Desecheo Island (117 ha) was historically an important seabird island in the Caribbean with 15 species recorded of which seven are known to breed, including major populations of brown boobies and red-footed bobbies. The introduction of invasive mammals, plus the use of the island as a bombing range, contributed to the extirpation of five of the seven known breeding populations of seabird species and vastly reduced numbers of the remaining two species. The island became a National Wildlife Refuge in 1976 and major conservation interventions have included the eradication of invasive goats, rhesus macaques, and rats between 1976 and 2016. Removing these critical threats from the island has allowed other active restoration goals to be realized, including restoring seabird colonies to the island.

Here we report on the installation of social attraction equipment in 2018 to augment bridled tern and brown noddy colonies and establish a species of conservation concern, the Audubon’s shearwater. We supported these actions through a review of historic seabird nesting and roosting on Desecheo. Motion-sensing cameras were installed to document activity at each social attraction site and evaluate the effectiveness of our methods.

During the two years of deployment and monitoring, a total of seven bridled tern nests were documented in new and historic sites for the species, two of them next to a decoy colony; however, no brown noddy visits or nests were detected. In 2018 and 2019, one and two Audubon’s shearwaters, respectively, were attracted to one of the sound system speakers, representing the first record for this species on the island.

Social attraction efforts on Desecheo appears to be a feasible activity that may help support seabird recolonization and support conservation goals for this National Wildlife Refuge.

Site and species selection

We conducted an extensive literature review of historic and recent seabird species records for Desecheo Island to guide where seabird restoration could be considered. Historic records were defined as occurring in the 20^th century while recent records occurred in the 21^st century. The primary resource for our literature review was the USFWS Caribbean Islands National Wildlife Refuge Complex Library, where both internal and public reports are digitized and filed. In addition to this, we conducted an internet search using Google and Google Scholar using the search terms “Desecheo” and “seabird” as well as reviewed internal reports from past field trips to the island completed by Island Conservation staff. For each report or article, we collected the following information: behaviour (breeding or roosting), number of individuals or nests observed, location on island, and the reference (author and year). Location data varied in quality and specificity; therefore, some general descriptions were interpreted by individuals most familiar with the island. With this information, we used ArcMap software (version 10.3.1, ArcGIS) to create polygons delimiting the areas where seabirds were reported and created a location map for each documented seabird species.

Of the 15 (12 documented historically, plus three recorded after 2008) species identified through the literature review, we prioritized the restoration of three species’ breeding populations: bridled tern, brown noddy, and Audubon’s shearwater. All three species are small (<300 gm), with ground- or burrow-nesting breeding behaviour, and thus, expected to have been most impacted by multiple introduced, invasive mammal species on Desecheo (Jones et al., 2018; Jones et al., 2008; Medina et al., 2011). In addition, we expected bridled tern and brown noddy populations to have a higher probability of responding to social attraction tools given that both species have been previously observed attempting to nest in low numbers on the island and on rock stacks offshore of Desecheo and in the nearby Mona channel, meeting necessary assumptions of a nearby source population to interact with any stimuli (Buxton et al., 2014). While we didn’t find published examples of social attraction projects for noddy species or specifically bridled terns and Audubon’s shearwaters, successes have been proven at the family level. Terns (family Laridae) are known to respond well to seabird restoration methods, a function of flexible breeding behaviour and low fidelity to breeding sites (Jones & Kress, 2012). Similarly, shearwater species have been demonstrated to respond well to acoustic social attraction techniques elsewhere (Jones & Kress, 2012), however, we had greater uncertainty about an Audubon’s shearwater source population to interact with the social attraction stimuli as no individuals were known to breed on Desecheo. Calling activity was detected on the southwest coast of the island in 2012 (Island Conservation, 2013). The nearest known source population is 80 km southwest on Mona Island (C. Figuerola-Hernández pers. comm., Dec 2020). However, the Caribbean population of the Audubon’s shearwater is small; the species has disappeared from a number of former breeding sites, mainly due to predation by invasive species like rats and feral cats, and has been classified as Near Threatened by Schreiber and Lee (Lee, 2000), making this species a conservation priority for the region (Bradley & Norton, 2009).

Using the maps created for bridled tern and brown noddy, we identified three and two potential social attraction sites respectively, where there was evidence of historic breeding, and we could safely access the site with appropriate equipment. For the Audubon’s shearwater, we utilized observations from the 2013 seabird monitoring trip to identify two social attraction sites with habitat similar to the species’ nesting sites on other Caribbean islands (W. Mackin pers. comm., Oct 2017) and could be safely accessed and monitored over time by personnel (Fig. 1). Beyond logistical considerations, the selection of the two sites was based on 1) prioritizing the area where the species had been heard calling while flying over at night in 2013 and 2) finding suitable nesting habitat for the species on the west coast of the island where calling was detected.

Equipment

Using a mix of audio and visual cues, we employed three social attraction methods for restoring seabird colonies on Desecheo Island: decoys, mirrors, and sound systems. For bridled tern and brown noddy, recycled high-density polyethylene plastic decoys were manufactured and painted by The National Audubon Society. The decoys were a 1:1 size scale and were placed both as individuals and in pairs, simulating roosting and nesting behaviour, facing the ocean. All decoys were glued to the rock using high-strength anchoring epoxy (Fig. 2). Mirrors were approximately 30 cm in height by 15 cm in width and directed at decoys to create a visual effect of a larger colony. Sound systems were manufactured by the National Audubon Society and consisted of one 240-watt pyle amplifier, a charge controller and a mp3 player as the main components of the sound system(Fig. 3b). In addition, each sound system had two outdoor speakers, four solar panels, and two deep-cycle marine batteries. The speakers were placed approximately 20 m apart from each other. The amplifier unit maximized the sound output of the recordings, which were collected from natural Audubon’s shearwater and brown noddy colonies in the Caribbean. The audio range for each system was estimated to be between 300-500 meters, depending on surrounding noise such as boats and waves, simulating the expected sound level of an active breeding colony.  Two sound systems were programmed to play the sounds of an Audubon’s shearwater colony for 12 of every 24 hours from dusk to dawn, and one sound system was programmed to play the sounds of a brown noddy colony for 12 of every 24 hours from dawn to dusk.  . To monitor responses, we deployed one to four motion-sensing cameras (Reconyx PC900 Hyperfire Professional IR and/or Browning Strike Force Elite HD) at each decoy colony site and two cameras -- one directed on each speaker -- at sound system sites.

Bridled tern

In February 2018, we deployed 30 bridled tern decoys, five mirrors, and five motion-sensing cameras at one site on the south of the island, covering 192 m2 total area (Fig. 2a, b). In March 2018, a large storm surge hit Desecheo and 29 out of 30 bridled tern decoys were lost, as well as all the mirrors and cameras. In February 2019, we removed the single remaining decoy and established two new decoy colonies on the west coast, one with 30 decoys, five mirrors, and three cameras and the second with 32 decoys, five mirrors, and two cameras.

Brown noddy

In February 2018, we deployed 18 brown noddy decoys, three mirrors, and five motion-sensing cameras at one site on the south side of the island (Fig. 2c, d). In addition, we deployed a sound system with accompanying cameras 30 m north of the decoy colony (Fig. 3c, d). Cameras at the sound system site were deployed from February to September in both 2018 and 2019. In March 2019, we added a second decoy colony of 10 brown noddy decoys on a rock outcrop on the west coast with two cameras and no mirrors.

Audubon’s shearwater

            In February 2018, we placed two sound systems with motion-sensing cameras on the western coast of the island 250 m apart (Fig. 1). Cameras were deployed from February to September 2018 and February to August 2019.