The Forest Unseen: A Year's Watch in Nature Read Online Free PDF Page A

fat stores by jabbing insects and seeds under flaking bark, storing food for later recovery. Carolina chickadees are particularly fond of caching food by poking it into the undersides of small branches. This habit may be a guard against thievery from less agile bird species. Nonetheless, caches are vulnerable to plunder, so each chickadee flock in the forest defends a winter territory from which neighbors are vigorously excluded. Non-caching chickadees in other parts of the world are much less territorial.
    Larger bird species often join chickadee flocks in the winter. Today, a downy woodpecker chisels for larvae in the bark of an oak tree, then flies after the chickadees when they flit east. A tufted titmouse also travels with the flock. The titmouse bounces among branches like the chickadees, but it is less agile, preferring to light on twigs without swinging from branch ends. All the birds call, keeping the flock together. The chickadees and titmouse chatter and whistle, the woodpecker gives high-pitched
pik
notes. This flocking behavior gives the group members safety from hawks, which are easier to spot when many eyes are vigilant. But chickadees pay a price for safety in the crowd. Tufted titmice are twice as heavy as chickadees, and the larger birds dominate, pushing the chickadees away from dead branches, higher twigs, and other preferred feeding locations. These subtle changes in location result in significant lost feeding opportunities for chickadees. In flocks where titmice are absent, chickadees are better fed. Survival in the winter mandala therefore requires not just sophisticated physiology but careful negotiation of social dynamics.
    Daylight is fading now. I move my chilled limbs and rub my ice-crustedeyes in preparation for the walk out of the forest. The birds will continue their search for food a few minutes longer, then they will head to their roosts. As light fails and the temperature drops, chickadees will gather in holes left by fallen tree limbs, sheltering from the wind’s heat-ripping power. The birds huddle in groups, giving a nod to Bergmann’s rule by creating a ball of birds with a large volume and a relatively small surface area. Then the chickadees’ body temperature will fall by ten degrees into an energy-saving hypothermic torpor. At night, as in the day, integrated behavioral and physiological adaptations give the birds an edge over winter. Torpor combined with huddling halves the chickadees’ nighttime energy needs.
    The chickadees’ adaptations to the cold are remarkable, but they are not always adequate. There will be fewer chickadees in the forest tomorrow. Winter’s chill hands will pull down many of these birds, dragging them deeper than the appalling emptiness I felt when I experienced the cold. Only half the chickadees that fed among the falling autumn leaves will live to see the oak buds open in the spring. Nights such as tonight cause most of the birds’ winter mortality.
    This week’s glacial temperatures will last just a few days, but the spike in bird mortality will change the forest in ways that extend throughout the year. Deaths on winter nights check the chickadee population, trimming any birds that exceed the scant supply of winter food. Carolina chickadees each require, on average, three or more hectares of forest to sustain themselves. This square meter of mandala therefore supports just a few hundred-thousandths of a chickadee. Tonight’s cold will remove any excess.
    When summer arrives, the mandala will be able to support many more birds. But because the abundance of resident species like chickadees is kept low by winter’s meager supplies, the food available in summer vastly exceeds the resident birds’ appetites. This great seasonal flush of food creates an opportunity that is exploited by migrant birds that risk long flights from Central and South America to feed on the excess in forests throughout North America. Winter’s cold is thereforeresponsible for