Plants and animals who choose to hunker down
The evergreen plants and birds that overwinter here in the Southern Appalachians have made fundamental “choices” in how their lives will be governed. Being aware of what those “choices” are provides a better understanding and appreciation of what they’re up to.
All plants in upland or northern environments face the double-edged dilemma of low temperature stress and lack of moisture in winter. Most opt to lay low or go underground. Herbaceous perennials die back completely and over-winter as dormant corms or regenerative root stock. Broadleaved deciduous trees, shrubs and various vines shed their leaves and assume other protective measures. Come spring, these plants really have to hustle to do their thing and produce seed or fruit during the growing season.
Evergreen plants, however, have “chosen” the other fork in the trail. They hunker down and make it through the winter with their foliage intact so as to obtain a head start when the growing season arrives. For this group of plants, photosynthesis can continue longer in the fall and begin earlier in the coming year; indeed, keeping their leaves (or needles) actually helps these plants survive because they can use them for photosynthesis on mild winter days. Come spring, energy that would otherwise be channeled into producing leaves is saved for direct reproductive efforts.
Individual evergreen species often have their own distinctive over-wintering devices. Everyone has observed how rhododendron leaves curl and droop in extreme cold. Drooping (a dormant posture also assumed during periods of drought) lessens exposure to wind, while curling temporarily shields and closes off air-circulation pores (stomata) on the undersides of the leaves.
Because they seem so delicate and vulnerable, we go out of our way to feed birds that overwinter with those of us who have also decided not to winter in south Florida. But our feathered friends long ago devised basic strategies for withstanding wind and cold. And they did so as a result of taking the same path as evergreen plants. They have discovered that it behooves them to hunker down and make it through the winter rather than migrate to faraway places.
The male bird that migrates probably won’t be able to keep up with his mate. So he’ll have to go through all of the rigmarole associated with attracting a new one.
He will also have to expend a considerable amount of energy and time establishing a new territory.
On the other hand, the birds that choose to overwinter — chickadees, cardinals, nuthatches, titmice, crows and so on — are good to go when spring arrives. The tremendous advantage to this strategy is that it gives them a much wider window for breeding purposes.
For instance, the phoebe, a bird that many of you are familiar with, is the only flycatcher species that overwinters in the Smokies region. Arthur Stupka, the first naturalist in the Great Smoky Mountains National Park, cites instances in his Notes on the Birds of the Great Smoky Mountains National Park (1963) when newly hatched birds were observed with regularity in early April and in a few instances in early March. On the other hand, those flycatcher species that migrate (i.e., great-crested flycatchers and eastern kingbirds, Arcadian and least flycatchers and eastern wood pewees) have only started arriving in late April.
If things go well, a female phoebe can easily raise two broods, with instances of as many as three. If she has a nest failure due to, say, predation by a black snake, she has plenty of time to compensate because she’s not obligated to migrate somewhere.
Like the evergreen plants that also overwinter, the birds that do so have evolved ways to combat the challenges cold weather presents.
It’s not unusual to observe birds preening themselves with their bills and feet to rearrange and oil their feathers. They do so, in part, to maintain flight capabilities, but in winter the process is essential for heat regulation.
Just making it through the night is the most challenging task facing birds during the winter months. Like humans, they shiver involuntarily as a warming reflex, and when all else fails they huddle and snuggle. Finches, sparrows, crows, jays and doves roost in dense conifers to reduce heat loss. Species such as brown creepers, white-breasted nuthatches, winter wrens and bluebirds sometimes join one another in bird boxes or tree cavities.
There are birds in other parts of the world that actually hibernate like woodchucks, snakes and other animals. Here in our region, the chickadee is the bird that comes closest to utilizing this technique — which has been called “controlled hypothermia” and “overnight hibernation” — reducing the rate of heat loss from the bird by reducing the temperature difference between a bird’s body and the surrounding air. Shivering is stopped so that body temperature drops until a level of hypothermia is reached. On a cold night, a chickadee can allow its temperature to drop up to 12 degrees, resulting in a large overnight energy savings. The only problem is waking up quickly enough from this torpid state when a predator happens along.
I’ve perhaps over-emphasized the analogy between the “choices” made by some plants to be evergreen and some birds to overwinter. But I do think it’s an analogy that helps us appreciate more fully what it means in their worlds to hunker down and make it through the winter.