Loon

Because their legs are set very far back on the body, near the tail. This position turns the legs and large webbed feet into an efficient rear propeller that drives the bird through the water and lets it manoeuvre after fish, and it is the same solution used by grebes, cormorants, and penguins. The cost is that the legs are in the wrong place for standing and walking, so on land a loon cannot hold itself upright or take normal steps and instead pushes itself along on its breast in an awkward shuffle. Loons therefore build their nests right at the water's edge, often on a small island or a floating mat of vegetation, so they can slip on and off with minimal time ashore.

The common loon has four principal calls, each with a purpose. The wail is a long, rising and falling, mournful cry used mainly by members of a pair to find and reconnect with each other across a lake, especially at night. The tremolo is a wavering, laughing call, the classic loony laugh, given when the bird is alarmed, when an intruder enters the territory, or in flight, and it can signal stress from a boat or a person approaching too close. The yodel is a loud, complex, rising call given only by territorial males, and each male's yodel is individually distinctive and changes if he moves to a new territory, effectively acting as a name and a claim. The hoot is a short, quiet call used at close range between mates and between parents and chicks.

Loons routinely swallow small pebbles from the lake bottom, which sit in the muscular gizzard and help grind up food, and they cannot easily tell a lead fishing sinker or jig head from a suitable stone. A loon can also ingest lead tackle that is still attached to a fish it has caught or scavenged after an angler has broken the line. Once in the gizzard, the lead is slowly ground down and absorbed, and even a single small sinker delivers a fatal dose over one to three weeks, causing the bird to become weak, lose coordination and the ability to fly and dive, and eventually die. Lead poisoning from tackle has been found to cause a large share of recorded adult loon deaths in parts of the northeastern United States, prompting bans on small lead sinkers and jigs in several states.

All three are diving waterbirds with legs set far back, but they belong to different families and differ in build and behaviour. A loon is a large, heavy, thick-necked bird with a straight, dagger-shaped bill, striking black-and-white breeding plumage in most species, and a wailing voice, and it is confined to the Northern Hemisphere. A grebe is generally smaller and slimmer, with a thinner neck, a small pointed bill, lobed rather than fully webbed toes, often ornate head plumes in the breeding season, and it builds a floating nest. A cormorant is a long-necked, hook-billed, mostly dark bird that often perches with its wings spread to dry, dives from the surface or from a plunge, and is found worldwide including in the tropics. In Britain and much of Europe, loons are called divers rather than loons.