They hunted fish on the seas and they used the beaches as refuges from the predators.
Hesperornis appeared in Primeval, it came through an anomaly that flooded a basment and killed a plummer before presumably retuning to the Cretaceous. When Nick passed into the Cretaceous through the anomaly, he spotted several Hesperornis in the Cretaceous sea and he saw more on the shore and the ugly birds stare at him. When he found his wife Helen, she throws a rock at the Hesperornis and she explained that they're dangerous, but dumb.
In Sea Monsters: A Walking with Dinosaurs TrilogyEdit
Hesperonis also appeared in the third episode of Sea Monsters, "To Hell... and Back?". It was one of the many inhabitants of the Most Dangerous Sea Ever, or, Hell's Aquarium, in the Cretaceous. One individual had been eaten by the Xiphactinus during the underwater bloodbath scene. Nigel Marvin also witnesses many Hesperonis on a beach during one part of the program.