Winter mornings begin at David Sibley’s Deerfield, Mass., household as they do at my similarly rural place 90-something miles to the west and south, across the New York line: with filling the bird feeders.

“Baby, it’s cold outside” takes on a new meaning when many in your customer base weigh barely as much as a few pieces of pocket change.

The sunflower hearts — shelled sunflower seeds — that we both offer are happily devoured, but they are not the key ingredient in the birds’ survival. What the American goldfinches, white-breasted nuthatches, house finches and the rest cannot live without in these coldest months is a suite of physical and behavioral survival strategies hard-earned over millions of years of evolution.

These tactics, and the birds themselves, are the subject of “The Courage of Birds: And the Often Surprising Ways They Survive Winter,” the recent book by Pete Dunne, a prolific author about all things bird and former director of the Cape May Bird Observatory in New Jersey. It was illustrated by Mr. Sibley, a naturalist, author and illustrator perhaps best known for “The Sibley Guide to Birds.”

“Welcome to winter, nature’s proving ground,” Mr. Dunne writes, “where there is no prize for second place during these four months. Across the Northern Hemisphere, it’s ‘survivor take all.’”“Just walking through the woods this time of year,” Mr. Sibley said, “you can walk for an hour and not see a bird.”

Many colorful species are absent or in dulled-down winter plumage (yes, blue jays and male northern cardinals, we see your defiant exceptions). Song, too, is at a minimum now. But in each sign of bird life out there, we onlookers are treated to a vivid display of resilience. Get out your binoculars and really watch.

The birds are “meeting the season beak-on,” Mr. Dunne writes, and take note: A bird’s bill is not insulated. Nor are its legs and feet. So all those vulnerability points tend to be smaller in species that winter in cold zones — scaled down as a result of the natural selection process across countless generations.Mr. Dunne calls the feathers on these miniature, modern-day dinosaurs “the evolutionary edge.”

Feathers are the first line of defense against weather, Mr. Sibley said in a recent conversation, and besides enabling flight, “they’re streamlining, waterproofing, windproofing, coloration — all those things.” And down feathers, the soft, fluffy kind closest to the bird’s body, he added, are “the most effective insulation known.”

Using tiny muscles where their feathers attach to skin, birds can raise and lower them, thickening the insulating layer around their bodies, he said, “like putting on an extra jacket or getting into a sleeping bag.”Also thanks to feathers, a bird can tuck in its most vulnerable body parts, particularly overnight. Heads are turned so beaks can be buried into the shoulder-like scapular feathers atop a wing “to reduce heat loss and recycle warmth in the same way people do when breathing into cupped hands,” Mr. Dunne writes. By perching on one leg, the bird can pull the other up into safety, conserving more heat.

Watch for other protective behaviors: as birds turn their backs to the sun to soak in maximum warmth, for instance, or as they cuddle, clustering together to share warmth.

Overnight, many birds roost in tree cavities, nest boxes or dense vegetation, sometimes together. Certain species, including the black-capped chickadee — a familiar bird in roughly the northern half of the country and in portions of Canada and Alaska — may get through a cold night by lowering their metabolism, respiration and heart rates, as well as their body temperature, even down to 50 degrees, to induce a hibernation-like state of torpor.

Another cold-defying strategy of birds is shivering on demand to raise their body heat — that’s what chickadees do to emerge from torpor.Whoever came up with the expression “eats like a bird” really wasn’t watching. Oh, do they feed — especially in anticipation of nightfall or an approaching storm. Anyone with a wintertime feeder has seen birds swarming it in the hours before dusk or severe weather.

“They fuel up and then they can go find a nice, sheltered spot out of the wind and precipitation,” Mr. Sibley said, “and hunker down and maybe stay a day or two if they have to, just living off of the extra food that they took in.”

Small birds lose about 10 percent of their body weight each night year-round while at rest, he writes in his 2020 book, “What It’s Like to Be a Bird: From Flying to Nesting, Eating to Singing — What Birds Are Doing, and Why.” Half is from burning fat and evaporating water, the other half from defecating.

For perspective, Mr. Sibley cites one of the tiniest North American species, the golden-crowned kinglet, who despite weighing barely as much as a nickel can withstand temperatures down to minus 40 Fahrenheit.Its cold-season daily requirement of about eight calories? That would be like 67,000 for a 100-pound human, Mr. Sibley writes, or about 27 large pizzas or 26 pounds of peanuts, to get through a day. “Most of the day — up to 85 percent of daylight hours — is devoted to searching for food,” he adds.

To his mind, one bird takes stocking up to an exceptionally impressive extreme.

The Canada jay does not visit us; its year-round range is in the forests of northern North America, even into Alaska. “One of their tactics for surviving the winter is to spend all summer gathering food and hiding it,” he said, “thousands and thousands of hidden bits stuck in bark crevices and clusters of spruce needles. They hide food, and then go back and consume it during the winter.”

The Cornell Lab of Ornithology’s All About Birds website says that this food storage behavior — caching future provisions high enough in trees to be above the eventual snow line — may be what allows the jay to survive winters so far north.

The species nests as early as February, which is exceptional in such a cold environment but means fledglings will be ready to forage independently by summertime. “They get pushed out of the adult’s territory in April, May, to find their own territory,” Mr. Sibley said, “but it also gives them the entire summer to gather and store food for the winter.”