Bird of the Month: Cocos Booby
Written by Andy McCormick
Andy McCormick, Volunteer and Former Board President of Eastside Audubon
This South Pacific breeding seabird is now a separate species and has expanded its range northward along the Pacific coast and has become a more frequent visitor to Washington waters.
Cocos Booby resembles Brown Booby (S. leucogaster) from which it was split in 2024 by the American Ornithological Society (AOS). The females of the two species are similar. Both have brown head, chest, back, and tail, and a clean demarcation between the brown upper chest and the white lower chest and belly. Both have yellow skin on the face.
The adult male Cocos Booby differs and has a frosty white head and neck with blue facial skin. Both the male and female become whiter on the head as they age but the white on the male is more extensive and includes the neck. Photos, vocalizations, and videos of Cocos Booby can be found at the Macaulay Library.
cocos booby
Scientific Name: Sula brewsteri
Length: 28”
Wingspan: 57”
Weight: 2.4 lb (1100 g)
AOU Alpha Code: COBO
ISLAND NESTER
Cocos Boobies prefer rocky terrain for nesting, but the structure of nests varies considerably. In some locations nests are constructed with various forms of vegetation, in others, there may be little more than scrape, or eggs may be laid directly on a rock or hard ground.
Typically, two eggs are deposited in the nest and both parents will incubate by placing the eggs on their feet. The feet of boobies is highly vascular, and they are efficient in transferring heat to the eggs. Incubation can extend for 45 days. The hatchlings emerge from the egg naked and feeble and often cannot hold their head up. Natal down develops over the first few weeks and by 14 weeks juvenile plumage has been acquired. The first chick is more likely to survive than the second, which can sometimes become a victim of the first in what is known as siblicide (Mlodinow et al).
The breeding cycle of Cocos Booby is not fixed and varies by location. On islands near Costa Rica breeding occurs between October and February, but at the southern end of the breeding range in the Gulf of Panama and near Columbia, breeding occurs all year long (Mlodinow, et al).
DISTRIBUTION AND MANAGEMENT
Cocos Booby nests in the Gulf of California and on islands off the coast of Mexico, Costa Rica, Panama, and south to Columbia. However, in the 1990s the species expanded its range to Baja California and even Hawaii, which is unusual for this bird which has been considered a “near shore” bird (Mlodinow et al). In recent years, the nonbreeding range of the Cocos Booby has expanded north to Washington and west to parts of Japan.
BirdLife International has not evaluated the separate status of Cocos Booby, but Previous assessments which include it together with Brown Booby rated the birds as of Least Concern. There are no management plans in place. However, nesting was disturbed in the 19th and early 20th centuries by the collection of guano. Following the termination of guano harvesting, the booby colonies in the Pacific Ocean recovered by the 1960s and are now stable with a total population of 65,000 pairs at any time during the year. Maintaining healthy fish populations is an important goal for the current population of these piscivores.
A NOTE ON TAXONOMY
When originally described by Nathanial Stickney Goss, a colonel in the Kansas militia and amateur ornithologist, this bird was called Brewster’s Booby. Goss named it after his colleague William Brewster, and it was recognized as a separate species in 1889 by the AOS (Wikipedia). However, in 1944 the AOS lumped it under the Brown Booby until 2024 when, after genetic testing and documentation of the lack of hybridization when the two species breed in the same location (breeding sympatry), it was reinstated to full species status as Cocos Booby (Mlodinow et al). It is now named for Cocos Island of Costa Rica which sits atop the Cocos tectonic plate. Interestingly, the Cocos Plate abuts the Nazca Plate which is named for a region in southern Peru for which the Nazca Booby (S. granti) is also named.
Photo credit: Quinn Diaz, Pitta Nature Tours
References available upon request from amccormick@eastsideaudubon.org.