As a continental glacier retreated northward long ago, it left a jumble of ice blocks at its edge, and where it stood still for a time, great quantities of rock and soil built up at the southern end to form a moraine. When huge chunks of ice were buried by this debris, kettle holes formed as the ice finally melted. Swan Pond is one of several shallow, water-filled kettles in this hilly moraine area, about 2 to 3 feet deep. Because kettle holes have no stream supplying them, Swan Pond's water level fluctuates greatly through the year. In March and April, when rains and melting snow swell the pond, flocks of snow-white whistling swans stop there on their trek from the Atlantic seaboard to breeding grounds in the Yukon. The pond gradually shrinks in summer, and newly exposed mud flats quickly sprout thick growths of smartweed and other shore plants whose seeds feed the next spring's migrating waterfowl. Shore birds flock there in summer, including the long-legged, robin-sized killdeer, named for its call. As water rises in fall, muskrats harvest the plants and create open water again for waterfowl, and by the time the pond freezes, they have piled the plants into domed huts, surviving winter by eating their houses from the inside out. Swan Pond, on private property, is an excellent example of the compatibility of farming with preservation of wildlife habitat.