Which bird is the deepest-diving bird species?

The emperor penguin is the deepest-diving bird species.

Emperor penguins have been recorded diving to depths of more than 500 metres, with a documented maximum near 564 metres. They dive beneath Antarctic sea ice to pursue fish, squid, and crustaceans, often remaining underwater for many minutes.

Their bodies are adapted for repeated deep dives. Dense bones reduce buoyancy, oxygen is stored in blood and muscle, and the birds can slow their heart rates while submerged. A streamlined shape also reduces energy use as they descend and return to the surface.

Emperor penguins breed during the Antarctic winter, when temperatures and winds are exceptionally severe. The male incubates the single egg on his feet while the female travels to sea to feed. Their deep-diving record is distinct from the question of which penguin is largest: emperor penguins hold both distinctions among living penguins.