Breeding status of the Red-tailed Tropicbird (Phaethon rubricauda) and threats to its conservation on Easter Island (Rapa Nui)
By Marcelo Flores*, Pedro Lazo, Graciela Campbell and Alejandro Simeone
Abstract Easter Island is located in the SE corner of Polynesia. This 163.6 km2 volcanic island is one of the most isolated places on Earth. During spring 2014 and 2015, we studied a breeding colony of Red-tailed Tropicbirds (Phaethon rubricauda), located on Rano Raraku volcano. We specifically aimed to determine: 1) main nest types used by tropicbirds, 2) nest distribution and abundance, 3) breeding success and 4) the threats faced by tropicbirds at this colony. Results showed the presence of four nest types: rock caves, moai nest, rock-protected and vegetation nests. Nests were strongly associated with unfinished moai statues. Breeding success ranged from 37% in 2014 to 26% in 2015. Camera-traps and direct observations showed interactions with five invasive alien species: Chimango Caracara was the most severe predator of unattended eggs and chicks; cats were observed close to adult birds; dogs killed fledglings; ants predated on newborn chicks; rats were recorded inspecting both occupied and unoccupied nests. We suggest that this is a relatively new and expanding colony. Urgent measures to control and/or eradicate invasive alien species on Rano Raraku are needed to improve the breeding success and persistence of this colony in the future.
*Corresponding Author E-mail: [email protected]
Pacific Science, vol. 71, no. 2 December 9, 2016 (Early view) Introduction
Tropicbirds (Phaethontiformes: Phaethontidae) include three closely related species in the single genus Phaethon (the White-tailed Tropicbird P. lepturus, the Red-billed Tropicbird P. aethereus and the Red-tailed Tropicbird P. rubricauda). All species are medium-sized; with predominantly white plumage and long tail streamers (Brooke 2002). The distribution of these species includes warm tropical and subtropical waters of the Indian, Atlantic and Pacific Oceans (Orta and Bonan
2016).
Red-tailed Tropicbirds breed mostly on islands of tropical Indian and Pacific oceans (Harrison
1988, Schreiber and Schreiber 1993). However, little is known about the species in the SE
Pacific, particularly on those islands at the easternmost range of its distribution. Most information available for Easter Island or Rapa Nui (Millie and Johnson 1970, Harrison 1971,
Marin and Cáceres 2010, Flores et al. 2014), Salas y Gómez Island or Motu Motiro Hiva
(Harrison and Jehl 1988, Vilina and Gazitúa 1999, Flores et al. 2014) and Desventuradas Islands
(Aguirre et al. 2009) is based on short visits (some of them lasting only a few hours) and is mainly of anecdotic nature. There is hardly any information on population size, breeding ecology and threats for populations of this species at this marginal distribution.
Easter Island (27º09’S; 109º26’W) is located in the SE corner of Polynesia at 2,250 km to the east of Pitcairn Island and at 3,700 km to the west of South America. This 163.6 km2 volcanic island is one of the most isolated places on Earth, motivating the interest in its history, evolution and the mysteries of their megalithic sculptures or moai carved in the volcanic tuff by its ancestral inhabitants ca. 400-1500 A.C (Van Tilburg 1994, Arana 2014). However, in contrast to the rich archaeological and cultural heritage, little attention has been given to its native fauna
(Schlatter 1987, Fernández and Hormazábal 2014).
2 Johnson et al. (1969) first reported Red-tailed Tropicbirds nesting at the small islet Motu
Maratiri, in the Southeastern end of the island. Harrison (1971) states that “…apart from a few red-tailed tropic-birds Phaethon rubricauda, the whole avifauna is centred on four little islets off the east coast, three close together near the main island's only settlement and harbour…” In the austral summer 1993-1994, the first active nest of Red-tailed Tropicbird is observed at Rano
Raraku (Pedro Lazo, pers. obs.). In the next decade, the use of the colony was irregular until
2007 when several nests began to be used regularly. Jaramillo et al. (2008) reported active nests in Poike Peninsula and the cliffs of Rano Kao volcano in 2001 and 2003. Marin and Cáceres
(2010) reported 13 active nests in Rano Raraku in August 2009.
In this paper we characterize the breeding colony of Red-tailed Tropicbirds located at Rano
Raraku Volcano, the largest breeding site for this species on the island. We specifically aimed to determine: 1) main nest types used by tropicbirds, 2) nest distribution and abundance, 3) breeding success and 4) the threats faced by tropicbirds in this colony.
Methods
Study Area
Rano Raraku Volcano (27°07’19”S; 109°17’21”W) is formed by volcanic tuff; the sides of the crater are high and steep except on the north and northwest, where they are much lower and gently sloping. With a diameter of ca. 0,8 km and a maximum altitude of 120 m, it possesses a vertical cliff towards the ocean (Cauwe 2011). This area is also known as the Quarry where the moai-sacred megalithic statues-were carved, modifying the tuff walls and creating terraces, cracks and flat areas. It is part of the Rapa Nui National Park and it thus administered by the
Chilean Forestry Service (Fig. 1).
3 <
Nest type and location
During spring 2014-2015, we conducted an exhaustive search of nests in the inner and outer slopes of the crater located on the southeastern and southwestern part of the volcano. For ease of counting, we divided the area into four main sectors (A, B, C and D, Fig. 2). All four areas are characterized by tuff slopes with an inclination between 40-60° with some partially buried rocks.
The vegetation cover is dominated by the introduced molasses grass or “Mauku piro” (Melinis minutiflora) (Dubois et al. 2013).
<
Each nest found was photographed using a Canon digital camera and georeferenced with a
Garmin GPS. Each nest was also categorized according to its location and construction materials in: (a) rock cave: natural caves or crevices in the rocks or cliff; (b) moai: associated to the monolithic statues sculpted but unfinished in the quarry; (c) protected by rock: nest with the protection of one or more rocks on their sides and, (d) vegetation: nest built in the vegetation
(Fig 3).
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Considering factors like accessibility and landscape perturbation in a sacred and highly touristic place, we selected nine active nests to be monitored continuously using camera traps. All nests were located in sector A. We used three different types of Bushnell Trophy Cams: Five units
Model 119576, two units Model 119436 and three units Model 119636. All camera traps were set with the configuration to take 3 photographs per activation event with a delay of 30 seconds between events, following the configuration suggested by Hervías et al. (2013). Due to the white coloration of the birds and the need to install the cameras just a few centimetres away from the nest to cover both the entrance and the interior, the sensors were covered with white foam in
4 order to soften the intensity of infrared led and avoid overexposed images (Fig. 4). Cameras were checked every 2-3 days for battery levels and memory card capacity. Data collected included nest content (adult, egg, chick, fledgling); presence of invasive alien species and breeding activity
(incubation, parent switching, chick exercising).
<
In addition to camera trapping, we observed Sector A from three selected points (Bibby et al.
2000). Using 10x42 binoculars, observations were made in a radius of 100 m every 10 minutes.
Data collected included date, hour, invasive species and their locations in the breeding and adjacent areas (Fig. 2). Observation efforts were made in September-November 2014 and
October-December 2015.
Spatial distribution
In order to test the null of hypothesis of random spatial distribution (i.e. Poisson point process) by land type; rock (i.e. rock cave, moai and protected by rock) or vegetation, we used the quadrat.test function in the Spatstat Package (Baddeley & Turner 2005) of R. This analysis split the prospected areas (Fig. 2) by land type and compared the χ2 statistic from the observed number of nests by land type, with the expected values if nests were distributed randomly accordingly to the Monte Carlo method (10000 random simulations). Prior to the analysis we rasterized the prospected areas by their land type (rock or vegetation) using satellite images and the raster package (Hijmans 2016) of R (R Development Core Team 2011).
Breeding success
Breeding success was estimated with the information provided by camera-traps along with direct observations of nest activity. We considered success when a nest produced a fledgling. To test for differences in breeding success between nest types, we used a GLM with binomial distribution.
To analyse breeding success between years, a Chi-square analysis was used.
5 Invasive alien species
The presence of invasive alien species in the study area was assessed by direct observations with
10x42 binoculars and Bushnell camera-traps to see inside the nests as detailed above. In the case of ants, their presence was recorded when we searched for and characterized the nests.
Results
Nest type and location
A total of 53 nests were detected at Rano Raraku in the surveys conducted in 2014 and 2015
(Figure 2). Most nests were concentrated in Sector A (n=43), followed by sector B (n=7) and sector C (n=3). No nests were detected in Sector D. Nests in sector A were distributed alongside the limit between the cliff and the slope, in strong association with the moai niches. The major aggregation of nests (n=19) was located in the southeastern part of the area where 6 moai remain horizontal in their niches (Fig. 5)
Tropicbirds showed a strong preference to nest on rock ground compared to a random spatial process (quadrat.test χ2=250.46, df=1, p-value < 0.0001). Nests were located on the ground using natural scrapes and cracks on the volcanic cliff faces (4%), cavities created by moai carving
(26%), protected by rock or cliff faces (62%) and under vegetation (8%). With exception of the summit of the cliff wall in sector C (Te Pu Makari), all nests were located in areas where the extensive dense growth of Molasses grass (Melinis minutiflora) provided abundant cover. Figure
5 shows the nest distribution at the “moai stairway” in sector A.
Breeding Success
A Pearson’s Chi-squared test comparing unoccupied nests versus occupied nests for 2014 and
2015 showed significant differences between years (χ2= 5.76, df=1, p=0.016)
6 In 2014, 29 nest sites were identified with an occupation rate of 55%. All active nests were located in sector A. Breeding success was 38%, while the remaining nests were predated by alien species. In 2015, 53 nest sites were identified with an occupation rate of 28%. Breeding success was 27%, failed nests 46% and predated nests 27%. A single nest (rock cave) failed due to an adult having broken the egg. Successful nests were located in sector A (n=3) and sector B (n=1).
Table 1 summarizes the number of nests per category for 2014 and 2015.
<
A total of 5 invasive alien species were observed interacting with Red-tailed Tropicbirds in the study area. Stray cats (range 3-7 ind.) were observed permanently associated with picnic areas where they interacted with tourists and local tour guides, begging for food. Camera traps showed the presence of a cat close to an adult nesting in 2014. Although no direct attack was recorded, the nest was abandoned within the next 8 hours after the event (Fig. 7a). Like cats, free-ranging dogs (Canis familiaris) were observed permanently in the picnic area begging for food. Although they were not registered by the camera-traps, an attack was observed in October 2015 when two fledglings were killed and partially consumed. The introduced Chimango Caracara (Milvago chimango) is the only raptor on the island. It was found nesting in eucalyptus trees in the picnic area of the park, as well as on the ground next to Tropicbirds. Camera-traps showed that
Chimango Caracara was the main predator of Red-tailed Tropicbirds at Rano Raraku volcano, preying on both unattended eggs and chicks (Fig. 7b). We observed Chimango Caracaras interacting with Tropicbirds in aerial displays and patrolling the area, following the tropicbirds to their respective nests. Two active Chimango Caracara nests were found less than 2 m away from active tropicbird nests. Rats (Rattus sp.) were observed in nests with adults incubating eggs, fledgling and recently abandoned, however no direct interaction between the species was observed (Fig 7c). Finally, Argentinian ant (Linepithema humile) was present in all nests. Direct
7 observations showed several disturbed adults covered with ants and a single chick was found agonizing and partially predated by ants. A necropsy performed on this chick showed ants in the rectum, throat and nostrils (Fig. 7d).
<
Discussion
Easter Island, Salas y Gómez Island and Desventuradas Islands are likely the farthest colonies where Red-tailed Tropicbirds breed within the SE Pacific (Nelson 2005, Schreiber and Schreiber
2009, Flores et al. 2014, Orta et al. 2016). The particularity of the colony on Rano Raraku volcano, Easter Island, in terms of location, association of nests with the carved moai statues and the presence of the pool of alien species, make this colony unique.
Nest type and location were similar to ones reported previously for other colonies in the North
Pacific such as Kure Atoll and Oahu in Hawai’i (Fleet 1974, Vanderwerf & Young 2014), with nests made in natural holes and cracks, as well as made of vegetation (Nelson 2005, Schreiber and Schreiber 2009, Orta et al. 2016). However we found an interesting preference for nests associated with moai statues. The space created under the unfinished moais provides protection against weather, particularly against rain and solar radiation. Since Rapa Nui people consider moais sacred, and they are also protected by Chilean laws (Supreme Decree 72/1995 Ministry of
National Assets), contact with the statues is restricted for both tourists and islanders. However, a conflict looms due to the potential damage caused by seabird faeces on the statues (Susana
Nahoe pers. comm.). This must be studied promptly in order to determine its real effect, considering other terrestrial and alien birds, like Chimango Caracara, Rock Dove (Columba livia) and Common Diuca-Finch (Diuca diuca), which also defecate and perch on the statues.
Breeding success in Easter Island was similar to the 34% reported for Kure Atoll by Fleet (1974) and 36% recently reported by Sommerfeld et al. (2015) for Christmas Island, both islands with
8 alien species. Among causes of failure, new breeders stand out due to non compatible incubation shift durations, deserting and leaving the egg or chick unattended and available for predators
(Schreiber and Schreiber 2009). In Rano Raraku we observed eggs and chicks unattended during incubation, eggs broken by an adult during incubation and most importantly, eggs and chicks preyed upon by alien species.
Predation by Polynesian rats (Rattus exulans) has been reported for Kure Atoll (Fleet 1974), while black rat (Rattus rattus) predation was reported on Aldabra Atoll (Diamond 1975). Orta et al. (2016) indicated that human exploitation for food as well as the tail streamers taken for ornamental purposes are factors affecting the breeding success in many of the Pacific islands.
Although these threats were not observed on Easter Island, the long tail streamers are used by the islanders for making typical costumes for the local carnival (Flores pers. obs.).
Alien species on islands are normally subsidized by human activities (Gomper et al. 2008). Cats and dogs interact constantly with tourists and local guides who feed them daily. During daytime, both species remain close to picnic areas and park entrance gates, but at night, they actively search for seabird nests. Medina et al. (2011) reviewed the effect of cats including predation and extinction of several species of reptiles and birds around the world. A case of sublethal effects could be produced by a cat due to survival of the bird but with a failed nest The presence of a cat near a nesting bird could be a case of sublethal effects due to survival of the bird which later abandoned the nest (Bonnington et al. 2013). The episode of a dog killing a fledgling during daylight corresponded to a dog abandoned by local islanders after a strike and takeover of the park. Dogs also can predate upon a wide range of taxa (Ritchie et al. 2014) causing a decrease in both population and breeding success (Clinchy et al. 2013).
Chimango Caracaras were introduced around the 1930s to control rat infestation (Harrison 1971,
Marin and Cáceres 2010), however the results were not as expected. In its natural distribution in
9 South America, Chimango Caracaras are well known to be opportunistic and generalist predators, feeding on eggs and chicks of other species (Willson et al. 2010). On Easter Island,
Chimango Caracara is known as “Manu toke-toke” in Rapa Nui language which literally means
“thief bird”. Since its introduction, this Caracara has been reported to take food from islanders’ houses and feed opportunistically on eggs and chicks of nesting seabirds, roadkills, on garbage at the city dump and occasionally on other invasive species such as lizards, snails and leftovers. As a result, most of the community perceives the Chimango Caracara as a harmful invasive species and a bird that must be eradicated from the island. Cases of raptors introduced to oceanic islands are scarce in the literature. The Swamp Harrier (Circus approximans) was introduced in French
Polynesia where it predates on the Tahiti Petrel (Pseudobulweria rostrata) and Audubon’s
Shearwater (Puffinus iherminieri) (Wilcox 2009).
Ants were observed affecting Red-tailed Tropicbirds and are among the main threats to conservation of oceanic islands (New 2008; Rabitsch 2011). They can displace native species of invertebrates and vertebrates through predation or competition; they may also create favourable conditions for future invasions of other species and cause the disruption of whole communities
(Krushelnycky and Gillespie 2008; Kenis et al. 2009). Plentovich et al. (2011) reported decreased breeding success for Wedge-tailed Shearwater (Ardenna pacificus) in Hawai’i due to predation by tropical fire ants (Solenopsis geminata), although recent studies showed an increase of breeding success when predator control was applied (Vanderwerf & Young 2014).
The tourist attraction at Rano Raraku Volcano is a source of conflict due to maintenance actions like clearing of vegetation to better expose some statues. This causes that some nests to become exposed and easy to find for some alien species and therefore abandoned, especially those nests located near the trails.
10 Considering the number of nest sites available in Rano Raraku, the available habitat for nesting and sporadic records of possible nesting birds in cliffs of Poike Peninsula and adjacent islets
(motu), we estimate the breeding population of Red-tailed Tropicbirds at around 100 pairs on
Easter Island and we suggest that the colony at Rano Raraku is a relatively new and expanding colony. This number is similar to that reported by Champeau et al. (2011) for Hatu-Iti Island,
Marquesas Islands and far below the 500 and 1000 pairs reported by Brooke (1995) for Oeno and
Ducie atolls and Pitcairn Islands, respectively.
Aguirre et al. (2009) reported an undetermined number of Red-tailed tropicbirds breeding on
Desventuradas Islands (San Ambrosio island), but the prohibition of access the islands by the
Chilean Navy due to the presence of an operational base, has prevented further research. For the other oceanic islands, the information has been reviewed and updated by Flores et al. 2014.
Invasive alien species are widespread on Easter Island, including the small islet of Motu Nui where Chimango Caracaras and Argentinian ants are permanently present. This is important as eleven species of marine birds breed on this islet. In Rano Raraku, the presence of an assemblage of alien species including mammals, birds and ants may affect the colony seriously, therefore, measures to control and/or eradicate the invasive alien species of Rano Raraku, particularly cats, dogs and Chimango Caracaras, are urgently needed to ensure the breeding success of this colony.
11 Acknowledgments
We appreciate the help provided by Avitrek and CONAF Rapa Nui, particularly Enrique Tucki and Hotu Matu’a Pate, for their logistical support and permits that allowed our study within the
Rapa Nui National Park. We also thank Maximiliano Daigre and Paulina Arce for their valuable help in the field, Betsy Pincheira for her valuable corrections and comments to this manuscript and Cristobal Anguita for his support with statistical analysis. A grant to cover travel expenses and accommodation on Easter Island was provided by The PEW Charitable Trust. The Millenium
Nucleus “Ecology and Sustainable Management of Oceanic Islands” (ESMOI) provided grants to cover travel expenses and the loan of camera-traps. At the time this work was conducted, the first author was partially supported by a grant from the Dirección de Postgrado, Universidad
Andres Bello.
12 Table 1. Nest sites and activity in the Red-tailed Tropicbird colony on Rano Raraku for the 2014 and 2015 breeding seasons.
Year 2014 2015 Total Nest Sites 29 53 Unoccupied
nests 13 38 Occupied nests 16 15 Failed nests 0 7 Predated nests 10 4 Successful nests 6 4
We also tested for differences in the breeding success probability (mean ±95% CI) for moai, protected by rock and vegetation nests, but we found no statistical differences (GLM, binomial distribution, χ2 = 2.98, df=2, Pr (>χ2) = 0.23). (Fig. 6).
13 Figure 1. Location of Easter Island (Rapa Nui) in relation to South America. The map also shows the locations mentioned in this stud
14 Figure 2. Prospected areas (pale grey) and location of Red-tailed Tropicbird nests at Rano
Raraku Volcano (white dots). Note the major abundance of nests in sector A. No flying birds and nests were observed in sector D. White box shows the location of the “moai stairway” shown in detail in Figure 6. Modified from Google Earth©.
15 Figure 3. Nest type found on Easter Island. (a) Rock cave nest with an adult and a 3 day old chick. (b) Moai nest. The white arrow indicates the location of the nest and the pale grey lines mark the contour of the statues. (c) Nest protected by rock. The rock provides protection and offers shade for the nesting bird. (d) Fledgling in nest built in molasses grass (Melinis minutiflora) away from rocks and/or cliffs.
16 Figure 4. Nest protected by rock monitored with a Bushnell Trophy Cam. Note the camera at ground level and ca. 1 meter away from the bird.
17 Figure 5. Location of Red-tailed Tropicbird nests in sector A (white dots), particularly associated to unfinished moai statues highlighted with grey lines.
18 Figure 6. Breeding success probability of the Red-tailed Tropicbird on Rano Raraku showing no significant differences between nest types.
19 Figure 7. Introduced species detected by camera traps at Rano Raraku. (a) Stray cat (Felis catus) next to a nesting Red-tailed Tropicbird (Phaethon rubricauda). (b) Chimango caracara (Milvago chimango) close to an unattended egg. (c) Rat (Rattus sp.) exploring an empty nest. (d) Two-day old
Red-tailed Tropicbird chick being eaten by Argentinian ants (Linepithema humile).
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