Fairly large, long-winged storm-petrel with deeply notched tail. Complex and subtle variation across extensive range. Typical flight style is somewhat bat-like: bounding with jolting twists and turns. Plumage looks chocolatey-brown in good light; paler than Wilson's or Band-rumped Storm-Petrel and with more extensive buffy bar on upperwing. Rump color is variable; birds in the Atlantic have a mostly white rump, usually with a thin dark line down the center. Pacific birds are more variable, sometimes completely dark-rumped, especially "Chapman's" Storm-Petrel (southern California to South America). Occurs in the Pacific and Atlantic Oceans, mainly in the Northern Hemisphere. Fairly common but stays far offshore; rarely seen from land. Compare especially with Band-rumped Storm-Petrel; on Leach's, look for more obvious pale upperwing bar, more deeply notched tail, and bounding flight style.