アシナガウミツバメ 対 クロコシジロウミツバメ

The most reliable field mark to separate Wilson's Storm-Petrel from Band-rumped Storm-Petrel is the upperwing pattern: Wilson's shows a diffuse pale band along the upperwing coverts (all ages), while Band-rumped's upperwing is uniformly dark. Wilson's also has feet that project just past the tail in flight, a square tail, and a fluttering, pattering flight style, whereas Band-rumped is larger, with a longer wingspan, a slightly forked tail, and steadier, more shearwater-like flight. Both species have a white rump, but Band-rumped's is more extensive. Structural and flight differences are often as important as plumage, especially at sea.

1. Upperwing pattern: Look for a diffuse pale band along the upperwing coverts in Wilson's Storm-Petrel (all ages); Band-rumped's upperwing is uniformly dark. This is the single most reliable field mark in flight.
2. Tail shape: Wilson's has a square-ended tail; Band-rumped's tail is slightly forked, though not as deeply as Leach's Storm-Petrel. The fork can be subtle and is best seen when the tail is spread.
3. Feet projection: In Wilson's, the feet extend just beyond the tail in flight, often visible as tiny projections; in Band-rumped, the feet do not project beyond the tail. This is a subtle but useful mark in good views.
4. Flight style: Wilson's shows a fluttering, hovering flight with frequent pattering on the water; Band-rumped flies more steadily, with a gliding, shearwater-like action, rarely pattering.
5. White rump: Both have a white rump, but Band-rumped's is more extensive. However, rump extent is variable and overlaps, so use with caution.
6. Flank color: Wilson's shows white on the flanks as well as the rump; Band-rumped's white is confined to the rump.
7. Size: Band-rumped is larger and bulkier, with a longer wingspan and heavier build. This is most useful when both species are seen together.
8. Underwing: Neither species has a strong white underwing panel; Wilson's lacks the white underwing lining of European Storm-Petrel, and Band-rumped's underwing is uniformly dark.
9. Toe webbing: Wilson's pre-breeding birds have yellow and black-spotted toe webbing, but this is not a field mark except at extreme close range.
10. Breeding context: If seen near islands in the North Atlantic or Pacific, Band-rumped is more likely to be a breeder; Wilson's breeds in the Southern Hemisphere but disperses widely.
11. Nocturnal habits: Band-rumped is strictly nocturnal at colonies; Wilson's is not as strictly nocturnal, but this is rarely relevant at sea.
12. Voice: Wilson's gives soft peeping calls while feeding, but voice is rarely diagnostic at sea for either species.
13. Ship-following: Wilson's is more likely to follow ships and gather in large flocks; Band-rumped is less frequently described in this context.
14. Confusion with other storm-petrels: Be cautious not to confuse Band-rumped with Leach's Storm-Petrel, which has a more deeply forked tail and different rump shape; Wilson's is more easily separated by its upperwing band and pattering flight.
15. Pitfalls: In poor light or distant views, size and tail shape can be misleading; rely on upperwing pattern and flight style as primary marks.

Band-rumped Storm-Petrel is noticeably larger than Wilson's Storm-Petrel in both length and wingspan. Wilson's measures 16–18.5 cm in length with a 38–42 cm wingspan and weighs about 34 g [wikidata], while Band-rumped is 19–21 cm long, with a 43–46 cm wingspan and a mass of 44–49 g [wikipedia]. This size difference is often apparent when seen together, but can be subtle at a distance or in isolation. Band-rumped appears bulkier and longer-winged, while Wilson's is more compact.

For all ages, Wilson's Storm-Petrel shows a diffuse pale band along the upperwing coverts, which is absent in Band-rumped Storm-Petrel, whose upperwing is uniformly dark [wikipedia]. Both species have a white rump, but Band-rumped's is more extensive. Wilson's has a square-ended tail, while Band-rumped's tail is slightly forked. Wilson's also shows white on the flanks, not just the rump. The underwing of Wilson's lacks the distinct white lining found in European Storm-Petrel, but Band-rumped's underwing is not described as having any pale lining or panels. In pre-breeding Wilson's, the webbing between the toes is yellow with black spots, but this is not visible in the field except at extremely close range. There are no age-specific upperwing or underwing patterns described for Band-rumped, so the upperwing band of Wilson's is the most consistent mark across all ages.

Wilson's Storm-Petrel is famous for its fluttering, hovering flight with wings held high and its habit of pattering on the water surface with its feet as it feeds. This creates a distinctive, almost bat-like jizz, especially when compared to the steadier, more direct, and gliding flight of Band-rumped Storm-Petrel, which is described as more 'shearwater-like.' Wilson's often follows ships and is highly gregarious, while Band-rumped is less described in this context. On land, both are awkward, but Band-rumped is strictly nocturnal at breeding sites, rarely seen on land by day.

Both species are pelagic and can be seen far from land, but their breeding distributions differ: Wilson's is mainly a Southern Hemisphere breeder but disperses widely, while Band-rumped breeds on Atlantic and Pacific islands in warmer regions. At sea, their habitats overlap, especially in the North Atlantic. Band-rumped is strictly nocturnal at colonies, while Wilson's is more flexible in its activity.

Wilson's Storm-Petrel gives a soft peeping noise while feeding, but neither species is likely to be identified by voice at sea. Band-rumped has distinct vocalizations at colonies, but these are not typically heard at sea or during migration.