DOI: 10.5061/dryad.h9w0vt4hb
title: The use of social attraction techniques to restore seabird colonies on Desecheo Island, Puerto Rico

description: 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.
description: <b>Site and species selection</b> 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<sup>th</sup> century while recent records occurred in the
 21<sup>st</sup> 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 (&lt;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. <b>Equipment</b> 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. <b>Bridled
 tern</b> 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. <b>Brown
 noddy</b> 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.
 <b>Audubon’s shearwater</b>
 <b>            </b>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.