Temperatures in the low 40s last Sunday, with a forecast for more days above freezing this week, make it possible to imagine speaking of the winter of 2014-15 in the past tense. This past February ranks among both the coldest and the snowiest months in living memory, and the Wild Side, naturally, is eager to see how this convergence of miseries will affect the Island’s wildlife.
Everything that winters routinely on the Vineyard, of course, is capable of coping with some measure of snow and cold. And for many of these species, the ones that ride out winter by hibernating, the harsh season will have less effect than one might imagine. Once dormant, many of these species can survive far deeper cold than we’ve experienced. Mortality may be further lessened by the fact the season’s two irritants have worked, in some ways, at cross-purposes. A deep layer of snow serves as thermal insulation, moderating and stabilizing the temperature at and below ground level, where many overwintering insects reside.
Some insects, rather than entering deep hibernation, remain semi-active during winter, rousing themselves to feed and continue their growth during warm spells. For these insects, the most apparent effect of the winter will likely be delay in their life cycle. Instead of maturing and beginning to reproduce in early April, for example, the semi-active nymphs of the northern green-striped grasshopper will not get to work on the next generation until mid-May.
While a species like this grasshopper may not experience unusual mortality from a hard winter, there may still be consequences. Their eggs will be laid later in the season than usual; the weather may be hotter, and the days longer, than eggs or delicate young usually experience, which could translate to more mortality from desiccation. In other words, for some insects, this past winter may have indirect effects that won’t be evident until the next generation has matured.
For animals (mainly warm-blooded ones) that remain active during the winter, the past few weeks may have had profound effects. The insulating effect of snow is largely irrelevant for these animals; far from an advantage, snow cover vastly complicates the process of finding food. Combine that with low temperatures that drain the heat and energy from a warm body, and you have a recipe for stress and, for some species, high mortality.
Among birders, stories have been circulating of birds found dead in unusual numbers, and of unusual species turning up at feeding stations. The brown thrasher, for example, winters here regularly, though in very small numbers; evidently much warier than the related mockingbird, which flourishes amid human landscaping, thrashers generally occupy thickets in areas without much human disturbance. This year, though, has seen many reports of thrashers seeking out feeding stations, clearly driven by harsh conditions to overcome their usual aversion to humans.
How severe the effects are will vary widely from species to species. Barn owls, for instance, are notorious for suffering during rough winters. We are near the northern limit of their range, suggesting that our winter climate is, on average, about as demanding as they can tolerate. Barn owls are capable of successfully targeting their prey (mainly rodents) using hearing alone, which you might think would allow them to nail voles moving in tunnels beneath the snow. But there seems to be a limit to how much snow a barn owl can handle; they may simply not be large or strong enough to plunge through a thick layer. In any case, winters like this one invariably decimate the Island’s barn owls, conceivably even wiping them out entirely and leaving the Vineyard vacant until it is recolonized.
A more obvious avian victim of the winter will probably be the Carolina wren, a tiny brown bird that is ubiquitous in yards and gardens. Again, this is a bird near its northern limit, and it is a species for which significant snow cover apparently poses insurmountable challenges for foraging. Its song will be heard much less often this spring.
From the biological perspective, such high-mortality periods can be significant. The lucky survivors may find life especially easy in coming months, with few competitors for the best nesting sites and food resources. Both the wren and the owl can be highly fecund, producing multiple broods in a season. The result will be reestablishment of normal numbers within just a few years (barring more catastrophic weather) — but in genetic terms, changes may occur in a population growing back from such a small number of founders.
If the survivors benefited from some genetically determined quality, they will pass that on, perhaps producing descendants that are better suited to hard winters. But if it was mainly luck that determined survival, the “founder effect” may crimp the genetic diversity of the species, possibly leaving it more vulnerable to future challenges. So for many resident birds and mammals, this winter will go down as a “bottleneck event,” changing the genetic composition of the Island population.