Band-rumped Storm Petrel
Hydrobates castro
The band-rumped storm petrel is 19–21 cm (7.5–8.3 in) in length with a 43–46 cm (17–18 in) wingspan, and weighs 44–49 g (1.6–1.7 oz). It is mainly brownish black with an extensive white rump. Similar to Leach's storm petrel with the forked tail, long wings, but Leach's has a more deeply forked tail, a differently shaped (V-shaped or triangular) white rump, and a 'tern-like' flight, whereas the band-rumped storm-petrel has a more 'shearwater-like' flight.
ID text from Wikipedia, CC BY-SA 4.0
Fork-tailed Storm Petrel
Hydrobates furcatus
The fork-tailed storm petrel is small seabird that is 20 cm in length with a wingspan of 46 cm. Its foraging behaviour resembles other storm petrels, where it flies with short, stiff wingbeats close to the surface of the water. Despite its name, the forked tail in this bird is not always visible. The distinguishing feature of fork-tailed storm petrels is their overall bluish-gray plumage. They generally have a paler underside, which contrasts the darker colours under their wings. They also have a dark gray forehead, a black ear patch, and a small, black bill.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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Play today's Daily BirdPhotos and recordings are credited to their authors under the licence shown. Full profiles: Band-rumped Storm Petrel and Fork-tailed Storm Petrel.