Getting Ready For Winter: The Migrators And Hibernators


Last week’s abrupt change in weather had many of us scrambling to drain garden hoses and outdoor showers, store outdoor furniture, and other chores related to the onset of winter. It also prompted some friends to make winter migration plans, booking flights south to warmer climes.Wildlife are also getting themselves ready for the winter season, and it’s interesting to take note of the various strategies they employ. For those with the means and mobility, migration is an option. Flying south is a common tactic among the birds, particularly those that dine exclusively on insects, such as the warblers, but also includes many other avian species looking for an easier way to meet their daily energy budget needs.

Migration is also undertaken by a few insects. The Monarch butterfly is the best known among that group, but a few other butterfly species, as well as some of the dragonflies, also fly south to warmer climes. Some marine fish also migrate; not necessarily south but offshore to the warmer waters found in and near the Gulf Stream.

What about the marine mammals: seals, dolphins, porpoises and whales? While seals are found year round on Long Island, they are here in greatest numbers during the winter, which might appear to indicate a migration from cooler waters on the coast of Maine. But the bulk of the population of all four species of seals found here in winter remains further north. It’s possible that the movement of seals south in late fall is prompted by the movement of their preferred food fishes, not water temperatures.

This may be true for all the marine mammals, which are well adapted to live in cold water provided there is an abundance of food. Several whale species will migrate south to warmer waters in the Caribbean to give birth, their young lacking the thick protective layer of blubber to survive up here. That migration is timed such that the young are born in the winter months. Of note is the fact that, while the Caribbean has high marine species diversity, it lacks the high productivity and rich food resources found in colder waters up north. As a result, females go without feeding during their four-month stay in the tropics while nursing their young.

All seven of the bat species found on Long Island, as with the warblers, are insect eaters and migratory. However, unlike the warblers, they are not migrating south in search of insect prey but are migrating in search of winter hibernating sites in the form of caves or inactive mines that have a stable temperature range of 40 to 50 degrees Fahrenheit. These ideal hibernating sites are found north, west, and south of Long Island, so bats will migrate in all those directions. Very few overwinter here on Long Island.

Bats are “true hibernators.” Hibernation is another overwintering strategy that greatly reduces energy demands by finding a suitable microenvironment that minimizes exposure to freezing temperatures, lowering body temperature, heart rate and breathing, and essentially shutting down metabolic processes.

There’s much confusion in the literature regarding the term hibernation. The more we’ve learned about the physiology of hibernation, the more exceptions were found to the definition, “true hibernator.” Today, many researchers consider the hibernation strategy to be a long spectrum or continuum of degrees and variations on the theme of reducing energy demands. I agree with biologist Bernd Heinrich’s position as found in his excellent book, “Winter World,” “…making ever more precise or restrictive definitions does not generate greater precision in the understanding of any animal.”

All our reptiles and amphibians, our freshwater fish, and some insects, such as the mourning cloak butterfly, being cold-blooded and unable to regulate their body temperatures, utilize the hibernation strategy. With the onset of cold weather, they seek out suitable hibernacula (mostly underground below the frost line in the case of the herps, and seeps or springs that are least likely to freeze in the case of freshwater fish) and become inactive.

While snowshoeing over a well-frozen freshwater marsh some winters ago, I inadvertently wandered over a snow-covered spring and sunk in to my knees. Flopping onto my back to pull my snowshoes out of the watery muck and slush, I discovered that they had acted like a net. The webbing of both ‘shoes’ was covered with a dozen or so 2- to 3-inch-long fish. They must have been packed into that small spring area like sardines in a can.

Along the spectrum of hibernators in the mammal group, bats, our two species of jumping mice, and groundhogs (a.k.a. woodchucks) spend the most time in a state of torpor, but even they need to rouse themselves occasionally by elevating their body temperature and moving about in their winter quarters. Chipmunks will awaken to feed in their underground food storage chambers, and they might take advantage of a warm spell to leave the den.

Getting ready for hibernation involves finding a suitable microenvironment that is most protective against sub-freezing temperatures and, for some species, feeding to increase internal fat stores and excavating and modifying hibernacula.

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