Discovery of the breeding colonies of a critically endangered and elusive seabird, the Mascarene Petrel (Pseudobulweria aterrima)

Abstract

Seabirds are the most endangered group of birds and among them, the gadfly petrels (genera <em>Pseudobulweria</em> and <em>Pterodroma</em>) are the most threatened and least known. The Mascarene Petrel (<em>Pseudobulweria aterrima</em>) is endemic to Réunion Island and is one of the rarest birds in the world. This species was considered extinct in the mid-20th century but was rediscovered in 1970. The population is thought to be in decline because of predation by invasive predators, habitat destruction, and light-induced mortality. The first goal of this paper is to detail the methods that we used to discover the breeding colonies of this species and to determine the threats at these sites. The second goal is to present characteristics of the colonies we found, the threats occurring at these colonies, and the first conservation actions implemented at these sites. We first conducted an island-scale acoustic survey using autonomous recording units (ARUs) to locate the breeding colonies. We then used infrared thermal binoculars to precisely locate the places where birds displayed and landed. Because all discovered breeding sites were on vertical cliffs, we abseiled these cliffs to access the nests. Once burrows were discovered, we deployed infrared camera traps to determine the presence of alien predators or competitors (rats, cats, tenrecs) and to study the behaviors of the birds at the colony. The large-scale acoustic survey revealed the presence of 17 vocally active sites, 16 of which were investigated with infrared thermal binoculars. We observed petrel landings at five of these sites. Two of them were accessible and we abseiled to find the nests. We found 14 occupied burrows at one of these sites and eight at the other. Camera traps revealed the presence of rats and tenrecs at both sites, and cats were detected close to the colonies. The two colonies are on tall vertical cliffs covered with native vegetation, at elevations of 650 m a. s. l. and 1250 m a. s. l., respectively. These findings allowed us to implement conservation actions, such as invasive mammal control, and to start long-term monitoring and applied research for conservation. We are confident that the methods we developed could be used with great success at other places where finding colonies of a cryptic, rare, and nocturnal seabird is particularly challenging.