Nat Hocken's a hero because he's willing to do whatever it takes to protect his family from the murderous flock of birds that's descended upon the village. Unlike his neighbors, he takes the birds' threat seriously and tries to warn them of the impending danger. So as well as protecting his family, Ned also tries to protect his neighbors, even though they scoff at his warnings.
Ned had long since proved himself to be a hero during the war, when he was disabled. And he's clearly taken that heroism with him into civilian life. One thing he seems to have learned during the war is the importance of good morale. To that end, he shows himself remarkably adept at putting his family at ease, even as the bodies are piling up outside and the birds are smashing through the door with their sharp, deadly beaks.
In "The Birds," Nat Hocken is heroic because he is a fearless leader. Nat proves this when the birds attack his home for the first time. Despite being attacked by the birds on his window-sill, for example, Nat goes directly to his children's bedroom to protect them. After sending the children out of the room and into a passage, Nat faces the birds head-on and alone:
He seized a blanket from the nearest bed and, using it as a weapon, flung it to right and left about him in the air…How long he fought with them in the darkness he could not tell.
In addition, Nat is also heroic because he constantly reassures his family that they will be safe. In the aftermath of this attack, for instance, Nat soothes his wife by making her a cup of tea and distracts his children by maintaining their daily routine, like making breakfast and getting them washed. He also accompanies Jill to the school bus to ensure that she is safe. Furthermore, he removes all of the dead birds from outside of his house so that it does not upset or scare his family.
Nat continues to lead his family throughout the rest of the story. He does so despite his wartime disability, thereby demonstrating his commitment to the protection of those around him. It is this character trait which makes him truly heroic.