Is there a summer sound more haunting than the call of a male loon across a dark lake? Actually, haunting may not be the right word here. The varying sounds the loon makes can be magical, bloodcurdling, or almost maniacal. Birders categorize them as hoot, tremolo, wail, and yodel, and in American Indian legends they are considered both death omens and a sign of the bird's magical power. But however you describe it, the call is unmistakably, well, loony.

(A side note on the word "loony": While the bird's name comes from a Scandinavian word meaning clumsy and the word "loony" has its root in "lunatic," some etymologists maintain that the bird's crazy cry might also be involved. "Loonie" is also the nickname of the Canadian one dollar coin, on which the bird is depicted.)

The Common Loon breeds on North Country lakes all along the border of the United States and Canada. In breeding season, you'll know mature loons by the striking black-and-white checkered back, iridescent black head and bill, red eyes, prominent white "necklace" marking at the back of the neck, and smaller white "chinstrap" at the throat. In winter, that white coloring extends up and around the eyes.

But even more impressive than its plumage and voice is the loon's amazing swimming and diving prowess. With bones and musculature specifically designed for these activities, loons can swim fast, dive deep, and stay underwater for long periods of time. A special membrane protects their eyes in the depths and allows them to remain submerged for up to 15 minutes. The red eyes, which turn brown in winter, might play a role in mate-attraction and also help them to spy prey (including perch, trout, and dragonfly larvae) in the murky depths. Their solid bones are denser than the hollow ones of most flying birds, making them less buoyant and thus able to go deeper. Also, their legs have a unique adaptation—an intriguing arrangement of bones that sit far back on their body—resulting in increased strength, leverage, and flexibility. Loons don't use their wings when they swim, instead propelling and steering themselves adeptly with extra-large webbed feet and zooming like well-guided torpedos.

Loon design is an advantage underwater, but it's a hindrance on land—the poor birds can't even walk upright and scuttle along on their chests. Except when nesting, they generally avoid being on land.

As for flying, that heavy body requires a running start of about a quarter-mile on water to get aloft, so you won't find them on small lakes. Early freezes can even trap them in their summer homes. And while they are strong fliers, their winter plumage molts in late winter and the loss of critical feathers can leave them temporarily grounded. Really, they do have something to scream about.

This story has been edited from its original form (December 2010).