Also known as: Leach's Petrel
20 cm (8") Fairly common off Pacific coast, uncommon off Atlantic. Similar to Wilson's Storm-Petrel but does not often flutter on water with pattering feet. Erratic butterfly-like flight. Also it has a forked tail and dark line dividing its white rump band which does not extend below the bird like in Wilson's.
Images

Credit: Seabamirum