It's no longer surprising to see robins in winter. They're losing their reputation as harbingers of spring.

As long as there are berries, winter robins will hang around. They lack a crop but do have a generous throat pouch they load up with food for overnight metabolizing. When temperatures plummet, that storage system can make the difference between overnight starvation and survival.

A robin anecdote.

At the end of a day surveying birds for the local Christmas Bird Count, our team pulled over near a wetland in Dublin where we'd seen a few roadside juncos.

We lingered, knowing a great day birding was coming to a close.

We saw a few robins working the winterberry shrubs, plucking this native holly's red fruits. A lot of robins in winterberry wetlands would be counted that day.

We had chatted about barred owls when we passed places where we'd seen them prior years—but none this year.

I asked if anyone in the group had a good barred owl hoot. Perhaps we could get a response and add an owl to our list.

It was a rhetorical question. Good birders are good hooters.

A fellow visiting from North Carolina had joined our usual Dublin team, and he responded with a throaty "Who cooks for yoooouuuu? Who cooks for yooouuu-awwwllllll?"

It sounded perfect to our human ears. As testament to that perfection, robins emerged from high in pines at wetland edge, taking flight in response to a presumed predator.

Wow. We counted 15 or 20. A fine ending to a fine day.

Mark gave another barred owl rendition.

Another stream of robins emerged, and another, and another.

Sheepishly, I admit that we disturbed robins gone to roost in the cover of white pines. Birders are not supposed to disturb birds, especially when foraging conditions are challenging.

As flimsy excuse, it wasn't frigid that day and the winterberry crop was bountiful.

Those conditions changed the following week. Winter robins wander when a region's food becomes scarce, along winter's other confirmed fruit-eaters, cedar waxwings.

I saw a flock of over 40 waxwings in crabapples at the tennis club in Peterborough. They let me pass within 10 feet as they fed on, too cold to take a break.

There are other winter adaptations, some no doubt unknown to us humans. Chickadees are one of the most studied species, and we generalize what's been learned about them.

About chickadees, familiar backyard companions favored by many people, I often hear "Where are the chickadees?"

This year's local Christmas Bird Count found the second lowest number in the count's 45-year history.

I don't know what's going on, or if anything is going on. I certainly will share any insights learned.

In the meantime, birdseed offerings increase chickadee survival during bouts of record cold. Other times they do just fine.