Brown Boobies nest in colonies on predator-free tropical islands, especially coral atolls. They sometimes nest on hillsides but more often select sandy beaches or rocky cliffs without trees. When not nesting, they are birds of the open ocean, sometimes resting on the water when far from land but often returning to islets or manmade perches (buoys, channel markers, jetties) to rest. In the ocean, like many seabirds, Brown Boobies forage in greatest numbers where prey is concentrated, whether near the convergence of two currents or an area of upwelling. Because their prey is mobile, these birds often follow fish for long distances, even when foraging to provision chicks. Birds that disperse into inland areas forage in freshwater where prey is available and visible.Back to top
Brown Boobies eat mainly fish and squid. They are acrobatic birds, particularly for their size, able to soar when searching for prey and then pivot rapidly to dive. They dive from various heights and at various angles, using their narrow wings and long tail to change direction, speed, and angle as they dive. Often, they dive at a shallow angle (“skim-plunging”) and make several dives in rapid succession. They are fairly good swimmers below the water, using both feet and wings for propulsion. Brown Boobies usually finish swallowing prey as they reach the water’s surface. They have a varied diet that includes shrimp, prawns, squid, anchovy, sardine, flying fish, halfbeak, mullet, mackerel, sea catfish, queenfish, garfish, goatfish, squirrelfish, parrotfish, and various types of flatfish and blennies. Brown Boobies feed alone, in flocks, and in mixed-species flocks with terns, shearwaters, pelicans, or other species of booby. Like Red-footed Boobies, Browns sometimes fly alongside or in front of moving boats or ships, capturing flying fish and other prey fleeing the vessel. They also follow schools of feeding fish such as tuna, which often force smaller prey toward the water’s surface. They are less likely to perch on moving ships, as Red-footed Boobies often do.Back to top
Males usually select the nest site, a patch of flat terrain with very little or no vegetation.
Both male and female arrange whatever items are available in their environment, such as branches, grass, feathers, bones of dead birds, or trash into a rough nest that varies in width from 12 to 18 inches, with interior cup about 7 inches across and 2.5 inches deep. Some nests involve only a few pieces of vegetation.
|Clutch Size:||1-3 eggs|
|Number of Broods:||1 brood|
|Egg Length:||2.3-2.4 in (5.86-6.06 cm)|
|Egg Width:||1.6-1.6 in (3.93-4.09 cm)|
|Incubation Period:||42-45 days|
|Egg Description:||Pale bluish or greenish, with white outer layer.|
|Condition at Hatching:||Blind and helpless.|
The male selects a nesting territory from the air, then checks the site by landing and performing a display called “parading,” in which he holds the tail up and lists from side to side while walking with exaggerated steps. When a female appears, the male begins displaying, thrusting his bill skyward and calling. On the ground, with the female, the male also performs many stylized displays involving bill and wing movements, as well as bowing. Paired birds also preen each other with their bills (a behavior known as “mutual allopreening”). Mating occurs when the pair begins to construct the nest together and the female begins a display that looks like nest-weaving. Male and female share incubation and chick-rearing duties and greet the returning mate by waving the head, touching bills, and bowing. In some colonies, about half of returning birds re-partner with their mate of the previous breeding season and occupy the same nest site. Both sexes defend the small nest territory, first using threat displays such as calling, pecking at the ground, and bill-jabbing but then sometimes fighting, using both bill and wings.Back to top
Brown Booby populations declined greatly in historic times and have continued to decline more recently. The species’ current population is likely 90% lower than 100 years ago. Partners in Flight estimates a global breeding population of under 500,000 (the great majority of which occur outside North America). Partners in Flight rates the species a 12 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. The International Union for the Conservation of Nature (IUCN) notes a declining population but gives the species a ranking of Least Concern. Like most island-breeding seabirds, Brown Boobies are very vulnerable to introduced predators. They have disappeared from historical breeding islands now colonized by humans, along with introduced rats, cats, pigs, and other animals. Disturbance of colonies by boaters also has a negative impact on breeding success.Back to top
Kushlan, J. A., M. J. Steinkamp, K. C. Parsons, J. Capp, M. A. Cruz, M. Coulter, I. Davidson, L. Dickson, N. Edelson, R. Elliott, R. M. Erwin, S. Hatch, S. Kress, R. Milko, S. Miller, K. Mills, R. Paul, R. Phillips, J. E. Saliva, W. Sydeman, J. Trapp, J. Wheeler and K. Wohl (2002). Waterbird conservation for the Americas: The North American waterbird conservation plan, version 1. Washington, DC, USA.
Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.
Partners in Flight (2017). Avian Conservation Assessment Database. 2017.
Schreiber, Elizabeth A. and R. L. Norton. (2002). Brown Booby (Sula leucogaster), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.
Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.