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By Root 739 0

prepare for the seasons, is not known. We do know, though, that animals use star patterns for navigation during their migrations. Many birds migrate at night, mainly the small songbirds whose rate of energy expenditure is so great that they need the day to refuel more often than large birds do. They watch the stars and recognize the patterns of the night sky. We know from detailed experiments and observations that they orient to the North Star or, more likely (as we do), by the star patterns around it, such as the Big Dipper. On their northward migrations to their summer breeding grounds they fly toward the Big Dipper, just as slaves escaping north at night followed it, code-naming it the “Drinking Gourd.” When the birds return at the end of the summer, Polaris and the Big Dipper are on their backs or shoulders as they fly through the night sky.

Proximally, summer in the northern hemisphere is best defined, as already mentioned, by the period of sunlight and warmth that sustains active life. In the tropics “summer” is essentially endless; there are about 4,320 hours of daylight per year. Here in New England daylight is restricted to about 2,520 hours. And despite the much longer days in the arctic summer, there are fewer of them—not quite half those in New England. However, my calculation is an approximation only. I have for simplicity assumed (1) thirty-day months, (2) twelve months of summer with twelve hours of daylight per day in the tropics, (3) six months of summer with an average of fourteen hours of daylight per day in the temperate zone, and (4) two months of summer with twenty-four hours of light per day in the high arctic.

In early February the worst of the winter is yet to come, even though the days lengthen. On some days when the sun does come out, I hear chickadees singing, blue jays carousing, the great horned owl hooting, and woodpeckers drumming. But the weather, like these activities that depend on it, is unpredictable. In 2006 the spring was unseasonably cold and the fall was unseasonably warm. More snow fell in Vermont that April than had been recorded for more than 100 years. But in early February a raven pair that I know had already refurbished their nest, and the female was incubating a clutch of eggs. A pair of great horned owls then evicted the ravens and took over the nest, and in early April the owl perched on her eggs (or her small young, or both) in the nest mold, which became surrounded by a wall of snow a foot high. The ravens did not renest that year—there wasn’t enough time. They, like the owl, have a narrow window of time. They need to have their young independent by fall. So they get an early start on summer. They need all summer and then some. Nest building and incubation take at least a month; rearing the young takes another two months; and then the young adults need the summer to practice their hunting skills, while there are still numerous young animals around to catch.

Fig. 2. Leaf and flower buds (in Vermont) of quaking aspen (left) and red maple (right). In each pair of twigs the thinner one bears leaf buds and the thicker one (from the top of the tree) bears flower buds.

The trees prepare for the coming summer nine months in advance, starting in July of the previous year, when they manufacture embryonic shoots, leaves, and flowers and enclose them in buds. They could potentially wait until spring (some, like black locusts, which flower late, do), but for the northern native trees it is apparently better to have at least the shoot-leaf buds ready-made to burst forth at a signal. It is too cold to make them in the winter, and their final signal for the buds to burst forth is warmth. The hitch, though, is that they risk death if they are fooled by any warm spell, such as the usual January thaw. Insects also prepare to be active at specific times in the coming summer. For example, the giant silk moths (Saturniidae) overwinter in the pupal stage, and like tree buds, they shut down their development from pupa to adult through late summer, fall, winter, and spring.

Insect development