Midway Atoll National Wildlife Refuge
Pacific Region
 

Birds of Midway Atoll

Masked Booby / Sula dactylatra

photo of masked booby pair

Description

Largest of the boobies. Adult birds are white with dark brown or black flight and tail feathers. Bill color is yellow and feet are yellowish gray. Facial skin is gray-black. Sexes are similar in appearance. Juveniles are generally grayish brown with white underparts. Adult plumage is attained by the third year. Length: 74-86cm (29-34 in); wingspan: 152cm (62 in); life span: max known age 20 years

Masked Booby
Photo credit: Nancy Hoffman/USFWS
  

Midway Population

Masked Boobies nest in small numbers at Midway and no historical information indicates that they were previously more numerous. There are generally less than five nesting attempts per year.

Feeding Habits

Feeds by plunging head first into the water from the air, sometimes diving from as high as 30m. Feeding areas are usually many miles from land. Feeds primarily on fish and squid. Feeding birds are often associated with schools of large predatory fish, such as tuna. Feeding generally occurs during daylight hours.

Breeding

Breeding season occurs between February and August. Breeds colonially. Nests are located on the ground and consist simply of a shallow depression. Begins breeding at four years of age. Monogamous. Two chalky white eggs are layed. Lacks brood patch and instead incubates with feet. Parents share incubation duties. Average incubation shift is 9.5 hours. Incubation period averages 43 days. Siblicide exhibited by chicks. The first egg hatches four to nine days before the second. The first chick to hatch ejects the second chick from the nest shortly after it emerges from the egg. Chick feeding occurs once or twice a day. Fledging occurs 109-151 days after hatching. Post fledging care and feeding continues for one to two months.
Last updated: March 22, 2010