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). The population trend appears to be stable, and hence the species does not 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 c.1,200,000-2,100,000 individuals (Wetlands International 2020), which roughly equates to 800,000-1,400,000 mature individuals.
Trends differ among populations, with some having unknown trends, while others are increasing or fluctuating (Wetlands International 2020). Overall, in the absence of evidence for any declines or substantial threats, the species is tentatively assessed as being stable.
The Brown Noddy is a tropical seabird with a worldwide distribution, ranging from the Hawaiian Islands (U.S.A.) to the Tuamotu Archipelago (French Polynesia) and Australia in the Pacific Ocean, including colonies off the Pacific coast of north-west South and Central America, from the Red Sea to the Seychelles and Australia in the Indian Ocean including south-east Asia and in the Caribbean to Tristan da Cunha (St Helena to U.K.) in the Atlantic Ocean including a colony off the coast of Cameroon. Some colonies are also present in the sub-tropics with individuals from these colonies wintering in the tropics (del Hoyo et al. 1996).
Behaviour Although its migratory movements are poorly known and the species is present all year round at most tropical colonies, it is seasonally absent from subtropical colonies and is known to disperse to the open ocean after breeding (del Hoyo et al. 1996). The timing of breeding varies throughout the species's range (del Hoyo et al. 1996). It may breed colonially in groups numbering up to 100,000 or more pairs, although it also nests almost solitarily depending on the availability of nesting sites (del Hoyo et al. 1996, Higgins and Davies 1996). Even when not breeding the species remains gregarious and can occur in huge flocks in some areas, although it is more usually observed in smaller flocks of 50-100 individuals (Higgins and Davies 1996).
Habitat The species occurs around isolated, bare or vegetated, pantropical and subtropical, inshore or oceanic islands or coral reefs with rocky cliffs or offshore stacks and coral or sand beaches (del Hoyo et al. 1996, Higgins and Davies 1996). It forages in the inshore waters surrounding such islands, often along the line of breakers or in lagoons, and disperses up to 50 km out into the pelagic zone to forage (especially when not breeding) (del Hoyo et al. 1996, Higgins and Davies 1996). Out at sea it often rests on buoys, flotsam, ships and on the open water (del Hoyo et al. 1996).
Diet Its diet consists predominantly of small fish as well as squid, pelagic molluscs, medusae and insects (del Hoyo et al. 1996, Higgins and Davies 1996).
Breeding site The nest may be a simple layer of debris or a more elaborate construction of seaweed and sticks, and may be placed in a number of sites including flat shingle beaches, bare ground, cliff ledges, offshore stacks, low bushes and tall trees (del Hoyo et al. 1996). It nests in colonies that can be very dense or more open depending on the availability of nesting sites (del Hoyo et al. 1996).
House Rats Rattus rattus have caused declines in the Brown Noddy population on Dry Tortugas, US, and are thought to restrict the distribution on Ascension Island (Ascension Island Government 2015), and likely at other locations throughout the very large breeding range. Egg and chick collection of Brown Noddy has occurred for centuries, but is now relatively infrequent and restricted to certain parts of the range, where the take is not believed to cause significant declines (Reichel 1991, Chardine and Morris 1996).
Text account compilers
Calvert, R., Butchart, S., Ekstrom, J., Malpas, L., Martin, R. & Stuart, A.
BirdLife International (2021) Species factsheet: Anous stolidus. Downloaded from http://www.birdlife.org on 26/02/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 26/02/2021.