Black-billed Cuckoo vs Steller's Sea-Eagle

In short

The most reliable way to separate Black-billed Cuckoo from Steller's Sea-Eagle is by overall size and structure: Steller's is a massive, heavy-bodied raptor with a huge yellow bill, while Black-billed Cuckoo is a slender, medium-sized songbird with a slim, black bill. In flight, Steller's Sea-Eagle shows broad, paddle-shaped wings with striking white panels (in adults), whereas Black-billed Cuckoo has long, narrow wings and a long, graduated brown tail. Juvenile Steller's Sea-Eagle is uniformly dark with a white-mottled tail, while juvenile Black-billed Cuckoo is drabber than adults but never shows the bulk or wing pattern of the eagle. Even at great distance, the difference in size and flight style is unmistakable.

Identification tips

1. Size: Steller's Sea-Eagle is gigantic, easily 10–20 times the mass of a Black-billed Cuckoo. If the bird appears eagle-sized, it's not a cuckoo.
2. Bill color and shape: Steller's has a massive, thick, yellow bill (blackish in juveniles); Black-billed Cuckoo has a slim, black, slightly downcurved bill.
3. Tail shape and pattern: Steller's adult tail is diamond-shaped and pure white; juveniles have a white tail with black mottling. Black-billed Cuckoo has a long, graduated brown tail with no white.
4. Wing pattern (adult): Steller's Sea-Eagle adults have bold white patches on the shoulders and underwing coverts, visible in flight and at rest. Black-billed Cuckoo's wings are plain brown above, sometimes with rufous in juveniles, but never show white panels.
5. Wing shape: Steller's wings are broad and paddle-shaped; Black-billed Cuckoo's are long, narrow, and pointed.
6. Flight style: Steller's soars and glides with slow, powerful wingbeats; Black-billed Cuckoo flies with rapid, direct wingbeats and rarely soars.
7. Orbital ring: Adult Black-billed Cuckoo has a narrow red orbital ring (yellow in juveniles); Steller's Sea-Eagle has yellow eyes as an adult, dark brown as a juvenile, but no colored orbital ring.
8. Leg and foot color: Steller's has yellow feet (whitish in juveniles); Black-billed Cuckoo's feet are not conspicuous and are zygodactylous (two toes forward, two back).
9. Plumage contrast (juvenile): Juvenile Steller's is uniform dark brown with some pale tail mottling; juvenile Black-billed Cuckoo is drabber brown, sometimes with rufous in the wing, but never as dark or bulky as the eagle.
10. Perched posture: Steller's perches upright, often exposed; Black-billed Cuckoo is more horizontal, often hidden in foliage.
11. Habitat: Steller's is found near large water bodies in northeastern Asia; Black-billed Cuckoo is in North American woodlands and thickets.
12. Context: If seen hunting fish or waterbirds, or soaring over water, it's Steller's; if foraging quietly in dense foliage, it's Black-billed Cuckoo.
13. Voice: Not diagnostic from the notes, but Steller's gives harsh eagle calls; Black-billed Cuckoo has a soft, repetitive "coo-coo-coo".
14. Downy young: Black-billed Cuckoo chicks have white, sparse down and complex mouth structures; Steller's chicks are silky white, turning brown-grey, but this is rarely observed in the field.
15. Seasonality: Steller's is a year-round resident or migrant in its Asian range; Black-billed Cuckoo is a summer breeder in North America.

Size

Steller's Sea-Eagle is among the world's largest raptors, with an average adult mass of about 6675 grams [wikidata], dwarfing the Black-billed Cuckoo, which is a slender, medium-sized songbird (precise mass not given, but orders of magnitude smaller). In the field, Steller's appears enormous, with a thick neck, massive head, and huge bill, while Black-billed Cuckoo is slight, with a long tail and delicate proportions. Even a distant eagle will appear much larger and more robust than any cuckoo.

Plumage

Juvenile and immature Steller's Sea-Eagles are mostly uniform dark brown with some pale mottling on the tail and occasional streaking on the head and neck; their tail is white with black mottling distally. Adults are strikingly pied, with blackish body, white shoulders, thighs, tail, and underwing coverts, and a huge yellow bill. In contrast, Black-billed Cuckoo adults are plain brown above, white below, with a long brown tail and black, slightly downcurved bill; they have a narrow red orbital ring (yellow in juveniles). Juvenile Black-billed Cuckoos are drabber, sometimes with rufous in the wing, but never show the bold white patches or massive bill of the eagle. In flight, the eagle's wing and tail pattern is diagnostic at any age, while the cuckoo remains plain and slim.

Behaviour

Steller's Sea-Eagle soars and glides on broad wings, often seen near large bodies of water or perched conspicuously in tall trees or cliffs. Its flight is heavy and powerful. Black-billed Cuckoo moves quietly through dense foliage, foraging for insects, and is rarely seen soaring; its flight is direct and low, with rapid wingbeats. The cuckoo is much more secretive and less likely to be seen in open areas.

Habitat

Steller's Sea-Eagle is restricted to coastal regions, large rivers, and lakes in northeastern Asia, often near open water where it hunts fish and waterbirds. Black-billed Cuckoo prefers dense woodlands, thickets, and forest edges across North America, especially where caterpillars are abundant. Their habitats do not overlap, and seeing both in the same context is essentially impossible.