More details on nominees on the SFWA Nebula Awards 1973 page.
Gene Wolfe The Death of Doctor Island.
Winner Best Novella
Edward Bryant Shark
George R R Martin With Morning Comes Mistfall
Vonda N Mcintyre Of Mist, and Grass, and Sand
Winner Best Novelette
Harlan Ellison The Deathbird
Norman Spinrad A Thing of Beauty
James Tiptree Jr Love is the Plan the Plan is Death
Winner Best Short Story
Carol Emshwiller The Childhood of the Human Hero