January 8-14, 2019 (Week 2 of 52): The Marvels of Mammal Survival Through Winter

Wild mammal species found in the Buffalo-Niagara Region exhibit amazing adaptations that help them survive food scarcity and cold winter temperatures. The most remarkable adaptation is hibernation; true hibernation to be precise. Several species of mammals in our Region are true hibernators: five species of cave bats (big brown, little brown, northern long-eared, small-footed, and eastern pipistrelle [tri-colored]), woodchuck, meadow jumping mouse, and woodland jumping mouse. These true hibernators undergo incredible biological changes, triggered by a hormone-like protein, that bring them into a death-like state which allows them to conserve enough energy to survive the winter without eating or drinking. Their body temperatures fall to that of the surrounding environment (typically below 40° F) and their metabolic rates drop drastically. The heart rate of a hibernating mammal drops to just a few beats per minute and breathing rate reduces to as few as one breath every few minutes. These animals survive on body fat accumulated during heavy feeding in late summer and fall. Most hibernator species have relatively large amounts of brown fat, another marvelous over-wintering adaption. Brown fat is capable of greater heat production than white fat because it is packed with mitochondria which burn energy and produce heat. It surrounds vital organs and can provide heat on demand, by hormonal release. Finally, these mammals retreat for hibernation to relatively warm, protected areas (e.g., caves for bats, underground burrows for woodchucks) that further aid in conserving critical energy reserves.

Another group of mammals that retires to burrows and dens to endure the winter months are sometimes referred to as false hibernators. This group of winter dormant mammals includes raccoon, striped skunk, and black bear. These animals feed heavily in fall to build up body fat, then retire to burrows, tree cavities, and other protected areas to “sleep” for most of the winter. Raccoons and skunks may emerge occasionally during relatively warm periods to feed. False hibernators do not experience the radical biological changes experienced by true hibernators. Their body temperatures fall relatively little and metabolic rates drop less significantly than true hibernators. The most dramatic changes are displayed by black bears, which enter a state called carnivorous lethargy that is relatively similar to true hibernation. Their heart rates are significantly lowered but body temperatures fall only about 10°F (substantially smaller drop than for true hibernators). It is noteworthy that black bears utilize brown fat, like most true hibernators. Also, black bear cubs are born between mid-January and early February while sows are in this lethargic state.

Eastern chipmunks have a slightly different approach to winter dormancy than the above-listed species. They hoard food in their dens (versus building up large fat reserves) and then awaken from torpor frequently to feed. They may also forage aboveground during relatively mild weather.

All three species of tree bats found in our Region during summer months (eastern red, silver-haired, and hoary bats) migrate south for the winter, typically departing in late September and October as insect prey becomes scarce. These bats over-winter primarily in the southeastern United States. Many of these bats breed in their wintering area and return to our Region to give birth and raise young. Those in the northern part of their wintering range may hibernate for a portion of the winter. It is worth noting that many of our Region’s cave bats migrate relatively short distances from their summer ranges before they enter hibernation.

A large percentage of our Region’s mammals do not migrate, hibernate, or otherwise go dormant during winter months. They stay active all winter, focused on surviving long cold periods with limited food supplies by continuing to feed and conserving as much energy as possible. Behavioral changes and physical adaptations help them survive. Some of the behavioral adjustments start in autumn, with most mammals feeding actively to build up fat reserves under the skin and elsewhere (e.g., in the tails of beavers). In addition, some species cache food for consumption during lean winter months. For example, eastern gray squirrels and southern flying squirrels gather and stash or store acorns and other hard mast for winter feeding. Similarly, red squirrels form middens of pine and spruce cones. White-footed and deer mice stockpile small fruits and seeds in nests and nearby “pantries.” Beavers cut, transport, and cache branches in shallow water near their lodges for wintertime feeding. Muskrats have a similar practice of caching herbaceous vegetation near their lodges. Mink occasionally hoard fish and other prey for later consumption.

The most common behavioral changes of winter-active mammals focus on conserving energy. For example, white-footed mice and deer mice construct insulated nests in woodpecker holes, bird houses, squirrel leaf-nests, and old bird nests (rehabbed by adding a roof and insulation). Some small mammals, such as meadow voles and short-tailed shrews, adopt a subnivean (defined simply as “underneath snow”) lifestyle. There they are sheltered from extreme cold air temperatures because snow serves as an effective insulator, especially when it is relatively dry and fluffy. Virginia opossums, which stay active all winter, remain sheltered in burrows and other protected areas when conditions are especially cold. Many other winter-active mammals also den-up during particularly cold periods, including several species that do so in small groups to capitalize on shared body heat. Examples of these “huddlers” include striped skunks, eastern gray squirrels, red squirrels, beavers, muskrats, white-footed mice, and meadow voles.

White-tailed deer congregate in protected areas called “deer yards,” sometimes in large numbers, especially when snow cover is deep. Such areas are densely vegetated (often dominated by pines and other conifers) and therefore provide greater thermal shelter from wind-chill and accumulate shallower depths of snow. These yards are often located close to feeding areas which further helps deer conserve critical energy reserves during harsh winter periods.

Once cold weather sets in, winter-active mammals must produce enough metabolic heat from food or fat reserves to offset heat and energy lost to their cold surroundings. That is particularly challenging for shrews and other small-bodied mammals that are especially prone to heat loss. Shrews are remarkable in that they utilize brown fat, like most true hibernators and black bears, with its heat production benefits.

The most common physical adaptation found in winter-active mammals is a heavy coat of fur. For example, coyotes and red and gray foxes shed their thin summer coats and replace them with thick winter coats consisting of a dense wooly underfur overtopped by long coarse guard hairs. The winter coat of white-tailed deer is similar except that their guard hairs are hollow and therefore are particularly effective at insulating. Beavers step it up another notch with their extremely dense underfur plus their meticulous preening and oiling of the underfur to keep it waterproof. These specially designed coats of fur, combined with thick layers of fat built up during fall feeding, provide effective insulation that helps winter-active mammals minimize losses of body heat.

Another interesting physical adaptation is countercurrent blood flow in the tails of beavers, muskrats, and river otters which allows these animals to avoid losing excessive amounts of body heat and to keep tails from freezing. A dense network of veins occurs in close contact with arteries where warm blood going out to the tail meets cold blood coming in from the tail. The warm blood heats cold blood returning to the animal’s core, and the cold blood cools warm blood going out to the tail. In this way, the tail is constantly cold but warm enough to keep tissues alive. Moreover, less body heat is lost by reducing the temperature gradient between blood in the veins and cold water temperatures.

The next time you see a gray squirrel, red fox, beaver, or white-tailed deer, consider the remarkable adaptations that allow them to survive the typical food scarcity and cold temperatures associated with Western New York winters.

Below are highlights of what you can expect to find outdoors in the Buffalo-Niagara Region this week. Those in bold/italics are new or substantially revised highlights to watch for this week. Check out the list of 300 publicly accessible sites at https://bnnatureblog.com/nature-sites/site-lists/alphabetical-list/ to find areas to explore in your neighborhood and throughout the Buffalo-Niagara Region.

Average Sunrise/Sunset (Day Length):

  • 7:45 AM/5:01 PM EST (9 Hours, 16 Minutes)
  • 0 Hours, 15 minutes of daylight longer than at Winter Solstice

Typical Weather:

  • Normal High Temperature: 31.2° F  Normal Low Temperature: 18.7° F

Lake, Pond, Stream & Wetland Conditions:

  • The Lake Erie water temperature off Buffalo cooled to 37°F and the Lake Ontario water temperature off Greece (Monroe County) cooled to 38°F as of January 11, 2019.
  • Water levels in most interior wetlands and vernal pools are near seasonal highs after a long period of normal or better precipitation and greatly reduced evapotranspiration rates.
  • Similarly, water levels in most ponds are close to seasonal highs.

Fungi:

  • The hard frost/freeze we experienced over the past few weeks extinguished most fungal fruiting bodies. The fungal “roots” (mycelium network) will survive the winter and produce new fruiting bodies during the appropriate season next year. Interestingly, fruiting bodies of some species of fungi (e.g., oyster mushrooms) remain viable during the winter and may disseminate spores during warm periods or in early spring.

Ferns and Grasses/Sedges/Rushes:

  • The hard frost/freeze we experienced over the past few weeks killed remnant grass, sedge, and rush stems (thus known as a killing frost). The roots of these perennial plants will survive and sprout next spring.

Wildflowers:

  • The hard frost/freeze we experienced over the past few weeks killed remnant wildflower stems (thus known as a killing frost). Roots and rhizomes of perennial wildflowers will survive and sprout next spring. Seeds of annual wildflowers will do the same.

Trees and Shrubs:

  • Many American beech leaves remain clinging to limbs but have now changed to brown. Some beech trees will hold their leaves the rest of the winter.
  • Several native shrubs and vines continue to provide fruit (soft mast) that is an important winter food source for a variety of birds and mammals: winterberry, cranberry viburnum, staghorn sumac, and swamp rose.
  • In addition, one non-native species, multiflora rose, provides fruit consumed by wildlife.

Insects & Other Invertebrates:

  • The hard frost/freeze we experienced over the past few weeks killed most adult insects and other invertebrates that have not migrated or entered hibernation. The vast majority of insect species in our Region over-winter as eggs or larvae/nymphs, although some species over-winter as adults.

Fish:

  • Many species of fish have moved into shallower areas and are feeding more heavily as water temperatures have cooled, including muskellunge, walleye, smallmouth bass and schools of yellow perch.
  • Steelhead continue to occur in Great Lakes tributaries and the Lower Niagara River at this time. Spawning does not occur until late winter and early spring. Steelhead are an anadromous form of rainbow trout that spawn in streams but live most of their lives in Lakes Erie and Ontario. All forms of rainbow trout are native to Pacific coast watersheds.
  • Brown trout continue to occur in Great Lakes tributaries and the Lower Niagara River. Spawning typically occurs from late October to December in these tributaries. Brown trout were introduced from Europe.

Amphibians & Reptiles:

  • With the onset of winter weather, essentially all amphibians and reptiles are now hibernating.

Water & Shore Birds, Gulls & Terns:

  • The annual buildup of “sea ducks” and similar waterbirds that over-winter in the Great Lakes and Niagara River continues with additional arrivals of greater scaup, lesser scaup, canvasback, redhead, common goldeneye, bufflehead, white-winged scoter, surf scoter, black scoter, long-tailed duck, common merganser, and red-breasted merganser.
  • Watch for winter resident tundra swans congregating in open waters along the upper Niagara River (especially off Beaver Island, Buckhorn, and Niagara Falls State Parks).
  • This continues to be a good time to look for purple sandpipers feeding in rocky habitats above Niagara Falls.
  • This is an excellent time to watch for rare species of gulls such as Franklin’s, little, black-headed, California, Iceland, Thayer’s, lesser black-backed, Glaucous, Sabine’s, and black-legged kittiwake among more common species such as Bonaparte’s and greater black-backed gulls. Such rarities add to the remarkable diversity of gull species – 19 species total – that have been observed along the Niagara River and bordering Great Lakes. The peak time is typically between mid-November and mid-January.

Birds of Prey:

  • Bald eagles can be found along the upper and lower Niagara River where good numbers will over-winter.
  • Winter resident raptors will continue to arrive in the region, especially in areas with extensive open grassland habitat, including northern harriers, rough-legged hawks, snowy owls, short-eared owls, and long-eared owls.
  • Snowy owls are frequently found along Great Lakes shorelines, such as the Buffalo waterfront, where they feed on ducks and other waterbirds.
  • Great horned owls have started to pair-up, form pair bonds, and establish nesting territories. Listen for their vocalizations, especially near sunset and sunrise, including duets consisting of a female and male singing nearly in unison (with male calls noticeably lower in pitch than those of the female).

Upland Game Birds:

  • Watch for wild turkey flocks in farm fields, along forest edges, and near bird feeders.

Songbirds:

  • Watch bird feeders for the following songbird species that are part of this year’s “winter finch” irruption: purple finch, red crossbill, white-winged crossbill, common redpoll, hoary redpoll, pine siskin, evening grosbeak, and red-breasted nuthatch. Nyjer (AKA thistle) and black oil sunflower are the best seeds for attracting these species.
  • Bird feeders are also excellent locations to watch for arrivals of more typical migrant and over-wintering feeder birds such as dark-eyed junco, white-throated sparrow, song sparrow, and American tree sparrow.   Place seed such as white millet in ground feeders or directly on the ground to attract many of these migrants.
  • Bird feeders will continue to be active with year-round resident birds such as mourning dove, downy woodpecker, hairy woodpecker, red-bellied woodpecker, black-capped chickadee, tufted titmouse, white-breasted nuthatch, blue jay, northern cardinal, house finch, and American goldfinch.
  • Good tips for feeding birds are available from the Cornell Lab of Ornithology, online at http://www.birds.cornell.edu/AllAboutBirds/notes/BirdNote01_WinterFeeding.pdf
  • If you don’t have a feeder of your own, consider visiting a local nature center (see the 2nd to last column in the tables of nature viewing sites found under the “B-N Region & Sites” tab on this web page).
  • The northern shrike, a predatory passerine that breeds in Canada and Alaska, will continue to occur in the Region. Watch for them on prominent perches overlooking open and brushy habitats.
  • Most eastern bluebirds, American robins, eastern meadowlarks, red-winged blackbirds, common grackles, brown-headed cowbirds, and summer resident sparrows have now left the Region for southern climes.
  • While most American robins have migrated south, small to medium sized flocks may still be encountered.
  • Small flocks of horned larks are being joined in open farmland and other tundra-like habitats by snow buntings and Lapland longspurs. Many will over-winter in our region.
  • To stay abreast of bird sightings in the region, consult eBird, Genesee Birds, and Dial-a-Bird (see the “Resources” tab on this web page for more details).

Mammals:

  • Resident species of cave bats (big brown, little brown, and eastern pipistrelle [tri-colored] bats) have entered hibernation. Most woodchucks have also started their winter hibernation. Two other species of true hibernators, meadow jumping mouse and woodland jumping mouse, have also begun hibernation.
  • False hibernators such as the raccoon and striped skunk are mostly dormant, other than occasional feeding forays during relatively warm weather. In contrast, Virginia opossums stay active all winter other than brief denning during particularly cold weather.
  • Most eastern chipmunks are in a state of torpor. In this condition, which is not a true form of hibernation, chipmunks sleep but arouse frequently to feed on hoarded food. They may forage aboveground during mild weather.
  • Gray squirrels and southern flying squirrels continue to actively forage for cached acorns and other available food. Similarly, red squirrels feed from middens of pine and spruce cones.
  • Watch bird feeders after dark for nocturnal visits by southern flying squirrels.
  • White-footed mice and deer mice prepare for winter by building nests in woodpecker holes, bird houses, and squirrel leaf-nests. Some rehab old bird nests by adding a roof and insulation. These mice often cross paths with homeowners this time of year as they seek shelter in sheds, garages, and houses – along with non-native house mice.
  • Beavers continue to feed on cut branches they cached in shallow water near their lodges during the fall, and muskrats do the same with herbaceous vegetation cached near their lodges.
  • Ermine (AKA short-tailed weasel) have molted from brown to white pelage for winter months.
  • Black bears, an uncommon species in the Buffalo-Niagara Region but increasingly common to our south, are in carnivorous lethargy. In this state, which is not a true form of hibernation, a bear’s heart rate is significantly lowered but body temperature falls only about 10°F (substantially smaller drop than for true hibernators).
  • White-tailed deer bucks typically shed antlers between mid December and mid February in the Buffalo-Niagara Region.

Be sure to find an opportunity to get outside this week to discover signs of the season.

Chuck Rosenburg

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