Flight typically favours a lightweight skeleton, and most birds achieve this with pneumatized bones containing air pockets connected to the respiratory system. The common loon breaks that pattern: its bones are notably denser and less hollow than those of comparable flying birds, adding mass that helps counteract natural buoyancy from air trapped in feathers and lungs.
That extra density, combined with a streamlined body, rearward-set legs, and the ability to compress its plumage to expel trapped air, lets a loon slip underwater with little effort and pursue fish at depth. Recorded dives have reached over 60 metres, though most feeding dives are shallower and last well under a minute, occasionally extending past three minutes. Adults typically weigh between 2.2 and 5.5 kilograms depending on sex and season.
A persistent misconception is that loons cannot fly at all because they are so heavy and dive-adapted; in fact they fly well but need a long, open stretch of water as a runway, since their body plan sacrifices easy takeoff for diving power. The precise maximum diving depth remains uncertain, since the deepest records mostly come from birds recovered from fishing nets rather than direct tracking instruments.