Exploring the behaviors of wintering birds: Adaptations ensure survival in freezing temperatures

By Tom Litwin for the Gazette
January 24, 2025

Great grey owls will leave their boreal forest habitat when their population exceeds the supply of prey, which includes lemmings, shrews, weasels, mice, voles and small birds. Photo by Tom Litwin

During migration season this past fall researchers at the Cornell Lab of Ornithology, using NEXRAD weather radar,  tracked  approximately 4 billion birds migrating from Canada into the U.S. and 4.7 million birds leaving the U.S. for the tropics. Clearly one strategy for dealing with New England weather is to leave it behind. But other species’ strategies have traded the benefits and perils posed by thousands of miles of travel for the benefits and perils of northern winters.

A walk in Whately’s  winter woods confirms the warblers, vireos, thrushes and flycatchers have gone south. The leafless woods are quieter and starker, green giving over to grey. In the quiet, the calls of winter residents are more pronounced as they forage dawn to dusk. Their challenge is brutally simple: gain enough calories to fuel daytime activity and store enough calories to fend off freezing nighttime temperatures.

Overwintering birds have a number of adaptations to buffer themselves from winter’s cold. In the fall they pack on fat stores in anticipation of the coming cold. In winter fat stores can be used to activate muscles, causing them to shiver, which generates body heat. Fluffing their heavier winter plumage traps body heat in the same way as a down jacket. Finding nighttime shelter in an abandoned nest cavity or dense stand of conifers, out of the wind and precipitation, reduces heat loss. Some species can activate a state of torpor; the chickadee can lower their body temperature up to 20 degrees Fahrenheit, reducing metabolic demand.

Some of our winter residents—tufted titmice, downy woodpeckers, chickadees, white-breasted nuthatches— abandon their breeding territories and forage through the woods in mixed flocks. Their call notes alert others to predators and feeding opportunities. Along the woodland edge skittish cardinals go about their foraging, while a raucous “scold” of blue jays mobs and taunts a great horned owl. Having had enough, the owl makes a break for the deep woods. The owl will hunt at night when a wider variety of small mammals—mice, voles, skunks, opossums, raccoons– are active.

The signs of a foraging pileated woodpecker are obvious. On the trunk of a dead white pine a large rectangular excavation deep into the trunk exposes a carpenter ant nest. On the ground below a pile of splinters is mixed with scat composed of exoskeleton shards. The forager has moved on to easier pickings. It is hanging on a grapevine, plucking grapes. Below, on the ground, a small flock of dark-eyed juncos forage in the grass. These short distance migrants breed in Canada, but unlike the neotropical migrants, this population ends their fall migration in the northeast.

As winter progresses along with cabin fever, the woods and fields can offer a few surprises; a small flock of evening grosbeaks descend on a winterberry bush flush with berries. This grosbeak is a native of the Canadian boreal forest, that ranges from Newfoundland to Alaska. Considered the largest intact forest on Earth, more expansive than the Amazon, it is covered by vast areas of black and white spruce, tamarack, and balsam fir, interspersed with yellow birch, mountain ash, sugar maple and extensive areas of wetlands.

The evening grosbeaks’ 2018 appearance in Whately resulted from an “irruption” where  large numbers of a species leaves their northern habitats in search of food. This can result from either a crop failure in their preferred food, or a highly productive year in producing offspring and the inability of the food supply to meet the population’s demand.  In either case they drift south in search of food.

A yellow rumped warbler photographed during a local Hitchcock Center birding course led by photographer Scott Surner. Photo by Scott Surner

In New England, Audubon Christmas Bird Count (CBC) data has identified irruptions in a variety of common Canadian species, according to recently retired CBC director Geoff LeBaron. This includes the seed and fruit eating pine grosbeak and bohemian waxwing, and seed eaters including the redpoll, white-winged crossbill, pine siskin, purple finch and red-breasted nuthatch.

Irruptions are not limited to finches as seen in the periodic arrival in New England of two large predators, the snowy owl and great grey owl. The snowy owl, a resident of the arctic tundra, feeds on lemmings, mice, arctic hare, ptarmigan and any other prey that has the misfortune of encountering this opportunistic hunter. When they have a breeding season where their population exceeds available food resources, snowy owls move south, often hunting in large fields and open spaces of airports. Similarly, the great grey owl will leave its boreal forest habitat when their population exceeds the supply of prey, including lemmings, shrews, weasels, mice, voles, and small birds.

With the passing of winter solstice, the days are getting longer. As daylight increases, hormones produced by a bird’s endocrine system shifts their winter survival behavior to breeding behavior. In February, the cardinals will start territorial singing. The mixed flock will dissolve as each individual is drawn to establish and defend their own breeding territory. The snowy and great grey owls will hunt their way back to their breeding grounds. Irruption species will be drawn back to their northern habitats. And later in the coming spring, while we are asleep in our beds, millions of northbound neotropical migrants will pass overhead.

Tom Litwin (he/him) is a conservation biologist and former director of the Clark Science Center at Smith College. He is retired from the Jackson Laboratory in Bar Harbor, Maine and Farmington, Connecticut, where he served as the vice president for education, and continues as a visiting scholar.

Earth Matters has been a project of the Hitchcock Center for the Environment for 15 years. HCE’s mission is to educate and to inspire action for a healthy planet. Our Living Building and trails are open to all at 845 West St. in Amherst. To learn more, visit hitchcockcenter.org.

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