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
Least Storm-Petrel
Fork-tailed Storm-Petrel
Band-rumped Storm-Petrel
Wedge-rumped Storm-Petrel
Black Storm-Petrel
Ashy Storm-Petrel
Wilson's Storm-Petrel