ヒメクロウミツバメ 対 クロウミツバメ

The most reliable field mark to separate Swinhoe's Storm-Petrel from Matsudaira's Storm-Petrel is the presence of white bases to the primaries on Matsudaira's, which contrast with its otherwise dark wing and are visible in flight; Swinhoe's is uniformly dark brown with no such contrast at any age. Both species are all-dark, long-winged storm-petrels with forked tails, but Matsudaira's shows a longer, deeply forked tail and a more prominent, hooked bill. In the field, separating these two is challenging, especially at sea and in poor light, but careful attention to wing pattern and tail structure can be decisive. Both are pelagic and rarely seen from land, so context is rarely helpful.

1. Upperwing pattern in flight: Matsudaira's Storm-Petrel shows white bases to the primaries, creating a pale panel or contrast on the upperwing, visible in flight at all ages. Swinhoe's Storm-Petrel is uniformly dark brown with no pale in the wing—this is the single most reliable field mark.
2. Alula coloration: Matsudaira's has blackish alula feathers that stand out against the white-based primaries; Swinhoe's alula is not contrasting, blending with the rest of the dark wing.
3. Tail shape: Matsudaira's has a long, deeply forked tail, often appearing more pronounced than Swinhoe's, which has a forked but shorter tail. This can be subtle in poor views.
4. Bill structure: Matsudaira's has a noticeably longer bill with a pronounced hook at the tip, which may be visible in close or photographic views. Swinhoe's bill is shorter and less hooked.
5. Overall coloration: Both are all-dark, but Swinhoe's is described as essentially dark brown in all plumages, while Matsudaira's is blackish. In practice, this is rarely diagnostic at sea.
6. Rump pattern: Both species lack a white rump, unlike Leach's Storm-Petrel. Do not use rump color to separate these two.
7. Size: Swinhoe's is slightly smaller, but size overlap and lack of direct comparison make this unreliable in the field.
8. Flight style: Both have fluttering, agile flight, but Matsudaira's may appear more elongated due to its longer tail and bill. This is subtle and easily misjudged.
9. Feeding behavior: Swinhoe's feeds by dipping and does not patter on the water; Matsudaira's feeding behavior is not described, so do not use this mark.
10. Habitat and context: Both are strictly pelagic and rarely seen from land; context is not helpful for separation.
11. Vocalizations: No diagnostic vocal differences are described in the notes; do not use voice for field separation.
12. Pitfalls: Beware of confusing Matsudaira's with Leach's Storm-Petrel populations that lack a white rump, but for Swinhoe's vs Matsudaira's, focus on the wing panel and tail shape.
13. Age and season: No age-specific or seasonal plumage differences affect the key marks; both species are all-dark at all ages, and Matsudaira's always shows the white primary bases.
14. When in doubt: If the upperwing is uniformly dark with no pale panel, Swinhoe's is likely. If you see a pale panel on the upperwing (white primary bases), Matsudaira's is the only option among these two.
15. Photographic review: For rare vagrants or uncertain records, photographs of the upperwing in flight are essential for confident identification between these two species.

Both species are small, all-dark storm-petrels, but Swinhoe's Storm-Petrel is slightly smaller (18–21 cm length, 45–48 cm wingspan [wikipedia]) and distinctly larger than European Storm-Petrel. Matsudaira's Storm-Petrel is not directly measured in the notes, but is implied to be similar or slightly larger, with a longer tail and bill. In practice, size alone is not a reliable separator between these two, especially at sea.

The key plumage difference is on the upperwing: Matsudaira's Storm-Petrel of all ages shows blackish alula feathers, but crucially, the bases of its dark primaries are white, creating a pale panel or contrast visible in flight. Swinhoe's Storm-Petrel, by contrast, is uniformly dark brown across all plumages, with no white in the wing or rump. Both lack a white rump (unlike Leach's Storm-Petrel), and both are all-dark below. There are no notes of age-specific variation in either species that would affect this contrast: the white primary bases on Matsudaira's are present at all ages, and Swinhoe's is always plain dark brown. Tail shape is also helpful: Matsudaira's has a longer and more deeply forked tail, while Swinhoe's is forked but less so.

Both species are strictly pelagic outside the breeding season and rarely seen from land. Swinhoe's Storm-Petrel feeds by dipping and does not patter on the water, while Matsudaira's feeding behavior is not described in detail. Swinhoe's is strictly nocturnal at breeding sites and avoids land on moonlit nights. Both have fluttering, agile flight, but Matsudaira's may appear more elongated due to its longer tail and bill. Neither species is known to follow ships regularly.

Habitat is not a reliable separator: both are oceanic, pelagic storm-petrels. Swinhoe's breeds in remote areas and is rarely seen from land except in storms; Matsudaira's breeds only in the Volcano Islands and winters in the Indian Ocean. Both are most likely to be encountered at sea, far from shore.