Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
The global population is estimated to number >200,000 individuals (del Hoyo et al. 1992). National population sizes have been estimated at c.10,000-100,000 breeding pairs and c.1,000-10,000 individuals on migration in Taiwan and c.10,000-100,000 breeding pairs, c.1,000-10,000 individuals on migration and c.1,000-10,000 wintering individuals in Japan (Brazil 2009).
The population is suspected to be in decline owing to disturbance and unsustainable levels of exploitation.
The Brown Booby can be found throughout the pantropical oceans with few exceptions. Breeding sites include the Carribean, the Atlantic coasts of Brazil and Africa, oceanic islands off Madagascar, the Red Sea, northern Australia, many oceanic islands in the western and central Pacific, as well as off the coast of Mexico and Peru.
This species is strictly marine, generally feeding on inshore waters. Its diet is comprised mainly of flying-fish and squid, but also some halfbeak (Hemiramphu), mullet (Mugil) and anchovy (Engraulis). Prey is usually caught by plunge-diving and it can also snatch prey off the surface of water. Kleptoparasitism has been observed, mostly by females. Breeding is seasonal in some areas, but elsewhere it breeds opportunistically or more or less continuously. Nests are built on the ground in the midst of vegetation on rocky islands or coral atolls. Individuals form colonies that are usually smaller than those of other Sula species (del Hoyo et al. 1992).
Brown Boobies are hunted for use as food and bait in some areas of their range (Carboneras et al. 2018); however, this is thought to have negligible effects on their population. They are also highly sensitive to human disturbance, with the presence of humans 10-20 m from the nest enough to cause the birds to leave (Borsa et al. 2010). Humans also indirectly affect the species; Verlis et al. (2014) found that most S. leucogaster nests surveyed contain at least one item of marine debris, with black and green items positively selected for as a nesting material. Further to this, Lavers et al. (2013) found that around 30% of the debris poses some entanglement risk to the birds. This is currently unlikely to represent a significant source of injury and mortality (Lavers et al. 2013), but is likely to increase due to the long term persistence of marine debris. In the past, organochemical pollution has caused a significant reduction in reproductive success due to the thinning of eggshells, often caused by DDT use in agricultural areas as well as, in one case, mosquito control for a tourist development. Currently, eggshell thickness varies in relation to individual characteristics and/or feeding sources, as opposed to actual exposure and, where records are available, eggshell thickness has recovered to pre-DDT conditions (Mellink et al. 2009). Introduced rats are also thought to impact on the species through reduction of reproductive success on invaded islands (Pitman et al. 2005. Carboneras et al. 2018).
Text account compilers
Fjagesund, T., Calvert, R., Butchart, S., Martin, R., Miller, E., Ekstrom, J.
BirdLife International (2020) Species factsheet: Sula leucogaster. Downloaded from http://www.birdlife.org on 11/08/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 11/08/2020.