Hesperornis was a large flightless bird that swam in the oceans and snared fish with a tooth-lined beak. Its small wings were held close in to the body and were of little use beyond possibly helping it steer through the water. Instead, Hesperornis relied on its powerful hind legs and webbed feet to chase prey and evade predators in the Cretaceous seas. A flattened tail may have helped the bird change depth and direction underwater.
In fact, Hesperornis was so adapted to diving and swimming that walking on land was an awkward proposition at best. Presumably, the bird only ventured onto solid ground to breed and lay eggs. Neither water nor land were safe for Hesperornis: Dinosaurs were terrestrial threats, and the aquatic mosasaur giant Tylosaurus was known to consider Hesperornis a tasty meal.
On the water, a long, slender neck gave Hesperornis a silhouette similar to a modern-day grebe. It probably fed and bred much like a penguin.