The common loon is distinguished from the black-throated loon (G. arctica) and the red-throated loon (G. stellata) mainly by its larger size. It usually has a steeper forehead and a bulging forecrown, somewhat similar to the black-throated loon. Its bill is heavier and the back is paler than its hindneck. It is more difficult to separate from the yellow-billed loon, but its breeding plumage has more white markings on the neck and the squares on its shoulders are usually smaller; the non-breeding plumage has darker neck-sides contrasting more sharply with pale areas and bill colour.
