オナガミズナギドリ 対 ミナミオナガミズナギドリ

Buller's Shearwater is best separated from Wedge-tailed Shearwater by its striking upperwing pattern in flight: a bold, broken black 'M' or 'W' across pale grey upperwings, unique among large shearwaters. Wedge-tailed Shearwater, in contrast, shows either uniform dark or plain grey-brown upperparts without any strong wing pattern, regardless of age or morph. Both have wedge-shaped tails and dark bills, but Buller's is slimmer and lighter overall. Use flight views and upperwing pattern first; perched or distant birds are trickier, especially for Wedge-tailed's pale morph.

1. Upperwing pattern in flight (all ages): Buller's Shearwater shows a bold, broken black 'M' or 'W' across pale grey upperwings, with blackish primaries and a dark secondary stripe separated by a pale panel. Wedge-tailed Shearwater (both morphs) lacks any such pattern; upperwings are plain grey-brown (pale morph) or uniformly dark (dark morph).
2. Underwing and body contrast: Buller's has a bright white underside and underwing, with sharp contrast to the upperparts. Wedge-tailed's pale morph is also white below, but lacks the sharp upperwing contrast; the dark morph is uniformly dark below as well.
3. Head and face: Buller's has a grey cap extending to eye level, setting off a bright white cheek that can look striking in good light. Wedge-tailed's head matches the upperparts—either grey-brown (pale) or dark (dark morph)—and lacks a sharply defined white face.
4. Tail shape and color: Both have wedge-shaped tails, but Buller's tail is blackish and matches the flight feathers, while Wedge-tailed's tail is the same color as the back (grey-brown or dark grey-brown).
5. Bill color and shape: Both have dark bills, but Buller's is slightly finer and more slender; this is subtle and not always reliable at sea.
6. Overall size and build: Wedge-tailed is bulkier and heavier; Buller's is slimmer and more elegant, but size is hard to judge unless seen together.
7. Flight style: Buller's often appears more buoyant and graceful, with long, arcing glides and less flapping. Wedge-tailed can look heavier and more direct.
8. Plumage morphs: Wedge-tailed comes in pale and dark morphs; Buller's is always the same pattern. Beware pale Wedge-tailed, which can look superficially similar below to Buller's, but never shows the upperwing 'M'.
9. Juvenile/immature birds: Juvenile Buller's look like adults, so the upperwing pattern is always present. Juvenile Wedge-tailed match their morph, with no special wing pattern.
10. Perched/rafter views: Separation is much harder when birds are sitting on water; rely on head pattern (Buller's: white face, grey cap) and overall color (Wedge-tailed: plain, never patterned). Tail shape is not always obvious when sitting.
11. Habitat and range: Both can be found in the tropical Pacific, but Buller's is more likely in temperate waters and off the Americas in late summer/autumn. Wedge-tailed is more strictly tropical.
12. Voice: Only useful at colonies; Wedge-tailed gives long, moaning calls. Not helpful at sea.
13. Pitfalls: The main error is mistaking a pale-morph Wedge-tailed for Buller's due to the white belly; always check for the upperwing 'M' in flight. Dark-morph Wedge-tailed is unlikely to be confused with Buller's.
14. Sex and season: No sexual dimorphism or seasonal plumage difference in either species; all field marks apply year-round.
15. Lighting conditions: The 'M' pattern on Buller's is most obvious in good light, but the pale panel is usually visible even in dull conditions. Always try to get a view of the upperwing in flight for confident separation.

Wedge-tailed Shearwater is the heavier bird (465 g vs. 326 g), and is the largest tropical shearwater. Buller's Shearwater is still large, but noticeably lighter and slightly smaller in bulk. Both are long-winged and similar in overall proportions, but Buller's can appear more elegant due to its slimmer build.

The most diagnostic difference is in flight: Buller's Shearwater (all ages from fledging onward) shows a distinctive upperwing pattern—a dark, broken 'M' or 'W' formed by blackish primaries and a dark stripe along the secondaries, separated by a pale silvery-grey panel. This is visible even at a distance and is unique among large shearwaters. The rest of the upperparts are bluish-grey, with a bright white underside and a grey cap that extends to eye level, setting off the white face. Wedge-tailed Shearwater never shows this pattern. Instead, it comes in two morphs: the pale morph has plain grey-brown upperparts and upperwings, with a white belly, while the dark morph is uniformly dark grey-brown all over. Both morphs lack any wing panel, 'M', or contrasting pattern—upperwings are plain. Juvenile Buller's already look like adults; juvenile Wedge-tailed resemble their respective morphs. Both species have wedge-shaped tails, but Buller's tail is darker and matches the flight feathers.

Both species are pelagic and often seen gliding low over the sea, but Buller's Shearwater is noted for its more buoyant, graceful flight, often with languid, arcing glides and less frequent flapping. Wedge-tailed Shearwater can appear heavier and more direct in flight, especially when compared side-by-side. Both nest in burrows, but this is not a field mark at sea.

Both are oceanic and can overlap in the tropical and subtropical Pacific. Wedge-tailed Shearwater is more strictly tropical, while Buller's Shearwater ranges widely across the Pacific, including temperate and even subarctic waters, and is more likely off the west coast of the Americas in late summer and autumn. Habitat is rarely diagnostic at sea, but context may help in marginal areas.

Wedge-tailed Shearwater is known for its long, moaning calls at colonies, with a distinctive inhaling and exhaling pattern ('OOO...err'), especially at night. Buller's Shearwater is less often heard at sea and is not noted for distinctive calls in field situations. Voice is not usually helpful for separation at sea.