BirdLife species factsheet for Red-billed Tropicbird
Red-billed Tropicbird (Phaethon aethereus) ranges across tropical waters of the Atlantic Ocean, the north-west Indian Ocean and the eastern Pacific. The species is mostly pelagic. It breeds in loose colonies on sea cliffs or on remote, rocky islands.
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 over 1,000 pairs along the Pacific coast of Mexico. 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.
Predation of eggs and nestlings by invasive terrestrial predators, especially rats and cats, poses the most severe threat throughout the range. The species breeds in burrows and crevices, lays only one egg and has a long incubation period; all these factors make it vulnerable to predation (Sarmento et al. 2014). The species is persecuted in parts of its range (Orta et al. 2019). Moreover, is suffers from habitat loss due to coastal housing development and road construction (Lee and Walsh-McGehee 2000).
The species’s current population trend is not very clear though. It is suspected to undergo 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, 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, and we are unable to determine the overall population trend for this species.
Red-billed Tropicbird is currently classified as Least Concern. However, growing evidence of this species’s susceptibility to predation by invasive species suggests that the rate of population decline may be larger than assumed, so that the species may warrant a change in Red List status. Therefore, we present here our reassessment against all criteria for the species.
Criterion A – The population of Red-billed Tropicbird is thought to be in decline. The predation of chicks and eggs by invasive predators has been identified as the most severe threat to the species, with the potential of driving steep declines. However, data on long-term and short-term population trends for this species are scarce. Currently, we only have data for a small number of colonies.
The large population in the Pacific Ocean off the coast of Mexico seems to be overall stable. Partners in Flight report stable trends between 1970 and 2014 (A. Panjabi in litt. 2017). Moreover, the largest colony in Mexico on Peña Blanca Island seems to be stable (Hernández-Vázquez et al. 2018). The small population of the Arabian Peninsula and the Indian Ocean is stable or in slow decline (Symes et al. 2015). Research conducted in the Caribbean, however, found high mortality of eggs and chicks related to predation by feral cats; the colony on Saba (former Netherlands Antilles) suffered from severe predation (B. Denneman in litt. 2014, Boeken 2016), while a colony on Dog Island (Anguilla) could re-establish after the eradication of rats on the island (Bright et al. 2014). In Brazil, the population already declined to small numbers (Orta et al. 2019); the colony in Abrolhos National Park faces a high probability of extinction within the next 100 years due to predation (Sarmento et al. 2014). In Cape Verde, the species has been undergoing a steep decline, when the population plummeted from 1,000 individuals in the 1960s to only 100 pairs in 1990 (Orta et al. 2019). We have no information about the status of the colonies in the Galápagos Islands.
Taken together, there seems to be evidence that the species is in decline particularly in some colonies in the Caribbean, where the largest part of the global population is breeding (B. Denneman in litt. 2014, Orta et al. 2019). However, we do not have sufficient data on the rate of decline. Moreover, we lack information from large parts of the range, especially from other colonies in the Caribbean, the Galápagos Islands, as well as from the populations in the southern-central Atlantic. We can tentatively assume that the species is in decline globally. Is there evidence that other colonies and populations are in decline? Is the decline in the Caribbean, Brazil and Cape Verde large enough to outweigh the stable trends in Mexico and Arabia?
In order to be listed as Vulnerable under Criterion A2cde+4cde, Red-billed Tropicbird would have to undergo a decline of ≥ 30% over three generations. In case the species approached this threshold, it might be considered Near Threatened; alternatively, if the rate of decline were lower, it would be retained as Least Concern. Therefore, information is urgently sought regarding the current rate of population decline for all colonies, to see whether the overall rate of decline in this species is large enough to warrant its listing under Criterion A2ce+4ce and possibly additionally A3cde.
Criterion B – The Extent of Occurrence (EOO) of this species has been calculated as 136 million km2. This is far too large for listing the species as Vulnerable and therefore, Red-billed Tropicbird may be considered Least Concern under this criterion. The Area of Occupancy has not been calculated; thus, the species cannot be assessed against Criterion B2.
Criterion C – The population of Red-billed Tropicbird is estimated to number 16,000-30,000 mature individuals. This is too large to meet the threshold for listing as threatened under Criterion C (<10,000 mature individuals). However, under the assumption that the true population size is closer to the lower end of the estimate, the species may approach this threshold. As such, this may warrant listing the species as Near Threatened under Criterion C, under the condition that it is declining and other conditions are met.
The species forms several subpopulations, and there is no evidence that the number of mature individuals is fluctuating. Several colonies number more than 1,000 mature individuals, e.g. on Saba (Boeken 2016) or on Peña Blanca Island (Hernández-Vázquez et al. 2018). Therefore, Red-billed Tropicbird does not meet any of the conditions for Criterion C2. Whether Red-billed Tropicbird qualifies for listing as Near Threatened under Criterion C1 thus depends on the rate of population decline of the species and on the nature of evidence for this decline. Is the species undergoing a continuing decline of ≥10% over three generations (30.3 years)? Is the population decline observed (i.e. during a census of all known individuals), estimated (i.e. from census data or abundance indices under statistical assumptions) or projected (i.e. extrapolated into the future) following IUCN Guidelines (IUCN Standards and Petitions Subcommittee 2017)?
If Red-billed Tropicbird should be undergoing a population decline observed, estimated or projected at ≥10 % over three generations (30.3 years), it may be listed as Near Threatened under Criterion C1. Otherwise, in case that there is no evidence for such a decline, the species may be retained as Least Concern under this criterion. We urgently request up-to-date information on the rate of population change of the different subpopulations, in order to be able to assess the species against this criterion.
Criterion D – The population size of this species is estimated at 16,000-30,000 mature individuals. This is too large for listing as Vulnerable and therefore, Red-billed Tropicbird may be considered Least Concern under this criterion.
Criterion E – To the best of our knowledge, no quantitative analysis of extinction risk has been conducted for this species. Therefore, it cannot be assessed against this criterion.
Therefore, it appears that the only criteria where the species might approach or meet the threshold for Vulnerable are A2cde+4cde and possibly additionally A3cde. To better assess the species against these criteria, we urgently request up-to-date information regarding the rate of decline of Red-billed Tropicbird.
Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of its Red List status. Therefore, please make sure your comments are relevant to the discussion outlined in the topic.
An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.
Boeken, M. 2016. Breeding Success of Red-Billed Tropicbirds Phaethon aethereus on the Caribbean Island of Saba. Ardea 104(3): 263-271.
Bright, J. A.; Soanes, L. M.; Mukhida, F.; Brown, R.; Millett, J. 2014. Seabird surveys on Dog Island, Anguilla, following eradication of black rats find a globally important population of Red-billed Tropicbirds (Phaethon aethereus). The Journal of Caribbean Ornithology 27: 1-8.
del Hoyo, J.; Elliot, A.; Sargatal, J. 1992. Handbook of the Birds of the World, Vol. 1: Ostrich to Ducks. Lynx Edicions, Barcelona, Spain.
Hernández-Vázquez, S.; Castillo-Guerrero, J. A.; Mellink, E.; Almaguer-Hernández, A. M. 2018. Colony Size and Breeding Success of Red-billed Tropicbird (Phaethon aethereus) on Peña Blanca Island, Colima, México. Waterbirds 41(2): 128-134.
IUCN Standards and Petitions Subcommittee. 2017. Guidelines for Using the IUCN Red List Categories and Criteria. Version 13. Prepared by the Standards and Petitions Subcommittee. http://www.iucnredlist.org/documents/RedListGuidelines.pdf.
Orta, J.; Jutglar, F.; Garcia, E. F. J.; Kirwan, G. M. 2019. Red-billed Tropicbird (Phaethon aethereus). In: del Hoyo, J.; Elliott, A.; Sargatal, J.; Christie, D. A.; de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona, Spain. https://www.hbw.com/node/52607 (Accessed 15 April 2019).
Sarmento, R.; Brito, D.; Ladle, R. J.; Leal, G. d. R.; Efe, M. A. 2014. Invasive house (Rattus rattus) and brown rats (Rattus norvegicus) threaten the viability of red-billed tropicbird (Phaethon aethereus) in Abrolhos National Park, Brazil. Tropical Conservation Science 7(4): 614-627.
Symes, A.; Taylor, J.; Mallon, D.; Porter, R.; Simms, C.; Budd, K. 2015. The conservation status and distribution of the breeding birds of the Arabian Peninsula. IUCN. Cambridge, U.K. and Gland, Switzerland, and Environment and Protected Areas Authority. Sharjah, U.A.E.