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 size may be moderately small to large, 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). 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). For these reasons the species is evaluated as Least Concern.
The largest colonies with several thousand pairs are found on the Galápagos Islands (Ecuador) and in the Caribbean (del Hoyo et al. 1992, Orta et al. 2019), followed by approximately 1,000 pairs along the Pacific coast of Mexico (Hernández-Vázquez et al. 2018, Piña-Ortiz et al. 2018). The population in the southern Atlantic Ocean numbers less than 3,000 pairs, and those on Cape Verde and around the Arabian Peninsula contain a few hundred pairs (del Hoyo et al. 1992, Orta et al. 2019). The population size is therefore preliminarily estimated at 8,000-15,000 pairs, equating to 16,000-30,000 mature individuals.
The species’s current population trend is not very clear. It is suspected to be undergoing a decline, mainly due to the severe consequences of predation by cats and rats. So far, however, this threat was assumed to only affect a minority of the global population. New evidence suggests that predation is particularly severe in Caribbean breeding colonies, where a large part of the global population occurs (B. Denneman in litt. 2014, Orta et al. 2019). Unfortunately, in most cases, there is not much information available about the decline. For some breeding colonies, we know the direction of the population change, but have no data on its intensity or rate. For many other colonies, we have no information about the trend. The current trend in most of the colonies is thus unknown. Whilst it is therefore not possible to confidently determine the overall population trend for this species, it is precautionarily suspected to be undergoing a small decline.
This species ranges across tropical waters of the Atlantic Ocean, the north-west Indian Ocean and the eastern Pacific. Breeding colonies are found on the Galapagos Islands, Ecuador, on Chanaral Island off Chile, off the Pacific coast of Mexico, in the Caribbean, Cape Verde, on islands in the southern Atlantic, off Senegal, and on the coasts of Yemen, Oman and Saudi Arabia (del Hoyo et al. 1992, Diop et al. 2019).
This species is found in tropical and sub-tropical seas and is mostly pelagic. It feeds primarily on small fish, especially flying fish, but will also take squid. Most prey is caught by plunge-diving but flying-fish are sometimes taken in flight. Breeding is seasonal in places but can be more or less continuous in others. It is loosely colonial, nesting in rocky crevices, or on the ground on small, remote oceanic islands preferentially on cliffs where take-off is easy. No regular migration is known, although individuals can undergo extensive dispersal out to sea (del Hoyo et al. 1992). On St Helena (South Atlantic), birds tracked with GPS during incubation and chick rearing undertook trips of 1000 - 2000 km, reaching as far as 400 - 750 km from their breeding colony, respectively (S. Oppel in litt).
The Red-billed Tropicbird population in Abrolhos National Park (Brazil) is under threat from high rates of predation on eggs and nestlings by invasive House Rats Rattus rattus and Brown Rats R. norvegicus (Sarmento et al. 2014). They are highly vulnerable to rat predation due to laying only one egg, their long incubation period, and the ease with which rodents can access the burrows and crevices in which they nest (Sarmento et al. 2014). This threat is of such a magnitude that this sub-population has a mean probability of extinction of 57.5% and a mean time to extinction of 75 years (Sarmento et al. 2014). On Saba island in the Caribbean, breeding success was zero at colonies most vulnerable to predation by cats, as compared with 65% in a colony with lower predation rates (Boeken 2016). However, predation only affects a minority of the global population. Red-billed Tropicbirds have also been impacted by road construction and housing development (Lee and Walsh-McGehe 2000), with the construction of coastal housing and sea walls excluding Tropicbirds from previous nesting sites (Lee and Walsh-McGehe 2000).
Text account compilers
Miller, E., Stuart, A., Symes, A., Smith, D., Fjagesund, T., Calvert, R., Hermes, C., Martin, R.
BirdLife International (2020) Species factsheet: Phaethon aethereus. Downloaded from http://www.birdlife.org on 29/01/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 29/01/2020.