There is no other sound quite like it, the tremulous wail that floats across a northern lake at dawn, rising and falling, carrying in its haunting notes a summation of wilderness.

Few people can listen to the call of a common loon without being moved. This is the signature sound of the North Woods, and for many visitors to the region hearing one laughing through the night is a high point of the trip.

The common loon, with its necklace of white and checkerboard pattern on the back, is the species that comes to most people's minds when they think of loons. There are four other species  the yellow-billed, the arctic, the Pacific and the red-throated  but only the common loon is found south of Canada in summer, with the southern edge of its breeding range stretching from the Adirondacks through northern Vermont and New Hampshire to Maine. It is strictly a warm-weather resident in the region's inland, migrating in late fall to the sea from the Maritimes to Florida.

A nesting loon.

Taxonomically, loons are ranked as among the most primitive of birds, yet there is nothing unsophisticated about them, from their elegant plumage to their many physical adaptations for an aquatic lifestyle. The legs, which are powerfully muscular, are set far back on the body; this provides maximum strength for swimming, but forces them to push along on their bellies when they are on land. The wings are kept folded underwater, and the loon propels itself with sweeps of its webbed feet; on the forward stroke, the flattened tarsi, or leg bones, cut resistance by the water.

In fact, almost everything about a loon's body helps it dive and swim. It has the ability to store unusually large amounts of oxygen in its muscle tissue just before plunging underwater. Many bones are solid, rather than hollow as in most birds, resulting in a specific gravity close to that of water, so the loon can slip under without a ripple when it needs to escape quietly.

Loons have been caught in fishing nets more than 200 feet deep, but most of their time is spent in shallow water, where they dive for small fish, their primary prey. Cruising slowly along the surface, the loon will repeatedly stick its head underwater, watching for fish. When it sees a school it dives with a quick thrust of both legs, arcing down like a dolphin. Although loons will catch gamefish like brook trout, they more often tend to capture slower prey, like small catfish and suckers.

Soon after they return from their wintering grounds, loon pairs pick a nest site, usually returning to the same secluded cove or backcountry pond they used in years past. The nest  a low, flattened bowl of sedges and grasses built right along the water  is finished, and the eggs (normally two) are laid around Memorial Day. Incubation takes about a month, and the chicks can swim almost from birth. For the rest of the summer they stay close to their parents, eventually learning to dive and hunt on their own.

Vocalizations play an important role during the breeding season, keeping loon pairs together and delineating their territory. Ornithologists recognize four or five different calls, including the famous"laugh" and a longer, more complex version known as a yodel; loons also wail, and have a number of low, short calls that they use to communicate when they are near each other.

Although you may hear loons call at anytime of the day during June, they will be most vocal at dawn and again at dusk; if there is a bright moon the loons may well call straight through the night. Some people can imitate a loon call well enough to draw a bird close, but this (or the use of tape-recorded calls) should be avoided, since it disrupts the loon's routine and may leave its nests or chicks vulnerable to predators.

Common loons breed throughout the northern half of the region, often picking remote backcountry lakes. What follows, however, are suggestions for fairly accessible areas with healthy populations of loons. Remember that June is the height of the breeding season, and you should never approach an area you suspect of harboring a loon nest.