How to identify the Brown Booby

To correctly identify the Brown Booby (Sula leucogaster), check each of these features. Birds can be tricky: habit, habitat, and call often confirm a sighting that a single photo cannot.

Size at a glance

Length

64-76 cm (25-30 in)

Wingspan

130-150 cm (51-59 in)

Weight

700-1500 g (1.5-3.3 lbs)

Physical features to check

1. 1

   Plumage

   Overall feather coloration and pattern.

   Adults have dark brown plumage on the head, neck, upperparts, and upper breast, sharply contrasting with pure white lower breast, belly, and underwing coverts. The flight feathers (primaries and secondaries) are dark brown. Juveniles are entirely dusky brown, gradually acquiring the adult pattern.

2. 2

   Beak shape

   Shape and color of the bill: a key field mark.

   Conical, pointed, and relatively stout. The bill color varies with age and sex: adult males typically have a bluish-gray bill base with a yellowish tip, while females have a more yellowish or greenish-yellow bill. Juveniles have a dusky bill.

3. 3

   Eye color

   Iris color and any distinctive eye markings.

   Yellowish-white to pale yellow in adults, darker in juveniles.

4. 4

   Leg color

   Leg and foot color.

   Dull yellow to greenish-yellow, with webbed feet.

5. 5

   Distinguishing features

   Standout features that separate this species.

   The sharp demarcation between the dark brown upperparts and white underparts is the most distinctive feature. The dark brown extends onto the upper breast, forming a 'hood' or 'vest' appearance. The pale bill and feet are also key.

6. 6

   Juvenile appearance

   How young birds differ from adults.

   Juveniles are uniformly dusky brown over their entire body, including the underparts, which are mottled with white. They lack the sharp contrast of adults. Their bill and feet are also darker, becoming paler with age.

7. 7

   Seasonal changes

   Plumage variation between breeding and non-breeding seasons.

   No significant seasonal plumage changes. Plumage remains consistent year-round once adult coloration is achieved.

8. 8

   Sexual dimorphism

   How males and females differ.

   Subtle. Females are generally slightly larger than males. The bill color can differ, with males often having a bluer tint at the base of the bill and females a more yellowish or greenish-yellow bill. The bare skin around the face (lores) is typically yellow in males and greenish-yellow in females.

Key field marks

• Plumage Pattern. Look for the sharp contrast between the dark brown head, neck, and upperparts, and the pure white lower breast and belly. The dark brown extends like a 'vest' onto the upper breast.
• Bill and Feet Color. Note the pale bill (yellowish to greenish-yellow, sometimes with a bluish base in males) and dull yellow to greenish-yellow webbed feet. This distinguishes it from Red-footed Booby (red feet) and Masked Booby (black flight feathers, black mask).
• Size and Shape. A medium-sized booby with a streamlined body, long pointed wings, and a wedge-shaped tail. Its flight is powerful and direct.
• Juvenile Identification. Young birds are entirely dusky brown, lacking the adult's white underparts. They will gradually acquire the adult pattern over several molts.
• Behavior. Observe its plunge-diving feeding technique. It dives headfirst into the water from a considerable height to catch fish.

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