What a warm winter portends for Martha’s Vineyard

A trio of mallards enjoyed a winter swim under sunny skies on ice free Lagoon Pond in Vineyard Haven this week. — Photo by Ralph Stewart

What would happen if we didn’t have winter? Perhaps we’re about to find out. Oh, it’s true we’ve had some cold days, and one significant snowfall (though, in classic Vineyard fashion, it had melted three days later). But as January ends, the winter of 2011-2012 has been a mild one, with few extended freezes, an unusual number of mild days, and little snow. You’d think we were in Virginia, not Massachusetts.

As always, the Wild Side wonders what wildlife thinks about this. In a general sense, winter is a formidable obstacle to survival: at our latitude, wildlife must migrate, go dormant, or change its behavior to adapt to sub-freezing temperatures and the likelihood that snow will cover the ground for weeks at a time. It’s a season of high mortality, as a dearth of food and the energetic demands of travel or staying warm sort out the fit from the less fit. So what happens when the level of stress is reduced?

At the most obvious level, the mild conditions are working out well for many species that remain active through the winter, most notably our resident and wintering birds. Paradoxically, this may mean that you’re seeing fewer birds around your house than you expect: many of our year-round species, like chickadees and nuthatches, appear to be managing just fine on natural resources in the woods, giving them little incentive to seek out concentrated food sources like bird-feeding stations. But many other species, from ducks on the bays and ponds to robins living on cedar and holly berries, are plentiful, readily observed, and apparently managing quite happily so far this season.

Meanwhile, at least a few lingering migrants have been found, birds that would be gone or dead by now in a more typical winter. For example, Island birders have found several ovenbirds latterly. Named for its odd domed nest, this ground-hugging warbler species is a common breeding bird and migrant here, but vacates our latitude entirely to winter mainly in the tropics. The recently observed individuals, either too weak to continue their journey or equipped with a substandard migratory instinct, clearly caught a break this season and could conceivably survive to nest here or migrate farther north in the spring.

In short, my guess is that most of our birds, finding plenty of food and having to spend less energy than usual keeping warm, will show high survival rates. Following a 2011 breeding season that was more successful than usual, large populations may give birds a chance to colonize new parts of the Island, providing a buffer against future threats. Generally speaking, this is a good thing.

In less obvious ways, however, a winter that largely hovers above freezing can cause problems for plants and animals, especially ones that are adapted to surviving a deep freeze. Gardeners know, for example, that a winter like this can wreak havoc among the perennial beds: wet soil that never quite freezes solid can result in lethally rotted roots and crowns, and repeated cycles of freezing and thawing can heave plants out of the ground. The result can be far more plants lost than you’d lose in a colder winter with more uniform temperature and, perhaps, a persistent layer of insulating snow.

The same may be true for some species of wild plants, and there are other ways that a mild winter can cause problems for wildlife. Diseases or fungal infections, for instance, can be a significant cause of mortality in insects, even dormant ones. A cold winter may help keep this threat in abeyance, while in a mild season, pathogens may flourish, causing a greater impact. And for some overwintering invertebrates, a winter that flirts constantly with freezing may result in frequent interruptions of dormancy; roused prematurely, these critters might expend their energy reserves before their real season of activity has arrived.

And there are ways in which high winter survival rates could result in undesirable outcomes from the human perspective. Does cold-weather mortality help control the numbers of insects, such as tent caterpillars and winter moths, that are capable of burgeoning into outbreaks that defoliate trees? I don’t know. But if it does, a winter like this one, with lower than normal mortality, might set the stage for explosive population growth over the next generation or two, helping trigger a new outbreak.

In short, I’d expect to find subtle but pervasive effects in nature if this winter ends as moderately as it began. As always, there will be winners and losers, unanticipated effects, and a wave of secondary effects resulting from skewed rates of survival. The results will be worth observing. This is the kind of thing that makes every year exciting to a naturalist. And, in the context of global warming, the effects of one mild winter may foreshadow the future.