A regionally significant population of White-tailed Tropicbirds Phaethon lepturus on Kurehdhoo (L...

A regionally significant population of White-tailed Tropicbirds Phaethon lepturus on Kurehdhoo (Lhaviyani Atoll), Republic of the Maldives.

October 2024

James C. Russell, Sebastian Steibl & Guy M.W. Stevens

Keywords: Eradication • Indian Ocean • Island • Rat • Seabird

Summary: Coral atoll islands provide critical nesting habitats for seabirds but are heavily impacted by human activity, including habitat loss and predatory mammals like rats. Seabirds, such as the White-tailed Tropicbird, have faced extirpation due to these threats. Despite poor documentation, some seabird colonies remain in the Maldives, especially on islands with low human disturbance and no introduced predators. In July 2023, a large, previously unknown colony of White-tailed Tropicbirds was discovered on Kurehdhoo Island. While the species is globally widespread, it is likely declining, including in the Maldives.

Seabirds play a crucial role in supporting both coral reefs and manta rays. Nutrients from seabird droppings enrich surrounding waters, fuelling plankton blooms that attract larger aggregations of manta rays. Recognising the vital link between land and sea, the Manta Trust is now supporting a University of Auckland-led research program, in collaboration with EPA Maldives, to identify, protect, and restore seabird nesting islands. The aim is to revive the reciprocal relationship between healthy islands and oceans.

Abstract

“Coral atoll islands around the world provide important nesting grounds for tropical seabird species (Berr et al. 2023). However, even among island types, coral atoll islands are small and have been disproportionately impacted by humans (Russell & Kueffer 2019). Seabirds on atoll islands have been harvested by humans for food (Mondreti et al. 2018) and extirpated throughout large areas of their range by the loss of habitat and introduction of predatory mammals, particularly Black Rattus rattus and Pacific rats R. exulans (Russell & Holmes 2015). However, rats can also be eradicated from islands (Spatz et al. 2022), and this can lead to spectacular recoveries in seabirds (Le Corre et al. 2015). The Maldives have been inhabited by people for approximately two and a half millennia (Knoll 2018), but the natural history of these islands has never been well documented. Early reliable natural history observations only date back to the early 19th century, and these records make little mention of seabirds, although earlier references note some islands with abundant nesting seabirds (e.g., La Harpe 1780). There is no reason to assume the coral atoll islands of the Maldives were not originally densely populated with seabirds as was, and still is, the case for other atoll islands in the Indian Ocean (including Chagos Archipelago to the south of the Maldives and Lakshadweep Archipelago to the north; Bourne 1971, Carr 2015, Pande et al. 2007). It is likely that seabirds were largely extirpated from most islands in the Maldives over its long history of human occupation before the modern natural history patterns were established and documented. However, nesting and roosting colonies of seabirds, although poorly documented, are still found on some islands scattered throughout the Maldives (Anderson & Shimal 2020); seabird presence likely coincides with low human disturbance and the absence of introduced mammalian predators. Given the number of islands in the Maldives and the potential number of seabirds they could support, identification of seabird breeding colonies, threats to these colonies, and the outcomes of management interventions would greatly benefit seabird conservation in this region (Anderson 1996). The White-tailed Tropicbird Phaethon lepturus, also known as the bosun bird among mariners, or dhandifulhu dhooni in Dhivehi, is a widespread tropical seabird species found on both volcanic and coral atoll islands. The global population is estimated at 400 000 individuals and is classified as Least Concern by the International Union for Conservation of Nature (IUCN), but the species is not well studied and is likely in decline globally (BirdLife International 2023). The main threats to White-tailed Tropicbirds are depredation by introduced mammalian predators (Saunier et al. 2022). The Maldives lies at the northern range limit for the species in the Indian Ocean, and although it has never been genetically tested, it is likely the nominal sub-species P.l.lepturus (Humeau et al. 2020). White-tailed Tropicbirds are recorded as a widespread breeding resident in the Maldives, but as is the case globally, they are also noted as likely in decline locally (Anderson & Shimal 2020). We visited Kurehdhoo (Lhaviyani Atoll, Maldives) in July 2023 where we discovered a large breeding colony of White-tailed Tropicbirds that, to date, had not been recognized. We surveyed the entire island colony in July 2023 and January 2024.”