LC
Masked Booby Sula dactylatra



Justification

Justification of Red List Category
This species has a very 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 has not been quantified, but it is not believed to 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.

Population justification
The global population size has not been quantified, but this species is described as 'fairly common' (Stotz et al. 1996).

Trend justification
The population is suspected to be in decline owing to predation by invasive species and unsustainable levels of exploitation.

Distribution and population

This species ranges widely in tropical waters, being found in every ocean on or off nearly every coast except the eastern Atlantic Ocean, northern Indian Ocean and the central-eastern Pacific Ocean (del Hoyo et al. 1992).

Ecology

This strictly marine species can normally be found over pelagic waters, preferring deeper waters than other boobies. It feeds on large species of shoaling fish, especially flying fish, but will also take large squid. Its breeding season depends on locality, forming small to medium-sized colonies of variable densities on rocky islands offshore. Nests are preferably built on cliff ledges, but a variety of other sites are used (del Hoyo et al. 1992).

Threats

Collection of eggs and hunting of adults from breeding colonies (Carboneras et al. 2018) is thought to be causing slow and significant declines. Masked Boobies are sensitive to human disturbance, with visitors passing within 10-20 m causing birds to leave their nest (Borsa et al. 2010); however, this is not believed to be significantly affecting the population. Invasive species, primarily House Rats Rattus rattus, also pose a threat in some parts of the booby's range, namely Clipperton Island, following the eradication of feral cats Felis catus (Pitman et al. 2005).

Acknowledgements

Text account compilers
Fjagesund, T., Butchart, S., Calvert, R., Martin, R., Ekstrom, J., Miller, E.


Recommended citation
BirdLife International (2019) Species factsheet: Sula dactylatra. Downloaded from http://www.birdlife.org on 19/03/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 19/03/2019.