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The most reliable field mark to separate Long-billed Murrelet from Spectacled Guillemot is the presence of bold white facial 'spectacles' on the guillemot, visible in all plumages, versus the plain face and subtle white eye ring of the murrelet. Juvenile and non-breeding birds of both species show white underparts, but the guillemot is much larger, heavier-bodied, and has striking red legs. In breeding plumage, the murrelet is brown and finely scaled with a pale throat, while the guillemot is sooty-black with its trademark spectacles. Bill shape and size, as well as overall bulk, are also helpful when birds are distant or in poor light.

1. White facial 'spectacles' (Spectacled Guillemot): The bold white eye-ring and connecting mark around the eye is diagnostic for all ages and seasons of Spectacled Guillemot. Long-billed Murrelet never shows this, only a faint white eye ring at best.
2. Bill shape and length: Long-billed Murrelet has a noticeably slender, longer bill relative to head size. Spectacled Guillemot's bill is black and straight but appears shorter and stubbier in proportion to its much larger head.
3. Leg color: Spectacled Guillemot has bright red legs, often visible when perched or taking off. Long-billed Murrelet's legs are dark and never red.
4. Size and bulk: Spectacled Guillemot is much larger and heavier-bodied (495 g vs. much less for murrelet), with a longer body and more robust profile. Murrelet is small and compact.
5. Breeding plumage: Long-billed Murrelet is brown with pale feather edges (scaly look) and a pale throat. Spectacled Guillemot is sooty-black with white spectacles and red legs; underparts are not scaly but solid dark.
6. Non-breeding plumage: Both have white underparts, but murrelet is white below with a black crown and nape, and a plain face with a white eye ring. Guillemot is white below but always with white spectacles and pale grey-brown tips on the underparts.
7. Transitional plumage: Spectacled Guillemot in transition shows white scaling on the underparts but retains the spectacles. Murrelet never shows spectacles or strong scaling in non-breeding plumage.
8. Juvenile plumage: Juvenile Spectacled Guillemots are separable from adults, but always show the spectacles. Juvenile murrelets resemble non-breeding adults but lack any facial spectacles.
9. Wing and flight pattern: No distinctive upperwing or flight pattern marks are noted for either species, so rely on facial and structural features in flight.
10. Habitat context: If the bird is seen inland or far from the coast, Long-billed Murrelet is much more likely, as Spectacled Guillemot is strictly coastal.
11. Behavior: Spectacled Guillemot may show red legs in flight or when perched on rocks. Murrelet never shows this.
12. Crown and nape: Murrelet has a black crown and nape contrasting with white underparts in non-breeding plumage; guillemot is more uniformly dark above.
13. Throat color in breeding: Long-billed Murrelet has a pale throat in breeding plumage, absent in Spectacled Guillemot.
14. Scaling and feather edges: Murrelet shows fine pale feather edges (scaling) in breeding plumage; guillemot does not.
15. Pitfalls: In poor light or at distance, size and bill shape may be hard to judge, but the presence or absence of spectacles is reliable if the head is visible.

Spectacled Guillemot is significantly larger and heavier than Long-billed Murrelet. The guillemot reaches about 38 cm in length and weighs around 495 g [wikidata], making it a robust, chunky auk. In contrast, Long-billed Murrelet is smaller at about 25 cm long and is about 20% lighter than Marbled Murrelet (which itself is much lighter than the guillemot) [wikipedia]. The guillemot's greater size is usually obvious when seen side by side or with familiar auks for scale.

Juvenile and non-breeding Long-billed Murrelets are white below with a black crown, nape, back, and wings; they show a white eye ring but otherwise a plain face. Spectacled Guillemot juveniles and non-breeding adults also have white underparts, but always show bold white 'spectacles' around the eye, and their underparts are tipped pale grey-brown. In breeding plumage, Long-billed Murrelet is mostly brown with pale feather edges (giving a scaly look) and a pale throat, while Spectacled Guillemot is sooty-black with white spectacles and red legs. Transitional guillemots show white scaling on the underparts, but the spectacles remain obvious. Both species have black bills, but the murrelet's is slender and longer relative to head size.

Both species are diving auks and may be seen swimming low in the water or diving for prey. Spectacled Guillemot's larger size and heavier build make its movements appear more robust and less agile than the smaller, more delicate murrelet. The guillemot's bright red legs may be visible when it sits on rocks or takes off from the water. Long-billed Murrelet is more likely to be seen alone or in small groups, sometimes far inland as a vagrant, while Spectacled Guillemot is more strictly coastal.

Long-billed Murrelet breeds inland in old-growth forests but is usually seen at sea or, rarely, as a vagrant on lakes and rivers far from the ocean. Spectacled Guillemot is a coastal species, rarely straying far from rocky marine shores. In winter, murrelets may appear in sheltered bays or even inland, while guillemots remain tied to the coast. If seen far from saltwater, a murrelet is much more likely.