Following a spell of warm, spring-like weather, biologists at the Vermont Fish & Wildlife Department observed black bears out of their dens early in March. In recent years, mild weather has frequently driven bears to enter their dens later and to emerge earlier.
“Bears are triggered to enter their den when food begins to become scarce in fall or early winter, which usually follows a heavy snowfall,” said Forrest Hammond, bear project leader for Vermont Fish & Wildlife. “Spring rains and warm temperatures cause bears to leave their dens in search of uncovered nuts and green shoots that start to emerge from the melting snowpack. Bears will be active as long as they can easily find food, but they will return to their dens if another deep snowfall covers their food supply.”
Shorter denning seasons in Vermont are consistent with reports from the American West, Scandinavia and Spain, where many brown bears have forgone hibernation altogether. Bears vary the duration of winter dormancy based on their latitude and altitude; bears that live further north or high in the mountains typically den for a longer period of time.
Vermont’s black bears are not the only species changing their behavior due to recent winter weather patterns. Many bird species have started to migrate, breed, and nest earlier in the spring in recent years.
John Buck, migratory bird project leader for Vermont Fish & Wildlife, says that the department has observed state endangered spruce grouse displaying courtship and breeding activity three weeks early as a response to low spring snowpack levels in recent years. “We’re concerned that the females may nest early and then see their nests buried under a heavy, late-season snowstorm, which would likely result in a high rate of nest failures,” he said.
The department has also observed that waterfowl are delaying their departure from Vermont for the winter because they continue to have access to open water, sometimes late into December or January.
Fish & Wildlife’s Steve Parren has been studying a population of wood turtles for the past 25 years. According to Parren, the turtles have historically emerged from hibernation in mid-April. “During the extreme warm spell that we had last winter, we saw wood turtles basking on March 17, nearly a month earlier than they are typically spotted,” he said.
Other amphibian and reptile species responded to a warm early-March rain this year by emerging from winter dormancy. Herpetologist Jim Andrews, of the Vermont Reptile and Amphibian Atlas, tracks the spring emergence of reptiles and amphibians in Vermont. Andrews reported seeing spring peeper frogs, spotted salamanders, and even some Eastern newts on March 12 this winter in the Champlain Valley.
Vermont’s insects also vary their emergence dates based on the spring temperatures. During warmer and drier springs, many will emerge early.
“Flowering plants, bees, butterflies – these species have evolved together based on a specific timing of events in the spring,” said Fish & Wildlife biologist Mark Ferguson. “Many of the state’s crops, including apple trees, require insects for pollination.” Last spring, many Vermont apple growers saw high levels of frost damage when unusually high temperatures pushed flower buds out before the last frosts of the season were over.
Milder winter temperatures can make controlling many forest pest species difficult. Vermont’s hemlocks are currently threatened by a non-native insect known as the hemlock woolly adelgid, which feeds on hemlock sap and may inject toxic saliva while feeding. The adelgid’s northward spread is limited by its inability to tolerate long stretches of temperatures below -20 degrees Fahrenheit, which have become less frequent in Vermont in the last decade.
As this pest spreads, it can cause devastating declines in Vermont’s hemlock forests. Hemlock forests are a critical habitat for many Vermont species; bears, bobcats and ruffed grouse all use hemlock for protective cover, and these forests are crucial as wintering habitat for white-tailed deer. Following the mild winter of 2011, hemlock woolly adelgid spread to seven additional towns in Vermont, more than doubling the number of towns in which the pest was previously detected.
While mild winters may benefit some species in Vermont, the weather’s unpredictability can prove difficult for wildlife. Mid-winter rains followed by deep freezes or March temperatures above 50 degrees Fahrenheit followed by heavy, late-season snowfalls can cause onerous conditions for many species.
“During a normal winter, bud break, insect hatching and birds returning to Vermont or establishing nests all occur at the same time,” added Buck. “Mild and unpredictable winters cause these events to get out of sync. Birds that don’t keep up with changing weather patterns return to Vermont to nest and find that the insects that they feed on have already hatched.”