Brown boobies, Sula leucogaster, are common residents of Puerto Rico and the Virgin Islands. The distribution is described as pantropical, between latitudes 30 degrees North and 30 degrees South, though it extends to about 34 degrees South in the central Pacific. Brown boobies occur in the Caribbean Sea, Red Sea, and the Atlantic, Pacific, and Indian Oceans. They also inhabit seas north of Australia.  The subspecies Sula leucogaster brewslei and Sula leucogaster etesiaca live along the Pacific coast of Mexico and the Pacific coast of Central and South America, respectively.

Although brown boobies may have historically inhabited the Florida Keys, clear evidence is lacking. Tropical storms occasionally blow individuals of this species well outside of their typical geographic boundaries - such an example is the October 2008 observation of a lone Sula leucogaster at Claytor Lake (Pulaski County) in southwestern (interior) Virginia.

Biogeographic Regions: neotropical (Native ); australian (Native ); oceanic islands (Native ); indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native )

occurs (regularly, as a native taxon) in multiple nations

United States

Origin: Native

Regularity: Regularly occurring

Currently: Unknown/Undetermined

Confidence: Confident

Type of Residency: Breeding

Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) BREEDS: islands from Yucatan, Florida Keys (formerly), and Bahamas (Sprunt 1984) south through Antilles to northern South America; Cape Verde Islands to Brazil; Pacific: Gulf of California to Colombia, Hawaii (Kure to Niihau and Moku Manu) to Australia; Indian Ocean. Ranges at sea generally in breeding range.

Adult feather coloration is similar between sexes. There are small differences among males of the subspecies S. leucogaster brewslei and S. leucogaster etesiaca.  Generally brown boobies have dark brown heads, necks, and backs with bright white underbellies. A sharp line separates the lower breast where the brown fades to white. The under wings are mostly white except for a variable dark bar that is relatively narrow and extends from armpit to carpal joints.  The subspecies mentioned earlier tend to have a light gray to white head that gradually turns brown at the neck. Overall their necks and upper backs are darker than other Sula leucogaster subspecies. Juveniles have no documented differences between sexes or subspecies. But are generally a brownish color on their head, neck, and back with a noticeable band across the lower breast which leads to the mottled brown and whitish underparts (underparts gradually attain full white color over first 2 years). The under wing-coverts (feathers that cover the bases of the quill feathers) are generally gray, which contrast significantly with the rest of the under wing. Adult soft part colors vary slightly between males and females, with no documented variation among subspecies. Males have gray-blue to steely-blue skin around the eyes and yellow to bright yellow skin at base of mandible. Females are much the same, except for face skin that is always bright yellow. In both genders the bills and feet range in color from bright yellow, bluish yellow, greenish yellow, light-pink, and gray.

Overall distinguishing booby species isn’t difficult, but juvenile red-footed boobies (Sula sula) look remarkably similar to juvenile brown boobies.  Overall length ranges from 64 to 85 cm, with females on average being a few cm longer. Their wingspan is in the range of 132 to 155 cm. There is a slight difference in weight between sexes, with females usually weighing about 300 g heavier. The ranges for males are 950 to 1700 g and for females is 1000 to 1800 g.

Range mass: 950 to 1800 g.

Average mass: 1300 g.

Range length: 64 to 85 cm.

Range wingspan: 132 to 155 cm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: female larger; sexes colored or patterned differently

Length: 76 cm

• Terrestrial
• Marine

Brown boobies use coral atolls and volcanic stack islands for nesting in tropical or subtropical waters. When faced with little or no competition for space, they prefer wide open spaces at sea level. They have, however, been found on cliffs and hillsides.

Range elevation: 0 to 15 m.

Habitat Regions: tropical ; saltwater or marine

Aquatic Biomes: coastal

Comments: Pelagic, but commonly observed from shore; often rests on buoys or rocky cliffs (Raffaele 1983). Usually does not range far out to sea; most numerous around small offshore islands (Stiles and Skutch 1989). Nests on ground or on cliff on island; nests in open area or vegetated site (Kepler 1978).

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: Yes. At least some populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

Generally remains near breeding island through year.

Brown boobies eat mainly fish. They consume most species of fish that are from 5 to 40 cm in length. Observed prey include flying fish (Exocoetus species), goat fish (Mullidae), squirrelfish (Sargocentron diadema), mackerel (Rastrelliger kanagurta), and ommastrephid squid (Ommastrephidae).

To acquire food, brown boobies use mainly plunge-diving. This is where they rise to 10 to 12 m above the water to search for prey before angling the body in such a way that it can enter the water as smoothly as possible. This position changes throughout the dive, starting with the initial descent. At the time of the descent the wings are folded in tightly next to the body, then thrust straight out over their back (prior to entering water) until their wings are touching over the center of their back. Depending on their altitude prior to diving, they are able to submerge themselves up to 2 m deep. They have also been known to pursue prey underwater using a combination of feet and wing motions.

Along with plunge-diving, some fledglings (about 1 in 100) and some adults (1 in 500) practice kleptoparasitism, where they steal prey from other seabirds. For example, brown boobies have been observed stealing prey from great frigatebirds (Fregata minor) as they transfer food to their young. It is thought that fledgling frigatebird calls for food are what attracts boobies.

Animal Foods: fish; mollusks

Primary Diet: carnivore (Piscivore , Molluscivore )

Comments: Food (fishes, squid) is obtained by diving from air to water surface; feeds more commonly in coastal waters (Terres 1980). Chicks are fed well-digested food by regurgitation, later given whole fish.

Richardson (2006) report that the presence of seabird nesting colonies (including Sula leucogaster on islands increases the productivity of certain trees (like mangroves) more than islands without nesting colonies.  Recently, this species, Sula leucogaster, has been found to be one of the many contributing factors (along with all other nesting seabird colonies) to eutrophication of the surrounding waters of the islands they inhabit. Their nutrient-rich feces causes lower species diversity and greater dominance of mostly epiphytic biota but also macro-algae and phytoplankton.  The rapid growth of the epiphytes blanket the sea grass beds and eventually causes them to die from lack of sunlight. Any organism in the area that depended on the sea grass that is unable to accommodate the change will soon die after it. However, no studies have examined direct effects of eutrophication on the nesting sea birds from presumed declines in fish in the surrounding water.

Brown boobies are also host to the parasite Babesia poelea. Work and Rameyer's (1997) research pointed to the possibility that avian piroplasms may be species specific and be transmitted by argasid ticks (Carios capensis).

Ecosystem Impact: biodegradation ; parasite

Commensal/Parasitic Species:

• argasid ticks (Babesia poelea)

Sally lightfoot crabs (Grapsus grapsus) are the only known predators of brown booby hatchlings. While these crabs mostly forage around nesting sites picking up feathers, egg shells, dead chicks, and dry bird excrement, they will on very rare occasions prey on recently hatched chicks that have been expelled from the nest. In these cases, adult birds do nothing to protect their young from the crabs and the crabs have never been seen going into the nests after a solitary chick. Brown booby young may also be preyed on by larger seabirds and nesting colonies may be threatened by introduced predators, such as rats, cats, and pigs.

Known Predators:

• Sally Lightfoot crabs (Grapsus grapsus)

Often rather gregarious (Hilty and Brown 1986). Feeds singly or in small groups but many may congregate at schools of fishes or around fishing boats (Stiles and Skutch 1989).

There has only been one study on the various calls of brown boobies. The results of that study suggest vocalizations are used for several different functions, including greeting mates, aggression, and chicks begging for food. Colonies as a whole are generally not noisy, as more communication is done through behavior. Males and females use vocalizations in different ways. Males emit a series of soft whistles while females are responsible for harsh quacking or honking. These sounds become more pronounced whenever they are faced with possible intrusion from another bird. Males calls become harsher and are held for longer while females emit a sound known as a “roar-call”. This is an emphatic shout that is intended to warn the intruder to stay away from the nest. When these birds greet their mates as they return from hunting, they use their typical sounds. However, these calls are made softly while still far out and tend to get louder as they approach the nest.

Hatchlings have their own range of sounds, starting with peeping while still in the egg. While very young there seems to be no discernible pattern to the food call, it is just a range of the noises they are able to make. As they get older it takes on a loud, raspy, repetitive craa-craaaa.

Most communication is accomplished through behaviors rather than sounds. An example of aggressive behavior will be seen mostly when claiming or maintaining breed sites. Aggressive behaviors are directed towards other birds as well as inanimate objects (shrubs or rocks). When performing aggressive behaviors against other birds, brown boobies hop towards the intruder with their neck and head stretched forward. When they get closer they bow and usually emit a honk. If the intruder does not back down the defending booby will perform a more exaggerated bow. If the intruder still doesn’t back down, then the territory owner may rattle its bill towards the intruder. This can escalate to bill jabbing with a downward slap of the wings. Sometimes the defender will grab onto others bill or neck.

Only males have been observed performing mating behaviors, which include sky-pointing and parading. Sky-pointing is done when a female first approaches. The male will stand up and throw his head into the air with neck stretched out as far as it can go. This is usually accompanied by a unique whistle. This ritual is slightly different from the other booby species as it doesn’t involve a use of the wings. Parading is really just an exaggerated form of walking. The body posture is unusually erect with an exaggerated sway. Other behaviors that are thought to reinforce pair bonding are mutual preening, bill touching, and bowing. Mutual preening is where each pair bond will clean the other’s feathers. Bill touching is done whenever mates return to their own territory after bowing. It has been noted that the mate reentering the territory will make their arrival call and then perform a slight bow (similar to bow described in aggressive behavior) unless the bird reentering is flying in and its partner is on a nest. In which case that bird will perform a seated bow. This action is thought to be a form of greeting.

Communication Channels: visual ; tactile ; acoustic

Comments: Usually feeds in daytime (Terres 1980).

Adult mortality per year has been reported to be between 3.3 and 7.7%. The maximum known lifespan in the wild is 27.2 years. There have been no studies to document average lifespans in the wild or captivity.

Range lifespan

Status: wild: 27.2 (high) years.

Brown boobies are monogamous. Males are the only gender observed performing mating behaviors, including sky-pointing and parading. In sky-pointing, when a female first approaches, the male will stand up and throw his head into the air with neck stretched out as far as it can go. This is usually accompanied by a unique whistle. This ritual is slightly different from the other booby species, as it doesn’t involve a use of the wings. Parading is just an exaggerated form of walking, in which body posture is unusually erect with an exaggerated sway.  Other behaviors that are thought to reinforce pair bonding are mutual preening, bill touching, and bowing. Mutual preening is where each pair bond will clean the other’s feathers. Bill touching is done whenever mates return to their own territory after bowing. It has been noted that the mate reentering the territory will make their arrival call and then perform a slight bow (similar to the bow described in aggressive behavior) unless the bird reentering is flying in and its partner is on a nest, in which case that bird will perform a seated bow. This is thought to be a form of greeting.

Mating System: monogamous

Brown booby breeding season is dependent upon food availability. Events such as El Nino can drastically shift normal breeding season for a few years. In the Caribbean and east Pacific, peak breeding months are December to February. In the central Pacific, the breeding season lasts from December to March, but it's not uncommon to find them breeding year round. In Hawaii brown boobies breed from March to May. After eggs are laid, parents take turns incubating for 42 days.  Usually brown boobies lay 2 eggs but only raise one chick past the fledgling stage. It is thought that having a second egg is for insurance purposes, in case one egg doesn't make it. The first chick hatches about 2 to 4 days before the second. This chick will then push the other young from the nest with no interference from either parent. The expelled chick will usually die from heat exposure, lack of food, or predation.  The surviving chick will continue to be cared for in the pre-fledgling stage for close to 100 days, during which time both parents continue to feed and protect it.  In the post-fledgling stage, it is not uncommon for young birds to go out on their own to learn how to hunt and socialize and then return to their parents nest to be fed. This sort of behavior has been observed up to a year after reaching post-fledgling stage, but it typically lasts 50 days. This variability is thought to be the result of food availability.

Breeding interval: Brown boobies breed once yearly.

Breeding season: Brown booby breeding times vary across the world, with most breeding occuring from December to March, although breeding may be year-round in some areas.

Range eggs per season: 1 to 3.

Range time to hatching: 40 to 47 days.

Average time to hatching: 42 days.

Range fledging age: 96 to 120 days.

Average fledging age: 100 days.

Range time to independence: 42 to 365 days.

Average time to independence: 50 days.

Range age at sexual or reproductive maturity (female): 240 (low) days.

Average age at sexual or reproductive maturity (female): 1278 days.

Range age at sexual or reproductive maturity (male): 240 (low) days.

Average age at sexual or reproductive maturity (male): 1278 days.

Key Reproductive Features: year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization (External )

Both male and female brown boobies incubate the eggs. Time on the nest is split evenly, with varying amount of hours per sitting. Once eggs hatch, both parents participate in feeding the young. There are no records about which gender feeds more often. When chick gets older and bigger the parents might roost away from nest but will often return to defend their nest territory and chick. This pre-fledgling care can continue up to 100 days.  Post-fledgling care can vary from 42 days up to 259 days with rare occurrences of juveniles returning to parents nest up to a year after to beg for food. This factor is thought to be controlled by local fishing conditions. If food is plentiful and easy for juveniles to catch and learn how to hunt efficiently, then it will be shorter.  There have been no documented cases of adult brown boobies showing post-fledgling chicks how to dive or hunt. However, in an experiment by Yoda et al. (2007), hand-raised chicks were often much slower to reach peak diving levels than chicks raised by parents.

Parental Investment: altricial ; male parental care ; female parental care ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female); extended period of juvenile learning

In West Indies and Hawaii, breeding season is long, may vary annually (Kepler 1978). Egg laying occurs early December-late April in Panama (Hilty and Brown 1986), nesting probably peaks September-April in Costa Rica (Stiles and Skutch 1989). Clutch size usually is 2; normally 1 chick survives. Incubation, by both sexes in turn, lasts 40-43 days. Young can fly by about 105 days. Not highly colonial.

Resident breeder.

Brown boobies are not considered endangered at this time and no programs promoting their conservation are in progress. Brown booby populations are considered globally stable currently.

US Migratory Bird Act: no special status

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

IUCN Red List of Threatened Species: least concern

United States

Rounded National Status Rank: NHB - Possibly Extirpated

Rounded Global Status Rank: G5 - Secure

Reasons: Still widespread, but with significant historical declines.

Global Short Term Trend: Relatively stable to decline of 30%

Comments: Most colonies not visited in past 3 to 5 decades, so rangewide trend information not available (Schreiber and Norton 2002). Caribbean population has declined over past several decades (van Halewyn and Norton 1984). Extirpated from four sites in Bahamas; from Pedro Cays, Jamaica; Desecheo, Puerto Rico; Grenadines and Grenada; and possibly 3-5 other sites in West Indies (Schreiber 2000). Hawaii population apparently stable (Harrison et al. 1984).

Global Long Term Trend: Decline of 30-50%

Comments: Populations may be only 1-10 percent of those before arrival of humans (Steadman 1989; Steadman et al. 1984, 1990).

There are no known adverse effects of Sula leucogaster on humans.

According to Spennemann (1999), at the turn of the 20th century brown booby feathers were in high demand from the European and American fashion industry. These feathers were used to adorn women's hats and the "take" of such feathers led to 2 million bird deaths (not all Sula leucogaster). This demand has since died down and regulations are in place to prevent over-harvesting for the fashion industry or any other products. Several countries (e.g., Japan and Madagascar) are turning to ecotourism as a source of income and have set aside several islands with very strict regulations to preserve the natural environment. Ichiki (2003) reports that the draw of ecotourism comes from tourists wanting to see animals in their natural environment. Conveniently, brown boobies nest on these protected islands (e.g., Ogasawara Islands, Japan) and its presence be another selling point for tourists with an interest in bird-watching.

Positive Impacts: body parts are source of valuable material; ecotourism