オオハシウミガラス 対 ウミガラス

The most reliable field mark between Razorbill and Common Guillemot is the bill: Razorbill has a thick, deep, blunt-ended bill with obvious grooves and a white line in breeding plumage, while Common Guillemot's bill is long, thin, and pointed. In flight, Razorbill shows a distinctly longer, wedge-shaped tail and a more compact, thick-set profile, whereas Common Guillemot has a short, rounded tail and a sleeker silhouette. Adult plumage is superficially similar (black above, white below), but Razorbill's face is darker in breeding season and shows a white line from the eye to the bill. Juvenile and first-winter birds are not strongly differentiated in the sources, so structure and bill shape remain key at all ages. Both species are colonial cliff-nesters, but Razorbill is slightly smaller and chunkier.

1. Bill shape is the single most reliable field mark at all ages and seasons: Razorbill has a thick, deep, laterally compressed bill with a blunt end and (in breeding) a white vertical line; Guillemot's bill is long, thin, and sharply pointed. This is visible even at a distance with binoculars.
2. Tail shape and length: Razorbill has a distinctly longer, wedge-shaped tail with central feathers projecting, giving a pointed look when perched or in flight. Guillemot's tail is short and rounded.
3. Head and face pattern in breeding plumage: Razorbill's head is jet black with a thin white line from the eye to the bill; Guillemot's head is black or dark brown (depending on subspecies) with no such line. Bridled Guillemots have a white eye ring and line extending back, not forward to the bill.
4. Non-breeding (winter) face: Both species develop white on the face in winter, but Razorbill's white is behind the eye and throat, and the bill line fades; Guillemot's white face has a dark spur behind the eye, and the bill remains thin and dark.
5. Body structure: Razorbill is thick-set, chunky, and compact, with a shorter neck and deeper chest; Guillemot is longer, slimmer, and more upright when perched.
6. Flight profile: Razorbill appears compact with a long tail and short neck; Guillemot looks longer, with a short tail and more streamlined body.
7. Bill color and grooves: Razorbill's bill shows vertical grooves and a white line in breeding; Guillemot's bill is plain dark and never shows grooves or white lines.
8. Plumage color: Razorbill's black is deeper and glossier on the head and back in breeding; Guillemot may appear dark brown, especially in southern colonies.
9. Juvenile/first-winter plumage: No strong differences are described; rely on bill and structure.
10. Leg color: Both species have dark legs; rare Guillemots may show yellow-grey, but this is exceptional and not useful for separation.
11. Feeding behavior: Razorbill may bring multiple small fish, Guillemot typically carries one fish at a time crosswise in its bill. This is most visible at colonies.
12. Nesting location: If seen at a nest, Razorbill prefers crevices or under rocks; Guillemot nests on open ledges.
13. Colony density: Guillemot colonies are much denser and noisier; Razorbill is more scattered and less numerous.
14. Size in direct comparison: Guillemot is larger and heavier, but size overlap is considerable; use only with caution.
15. Voice: Not reliably separable in the field with the information provided.

Razorbill is slightly smaller and lighter than Common Guillemot. Razorbill measures 37–39 cm in length with a mean weight of 505–890 g [wikipedia], while Common Guillemot is 38–46 cm long and weighs 775–1,250 g [wikipedia]. Guillemot is noticeably heavier, especially in northern populations. Wingspan is also greater in Guillemot (61–73 cm) compared to Razorbill (not specified, but wing length is shorter). In the field, Guillemot often appears longer and slimmer, while Razorbill looks more compact and thick-set.

There are no distinctive juvenile or immature upperwing patterns described for either species, so separation relies on structure and adult plumage. In breeding plumage, Razorbill has a black head, neck, and back with a white belly, and a diagnostic thin white line running from the eye to the end of the bill. Its head is darker than Guillemot's. In non-breeding plumage, Razorbill's throat and face behind the eye turn white, and the white line on the face and bill fades. Common Guillemot in breeding plumage is black (or dark brown in some southern birds) above and white below, with a thin, dark, pointed bill and a small, rounded black tail. In winter, Guillemot's face becomes white with a dark spur behind the eye, sometimes with dark flank streaks. Some Guillemots ("bridled" morph) show a white eye ring and line, but this is a thin, complete ring extending back, not a line to the bill. First-winter Guillemots look like adults in winter plumage, but may retain more white around the throat. Razorbill's bill grooves and white line are less marked in non-breeding plumage, but the bill remains deep and blunt. Guillemot's bill is always thin and pointed.

Both species are colonial cliff-nesters and spend most of their lives at sea, only coming to land to breed. Razorbill tends to nest in crevices or under boulders, while Guillemot prefers open cliff ledges. Razorbill pairs for life and is more likely to be seen in pairs or small groups, whereas Guillemot forms dense, noisy colonies. In flight, Razorbill appears more compact with rapid wingbeats and a longer tail, while Guillemot looks sleeker and flies with a straighter profile. Both dive for fish, but Razorbill can dive deeper (up to 120 m) and often spreads out to feed, while Guillemot is also a deep diver but typically carries only one fish at a time in its bill. Both species feed chicks with single fish, but Razorbill may deliver more frequent, smaller loads.

Both species breed on rocky cliffs and islands in the North Atlantic. Razorbill often chooses more concealed nest sites in crevices or under rocks, while Guillemot nests in dense aggregations on open cliff ledges. At sea, both are pelagic outside the breeding season and may be seen together in mixed flocks, but Razorbill is generally less numerous and more coastal in some areas. There is considerable overlap in habitat, so context alone is not diagnostic.

No diagnostic differences are described in the sources for field separation. Guillemot is noted for a loud, growling "murrrr" call at colonies, while Razorbill's calls are not described here. Voice is not a reliable separator based on these notes.