Masked Booby vs Nazca Booby

In short

The most reliable field mark between Masked and Nazca Booby is the bill color: Masked Booby adults have a yellowish bill, while Nazca Booby adults show a distinctly orange-yellow to orange-pink bill. Both species are large, white seabirds with blackish flight feathers and tail, but subtle differences in facial skin color and leg color can help. Juvenile Masked Boobies are more mottled brown above, while Nazca Booby chicks are white and fluffy, becoming grey as they fledge. Structural and behavioral differences are subtle, so focus on bare part colors and, for immatures, the pattern of the head and upperparts.

Identification tips

1. Bill color (adults): Masked Booby has a yellowish bill; Nazca Booby's bill is orange-yellow to pinkish, often brighter and more orange than Masked. This is the most reliable field mark in good light.
2. Facial skin: Both have a dark facial mask, but Masked Booby's is described as black or blue-black, while Nazca's is black. In practice, this is subtle and best used in combination with bill color.
3. Leg color: Masked Booby has yellow-orange or olive legs; Nazca Booby has grey legs. This is visible at close range or in photos.
4. Juvenile/immature upperparts: Juvenile Masked Booby is mottled or streaked brown above with a whitish neck collar; Nazca Booby chicks are white and fluffy, becoming grey as they fledge. If you see a brown-mottled young booby, Masked is more likely.
5. Underwing pattern: Both species have white underwings with dark flight feathers, not covered by white coverts. This is not a separator.
6. Tail color: Both have a dark (black or brown-black) tail in adults.
7. Sexual dimorphism: Both species are similar in plumage between sexes, but females are larger. Nazca Booby females have a slightly different bill color and squawk, while males whistle. Masked Booby females are heavier but not otherwise separable by plumage.
8. Iris color: Both have yellow irises as adults; not a separator.
9. Size: Masked Booby is slightly larger on average, but this is not reliable unless birds are side by side.
10. Habitat/geography: In the Galápagos and eastern Pacific islands, Nazca Booby is the default; elsewhere, Masked Booby is more likely. Always check local range.
11. Behavior: Both species show similar flight and feeding behavior. Courtship displays are not easily observed unless at a colony.
12. Vocalizations: Masked Booby males whistle, females honk; Nazca Booby males whistle, females squawk. Useful only at colonies.
13. Feet color in fledglings: Nazca Booby fledglings have grey feet, which may help if seen well.
14. Molt sequence: Not described as diagnostic between these two.
15. Pitfalls: Lighting can distort bill color, and immature Masked Boobies may appear paler-billed than adults. Always use multiple marks where possible.

Size

Masked Booby is the largest booby species, measuring 75–85 cm in length with a 160–170 cm wingspan and weighing 1.2–2.2 kg. Nazca Booby is also large and similar in structure, but the notes do not provide a direct measurement contrast. In practice, size is not a reliable field mark between these two unless birds are side by side, as both are robust, long-winged, and long-tailed.

Plumage

• Juveniles/Immatures: Juvenile Masked Booby is streaked or mottled grey-brown on the head and upperparts, with a whitish neck collar and dark brown wings. As they age, Masked Boobies develop a broader white collar and rump, with the head turning white by 14–15 months. Nazca Booby chicks are snow white and fluffy, turning grey as they fledge, with bill and feet also becoming grey. There is less detail on Nazca Booby juvenile upperwing pattern, but the main contrast is Masked Booby's mottled brown upperparts versus Nazca's cleaner white or greyish appearance in young birds.

• Adults: Both adults are bright white with black or dark brown flight feathers and tail. Masked Booby adults have a black or blue-black facial mask and yellowish bill, while Nazca Booby adults have a black facial mask but a distinctly orange-yellow to pinkish bill. Masked Booby legs are yellow-orange or olive, while Nazca Booby legs are grey. Both have yellow irises. Underwing pattern is similar: white with dark flight feathers not covered by white coverts.

Behaviour

Both species show typical sulid behavior: long, direct flights over open water, plunge-diving for fish, and territorial displays at breeding colonies. Masked Booby males display with a 'V'-shaped wing posture and descending whistles, while Nazca Booby courtship and territoriality are similar but not described in detail. In the field, behavior is not a strong separator unless vocalizations are heard or detailed displays are observed.

Habitat

Masked Booby is widespread across tropical oceans, while Nazca Booby is native to the eastern Pacific, especially the Galápagos and nearby islands. Both nest on open ground, often near cliffs. In the Galápagos and eastern Pacific, Nazca Booby is the expected species; elsewhere, Masked Booby is more likely. Habitat alone is not diagnostic if both species are possible.

Voice

Masked Booby is usually silent at sea but noisy at colonies: males give a descending whistle, females a loud honk. Nazca Booby males whistle and females squawk. These differences are subtle and mostly useful at breeding sites, not at sea or in flight.