Fr: Fou masqué
All: Maskentölpel
Esp: Alcatraz Enmascarado
Ita: Sula mscherata
Nd: Maskergent
Sd: Masksula
Port: Atobá-grande


Otto Plantema
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Text by Nicole Bouglouan


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Masked Booby
Sula dactylatra

Suliformes Order – Sulidae Family

The Masked Booby is the largest species of the family Sulidae. It was described first by the French naturalist René-Primevère Lesson, in 1831.
This species is strictly marine and usually occurs over pelagic waters, preferring deeper waters than other boobies.

Length: 81-92 cm
Wingspan: 152 cm
Weight: 1450 g

The adult has white plumage overall, except the brownish-black flight-feathers and rectrices. However, the bases of remiges and central rectrices are silvery-grey.
On the white head, the facial skin is black around the eyes, on the lores, and around the bill base, extending down to chin and upper throat. The skin is less dark on chin and lores.
The thick, long bill is bright yellow in males, with variable olive wash. It can be sometimes light horn with grey tinge at base. The eyes are golden yellow to amber. Legs and webbed feet are yellow-orange, sometimes olive.

Both sexes are similar but the female is slightly larger and she has dull greenish-yellow bill.
The juvenile has dark brown to blackish-brown head, neck and upperparts, including wings and tail. We can see a white hind collar between lower neck and mantle. The underparts are white. The eyes are grey at first. It can breed at 2-3 years old.

The Pacific races are larger than nominate, and vary in colour of bare parts. The nominate race (here described) is the smallest, and there is an increasing cline to the east.
The race “granti” is the largest. The Nazca Booby (Sula granti) is now a full species.  

S.d. dactylatra (here described) breeds in Caribbean and off N coast of S America (S to E Brazil), on Ascension and St Helena Islands in S Atlantic Ocean, the latter recently colonized.

S.d. melanops is found on islands of S Red Sea and W Indian Ocean. This race has duller bare parts with olivaceous yellow-green bill, dark olive-grey to khaki-olive legs and feet.

S.d. personata is found in E Indian Ocean, and on numerous islands in W and C Pacific Ocean. This one has yellow bill with greenish or bluish base, orange-yellow to amber eyes, bluish-grey to blackish legs with more brownish webs.

S.d. fullagari breeds in N Tasman Sea. It is similar to “personata”, but it has brownish to dark brown eyes, straw-yellow to greenish-yellow bill and dark khaki-grey legs.

The Masked Booby is strictly marine and pelagic. It forages in deeper waters than other Sulidae, and far from the coasts.
When on land for nesting, they are usually found on small, flat islands for easy take-off. They nest near cliff ledges or gentle slopes for the same reason.

The Masked Booby is silent at sea, but the breeding colonies are quite noisy. We can hear high-mitched whistling greeting calls. Other sounds such as hissing and quacking notes are usual on the breeding grounds. The female gives a more “honky” sounding call.

The Masked Booby feeds on flying-fish of up to 28 cm long. This species takes larger preys than other Sulidae. It also takes some squid.
It performs the typical plunge-diving from moderate to great heights, up to 30 metres. It may forage alone or in small groups. It is often victim of piracy by frigatebirds.
During the breeding season, they forage closer to land while feeding the chicks. Outside this period, they forage around 65 km from the coasts.

During the breeding season, the Masked Booby performs ritual displays. The male attracts a female by using some postures with neck and head outstretched. They also present small stones or feathers as gifts to one another.
The copulation occurs after a slow walk. The female starts to incubate as soon as the first egg is laid. The second one will be laid nine days later.

The Masked Booby adult remains in the vicinity of the colony all year round. But the young birds and some adults too, usually disperse over long distance, up to 1000 kilometres from a land.  
While flying, it performs several deep wingbeats interspersed with glides.

The breeding season depends on the range.
The Masked Booby nests in colonies established on the ground, near cliff ledge or slope for easier take-off and landing. The nests are 2-3 metres apart, but sometimes up to 100 metres. This is a shallow depression on the ground, surrounded by a circle of pebbles, debris or accumulated excreta.

The female lays 2 pale blue eggs, but only one chick is raised. Both parents incubate under their webbed feet during 40-49 days. At hatching, the chicks have sparse white down. They are fed by regurgitation by both adults. Usually, the older chick attacks and ejects the younger from the nest. The remaining chick fledges about 120 days after hatching. It is still fed at nest by parents for one or two months more.

The Masked Booby is fairly common in its range, but widely dispersed throughout tropical waters, with an estimated population of several hundred thousand individuals.
The race “melanops” is declining rapidly, and the remaining birds are threatened.
The breeding colonies are often subject to exploitation by humans who take the eggs and kill the adults. Introduced predators and tourism development are important threats too.
But currently, the Masked Booby is not globally threatened.