Leach's Storm-Petrel vs Swinhoe's Storm-Petrel

In short

The most reliable field mark to separate Leach's Storm-Petrel from Swinhoe's Storm-Petrel is the presence or absence of a white rump: Leach's typically shows a white rump patch (often with a dark central line), while Swinhoe's is uniformly dark-rumped. However, dark-rumped Leach's do occur in the Pacific and can be very difficult to distinguish from Swinhoe's, so careful attention to structure and subtle plumage details is essential. Both species are similar in size and share a forked tail and fluttering flight, but Swinhoe's is overall darker and lacks any white on the rump. In all-dark birds, identification may require close scrutiny of flight style and, in some cases, is not possible without genetic analysis.

Identification tips

1. Rump pattern is key: Leach's Storm-Petrel almost always shows a white rump patch, often with a dark central line; Swinhoe's is entirely dark-rumped. This is the most reliable mark in the Atlantic and for most Leach's globally.
2. Beware dark-rumped Leach's: In the north-eastern Pacific, a significant proportion of Leach's Storm-Petrels are dark-rumped and can be indistinguishable from Swinhoe's in the field. If you see a dark-rumped bird in this region, identification may not be possible without further evidence.
3. Overall plumage shade: Swinhoe's is uniformly dark brown, while Leach's is typically blacker with a paler face and sometimes a paler bar on the secondary coverts. These differences are subtle and unreliable at sea.
4. Tail shape: Both species have a forked tail, but this is not a useful separator between them.
5. Flight style: Both have a fluttering, buoyant flight and often patter on the water. Swinhoe's may feed more by dipping and less by pattering, but this is a subtle behavioral difference and not always visible.
6. Size: Both are the same size; do not use size to separate them.
7. Habitat and context: Both are pelagic and rarely seen from land. Swinhoe's is less likely to follow ships, but this is not a reliable field mark.
8. Geographic context: In the Atlantic, a dark-rumped storm-petrel is almost certainly Swinhoe's, as dark-rumped Leach's are extremely rare there. In the Pacific, both species can be dark-rumped, so caution is needed.
9. Age and season: There are no known age-specific upperwing or flight pattern differences between these two species; the rump pattern is consistent across all ages.
10. Voice: Calls differ but are not described in the sources and are rarely useful at sea.
11. Structural details: Both have longish wings and similar proportions; structure is not a reliable separator.
12. Pitfalls: DNA analysis has been used to confirm Swinhoe's in regions where dark-rumped Leach's occur. In the field, some individuals cannot be safely identified.
13. Summary for field use: If you see a storm-petrel with a white rump, it's a Leach's. If the rump is all dark, consider location: in the Atlantic, it's likely Swinhoe's; in the Pacific, it could be either, and further evidence is needed.

Size

Both Leach's and Swinhoe's Storm-Petrels are very similar in size, with lengths of 18–21 cm and wingspans of 43–48 cm (Leach's) and 45–48 cm (Swinhoe's) [wikipedia]. There is no practical size difference in the field between these two; both are distinctly larger than European Storm-Petrel, but not separable from each other by size alone.

Plumage

• Juvenile/immature/adult non-breeding: The key distinction is the rump: Leach's Storm-Petrel almost always shows a white rump patch, often bisected by a dark line, visible in flight and at rest. Swinhoe's Storm-Petrel is entirely dark brown, including the rump, with no white at any age or season. However, dark-rumped Leach's do occur, especially in the Pacific, and are rare elsewhere. Both species have similar overall dark plumage and a pale bar on the secondary coverts is sometimes visible on Leach's but is not a strong field mark for separation. Swinhoe's is more uniformly dark brown, while Leach's can appear blacker with a slightly paler face.
• Adult breeding: Same as above; the rump pattern remains the most useful mark. No age-specific upperwing pattern differences are noted in the sources for either species.
• Pitfall: Dark-rumped Leach's can be indistinguishable from Swinhoe's in the field, especially at sea.

Behaviour

Both species have a fluttering, buoyant flight and often patter on the water as they feed. Leach's Storm-Petrel is noted for its pattering and picking food from the surface, while Swinhoe's is said to feed mainly by dipping and does not patter as much. Both are strictly pelagic outside the breeding season and rarely seen from land except after storms. Swinhoe's is strictly nocturnal at breeding sites, avoiding landfall on moonlit nights to evade predators; this is not specifically mentioned for Leach's but is typical of storm-petrels.

Habitat

Both are highly pelagic and rarely seen near land except at breeding colonies or after storms. Swinhoe's is especially elusive, with breeding sites in remote areas and a tendency to avoid ships, while Leach's may be more likely to be seen from boats. Both nest in burrows and are limited to shuffling on land.

Voice

The calls differ, but no field-identifiable vocal details are provided in the sources. Swinhoe's call is distinct from Leach's, but in practice, vocalizations are rarely diagnostic at sea and are not described here.