Red-breasted nuthatch
Red-breasted nuthatch
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With October’s dwindling daylight and dipping temperatures, many of our summer birds have retreated south for winter. These are various species of warblers, flycatchers and sundry songbirds that rely primarily on insects for sustenance. The daunting feat of migration itself, navigating across stupendous distances and avoiding potential dangers, is a challenge that must be faced twice a year. For birds that nest here during Maine’s brief summers but spend the majority of their year in tropical climes, it begs a question: Should we regard them as indigenous Mainers or seasonal tourists?

Quantities of birds either arrive or remain throughout Maine’s winter months, however. These include year-round residents ranging in size from tiny golden-crowned kinglets to massive bald eagles. Several species of nomadic winter finches also visit us in certain years, depending on abundance of seed, cone and berry crops farther north. Flocks of waterfowl also relocate from the interior U.S. and Canada to winter on bays and open ocean. But over-wintering birds must manage two lethal threats to existence — freezing and starvation.

How do winter birds meet the challenges of staying warm? To start with, birds hold a higher internal body temperature than humans — generally about 105 degrees F. Their primary winter strategy involves preserving essential thermal resources by several means. Layers of insulating feathering are the most apparent physical barrier to cold. In severe cold, birds puff up their feathers, utilizing the trapped air within to raise the insulation factor. During overnights, chickadees and kinglets huddle and seek the relative protection of tree cavities. Studies indicate that temperatures within bigger living trees stay warmer at night than similar holes found in dead trees. On frigid mornings, you may occasionally spot a chickadee or titmouse at your bird feeder with tail feathers crumpled or bent by a tight overnight nest cavity.

These tiny creatures also reduce body temperature to conserve core body heat while they enter a temporary state of torpor. Cold feet? Chickadees’ feet probably stay cold most of the time in winter, but their feet don’t actually freeze. Foot temperature is regulated at near the freezing point so that the chickadee’s inner body heat is not depleted.

Winter survival is really a matter of maximizing calories ingested while minimizing calories spent. “Guilds,” social groupings of small winter birds, probably boost odds of survival to some extent. These roving woodland bands, often including titmice, kinglets, nuthatches and downy woodpeckers, are anchored by a few territorial chickadee members. It is theorized that having multiple sets of eyes to locate food bonanzas leads to better success. Comparative biologist Bernd Heinrich cites direct interspecies connections as chickadees glean fallen cone seeds from the ground that were dropped by foraging red-breasted nuthatches overhead.

How do water birds combat numbing water temperatures? Once again, tight interlocking feathering plays a role. The feathers of geese, for example, must also be preened and oiled daily as maintenance against cold weather. While preening individual feathers, geese use their bill to strip dirt, debris and water from feather surfaces. A quick shake or ruffling of feathers serves to dry additional moisture. How do loons, sea ducks and gulls withstand ocean waters when winter “sea smoke” veils the horizon? These birds have a counter-current heat exchange system in their legs. Veins and leg arteries are positioned very close together, permitting warm blood flowing from the heart to heat up the colder blood returning into the body central. On land, these birds have the added option of standing on one leg to reduce heat loss or lying down to shroud their feet with downy feathers.