Tuesday, Feb. 25, may not have been quite warm enough to pass for a truly fine day. But it was definitely a day that had the right idea. Lifted by a brisk influx of mild air from the south and nearly continuous sunlight, temperatures flirted with 50° across the Vineyard. After a few weeks that have averaged on the cold side, the moderating temperature was a mighty pleasant development. I arrived home from work to find a small flock of American robins feeding in our Oak Bluffs yard. That alone wasn’t surprising; robins winter regularly on Martha’s Vineyard, and they’ve been especially plentiful so far this winter. What struck me about these robins, as I watched them forage, was the prey they were taking: earthworms, definitely earthworms, tugged like reluctant rubber bands from the ground by the birds, and swallowed with evident relish.
The mild air, in short, was a game-changer for both the birds and the worms, thawing the top few inches of soil into a soggy but soft consistency. Worms by the score moved toward the surface, no doubt enjoying the warmth and the opportunity to wallow in the increased availability of oxygen. And the robins, which I expect were waiting with beaks poised for this phenomenon, were ready to take advantage.
It wasn’t just the prey selection that seemed new, however. Our robins, in varied, subtle ways, were behaving differently than they had been. The feeding was more frenzied than I had seen it recently, and there was more squabbling among the birds, as dominant individuals chased subordinate ones away from prime hunting spots. Quite abruptly, our amiable local flock of this species had begun to devolve into a looser, less friendly association of competing individuals. These birds, still migrants, won’t breed here, and will eventually continue northward (perhaps a long, long way; this woodland species breeds northward to the edge of where trees occur). But they were experiencing hormonal changes that drive the onset of breeding behavior, and in the subsequent waves of robins that will pass through, these changes will grow more and more evident until, finally, members of a local breeding population stake their claims.
The abrupt changes of late winter are real, not illusions or wishful thinking by humans, and three main factors align to drive the process. From its lowest pass at the winter solstice, the sun follows a path that is steadily higher in the sky as winter progresses. Reaching us from a higher angle above the horizon, the sun’s rays grow steadily stronger, because their path through the atmosphere grows shorter; less heat and light are absorbed by the air and the particulates it contains, and more arrive at ground level.
Meanwhile, the higher sun angle also translates to longer days. The sun appears to move at a steady pace across the sky, roughly 15° per hour, so as its path from horizon to horizon grows longer, the sun shines on us for longer each day. Sunlight, that is to say, doesn’t just grow stronger in late winter, it also grows more plentiful, and the warming effects of intensity and duration multiply each other.
Finally, the rate of change in day length accelerates to a maximum at the equinox. Don’t ask me to explain; spherical trigonometry is involved! But as we approach the spring equinox, each day is about three minutes longer than the preceding one. That may not sound like much, but over the course of a week, it adds up to about 20 minutes of extra daylight. Between all these synergistic effects of the calendar, it is no wonder that the natural world responds. Birds grow randy; plants grow green again; insects resume the process of maturation.
As a naturalist, I regret that our society has moved away from celebrating seasonal changes. Our holidays, for the most part, commemorate people or events (whether historical or religious), and we’ve drifted away from the pagan fondness for celebrating moments of inflection in the seasonal cycle.
Seasonal change, of course, is continuous. But the pace of change is not uniform. Certain moments represent tipping points, at which one alteration, insignificant in its own right, passes a threshold and triggers sudden, pervasive change. This year, Feb. 25 seemed to be such a moment — let’s call it “Earthworm Day,” the first day warm enough to bring worms to the surface for robins to feed on. To a naturalist’s eye, the whole world shifted.
Spring comes reliably every year, though in some years it takes its own sweet time to arrive. And to be sure, more winter could lie ahead; March and even the first half of April can produce deep cold or prodigious snowfall on the Vineyard. But at some point each winter, the overall trend changes direction, and the process of renewal grows inexorable.
Despite its complete predictability, driven largely by the unchanging orbit of Earth around the sun, every spring is unique. And despite my familiarity with the process — this spring will be the 66th I’ve had the pleasure of watching — it still always catches me by surprise.
