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.