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# Tahiti Petrel *Pseudobulweria rostrata* population decline at a nickel-mining site: a critical need for adapted conservation strategies

ANGÉLIQUE PAGENAUD<fnref n="1" />,<fnref n="2" />* ID, KAREN BOURGEOIS<fnref n="2" />, SYLVAIN DROMZÉE<fnref n="2" />, MARTIN THIBAULT<fnref n="1" />, GUILLAUME CHAGNEAU<fnref n="2" />, NICOLAS BARRÉ<fnref n="4" />, ÉLODIE BOUYÉ<fnref n="3" />, VINCENT CHARTENDRAULT<fnref n="5" />, NICOLAS DELELIS<fnref n="4" />, JULIEN DIJOUX<fnref n="2" />, MAGALIE PATROIS<fnref n="3" />, JÉRÔME SPAGGIARI<fnref n="6" /> and ÉRIC VIDAL<fnref n="1" />,<fnref n="2" />

(Received 09 August 2020; revision accepted 03 March 2021)

## Summary

New Caledonia hosts a large part of the world's breeding population of the Tahiti Petrel *Pseudobulweria rostrata*. This rare, cryptic and little-studied seabird nests locally in the mountains up to 1,200 m in altitude, particularly in ultramafic (i.e. nickel-rich) areas where mining activity is a major threat. The considerable development of mining activities in New Caledonia over the past decade raises concerns about its potential impacts on breeding populations through both direct habitat destruction and side effects such as pollution or repeated disturbances. This context calls for a dedicated assessment of the persistence of local populations to guide the design of an adapted conservation strategy and potential restoration programmes. We investigated the impact of mining activities on a Tahiti Petrel population when surveyed pre-mining (2004–2007) and following a period of full mining (2017–2018). The vocal activity was assessed at a total of 114 night-call count stations spread over the Koniambo massif. Areas with ground-originated vocal activity were then searched during daytime for nesting evidence. Finally, georeferenced aerial photos were used to estimate habitat degradation as the percentage of bare soil cover (PBSC) within a 400-m radius around each call count station. Our study revealed a dramatic decline in the Tahiti Petrel vocal activity and a desertion of breeding habitats during the full-mining period compared to the pre-mining period. In light of these results, we recommend designing safe breeding areas and combining restoration methods including social attraction, predation control and artificial burrows at mining sites.

**Keywords:** mining site, *Pseudobulweria rostrata*, vocal activity, restoration

<footnote n="1">¹UMR ENTROPIE, IRD, IFREMER, CNRS, University of Reunion, University of New Caledonia, BP A5, 98848 Nouméa Cedex, New Caledonia.</footnote>

<footnote n="2">²Aix-Marseille Université, CNRS, IRD, University of Avignon, IMBE, Centre IRD Nouméa - BP A5, 98848 Nouméa Cedex, New Caledonia.</footnote>

<footnote n="3">³KONIAMBO NICKEL SAS, Route Territoriale 1, Site de Vavouto - 98833 Voh, BP 679-98860 Koné, New Caledonia.</footnote>

<footnote n="4">⁴IAC - Institut Agronomique néo-Calédonien, Port-Laguerre, BP73, 98890 Païta, New Caledonia.</footnote>

<footnote n="5">⁵LPO Auvergne-Rhône-Alpes, délégation territoriale Drôme-Ardèche, 18 place Génissieu, 26120 Chabeuil, France.</footnote>

<footnote n="6">⁶SCO - Société Calédonienne d'Ornithologie - BP 13641, 98803, Nouméa cedex, New Caledonia.</footnote>

<footnote>*Author for correspondence; email: angelique.pagenaud@ird.fr</footnote>
## Introduction

Seabirds are one of the most threatened groups of birds, with an estimated 70% decline in the global population monitored over the past 60 years (Croxall *et al.* 2012, Paleczny *et al.* 2015). Among them, Procellariids are considered the most endangered order (Vié *et al.* 2009). Their decline is mainly due to invasive species (cats *Felis catus* and rats *Rattus spp*.), bycatch and destruction of nesting habitats (Croxall *et al.* 2012, Dias *et al.* 2019).

The Tahiti Petrel *Pseudobulweria rostrata* is a Procellariid seabird species whose breeding populations are known from only a few islands in the tropical Pacific, particularly in American Samoa, Fiji, French Polynesia and New Caledonia (Brooke 2004, BirdLife International 2020). The genus *Pseudobulweria* includes only four extant species. Three of them, Beck's Petrel *P. becki*, Fiji Petrel *P. macgillivrayi* and Mascarene Petrel *P. aterrima*, are considered on the verge of extinction and classified as 'Critically Endangered' on the IUCN Red List. The Tahiti Petrel is currently classified as 'Near Threatened' due to limited data and studies (Bretagnolle 2001, Brooke 2004, Gangloff *et al.* 2012, BirdLife International 2020). However, as highlighted by recent studies, this species has to cope with several at-sea and on-land threats such as invasive species, light pollution, bycatch, human disturbance, habitat destruction and plastic ingestion (Mareschal 2009, Faulquier 2014, Palmas *et al.* 2017, Berr *et al.* 2020). These threats undoubtedly contribute to its overall population decline (BirdLife International 2018).

In New Caledonia, Tahiti Petrels nest in three types of habitat: high elevation areas within forest or scrub on steep mountain slopes, rocky hill islets, and less frequently on sandy beaches (Pandolfi-Benoit and Bretagnolle 2002). Most breeding individuals are found across the large mountainous regions of Grande Terre (main island), especially in ultramafic (nickel-rich) areas (Villard *et al.* 2006). Nickel mining is a significant economic sector in New Caledonia (Wacaster 2017). Open-pit mining has steadily developed and mechanised since its beginnings in the second half of the 19th century and is expected to keep growing over the next 30 years (Baille and Jegat 2012, Wacaster 2017). Due to the overlap of petrel-inhabited and mineral-rich areas, this industry poses severe threats to the local Tahiti Petrel population (Pascal *et al.* 2008). These threats include habitat fragmentation and destruction (Reville *et al.* 1990, L'Huillier *et al.* 2010), light and noise pollution (Bayne *et al.* 2008, Francis *et al.* 2009, Saha and Padhy 2011, Duarte *et al.* 2015, Rodríguez *et al.* 2015), and a facilitated spread of invasive species, especially mammalian predators (Croxall *et al.* 2012, Rodríguez *et al.* 2019). Despite the known sensitivity of seabird colonies to such disturbances worldwide (Croxall *et al.* 2012, Dias *et al.* 2019, Rodríguez *et al.* 2019), the mechanisms underlying the observed population declines are rarely described. The lack of knowledge about the impacts of mining on the Tahiti Petrel, particularly on its biology, ecology, and population dynamics, hinders the development of appropriate restoration actions.

Nocturnal vocalisations can provide useful information on the relative abundance and general activity of night-active seabird colonies (Keitt 2005, Buxton and Jones 2012a). Because calling rates and seabird nest density are positively correlated (Oppel *et al.* 2014a), call count techniques are one of the most useful methods for monitoring long-terms trends in change in populations of nocturnal burrow-nesting seabirds (Borker *et al.* 2014, Oppel *et al.* 2014b). Here, we examined the changes that may have occurred on the Tahiti Petrel population at Koniambo massif after 10 years of developing nickel-mining activities and evaluated the current status of this population. We conducted temporal monitoring of the onsite vocal activity by call count stations, coupled with a ground survey of suspected breeding sites. Our results are discussed in light of the limited scientific information available for the species and argue for the urgent implementation of an adapted management strategy.

## Methods

### Study site

New Caledonia is an oceanic archipelago in the south-west Pacific. It is composed of the main island, Grande Terre, and several surrounding islands and islets. Grande Terre is a large elongated
<figure><img src="image_73.png" /></figure>

Figure 1. Location of the Koniambo massif in New Caledonia and spatial distribution of call count stations (n = 114) along the massif. Blue dots represent call count stations created in 2004–2007 (n = 95). Red dots represent new call count stations set in 2017–2018 (n = 19). Black lines delineate the nine mining sectors of the massif.

island (c.400 km long; 16,600 km²) with a central lengthwise mountain range (highest elevation: 1,627m above sea level) of which about a third of the surface is nickel-rich ultramafic soils (Jaffré 1992, Grandcolas et al. 2008).

This study was carried out on the ultramafic Koniambo massif (20 km long and 6–10 km wide), located in the north-west (21°30′S, 165°30′E) and culminating at 930 m above sea level. The Koniambo massif was mined from 1888 to 1947 by several small companies, resulting in only slight modifications of the massif (Bird et al. 1984, Nakache 1999). The massif was kept as a reserve between 1947 and 2008 when Koniambo Nickel SAS started exploiting it again. More modern mechanized and automated mining techniques allowed them to process larger volumes of ore. The industry reached its full operating capacity in 2014 (Horowitz 2004). The massif is currently subdivided into eight operating sectors (Bilboquet, Centre, Confiance, Guerioum Sud, Manguen, Tahafe, Trazy and Katepahie) and one non-operating sector (Coco) (Figure 1). Tahiti Petrels have been reported breeding on steep slopes at the highest altitudes of the massif, which also coincides with exploited areas (Villard et al. 2006, BirdLife International 2020, authors’ pers. obs.).

## Study species

The Tahiti Petrel is a cryptic and under-studied seabird species in New Caledonia and the Pacific, with a little-known breeding phenology and distribution. The few studies available provide preliminary data on its breeding behaviour, biology and phenology (Villard et al. 2006), describe its vocal repertoire (Rauzon and Rudd 2014) or refer to on-land spatio-temporal distribution and habitat use (Titmus 2017). Like most petrels, this species is nocturnal with hypogeous nesting, digging a burrow under rocks or tree roots in high elevation forests (up to around 1,000 m asl) or on
rocky slopes (Villard et al. 2006, Titmus 2017). Nocturnal vocal activity is by far the dominant form of communication for these petrels. A higher peak of activity is observed at the beginning of the night and near breeding sites as adults return to their colonies for mating, chick-rearing or to find a mate (Bretagnolle 1996, Rauzon and Rudd 2014, Titmus 2017). Tahiti Petrels nest in small and loose colonies of up to 10 pairs, with sometimes isolated pairs, rather than in dense and large colonies. The species is suspected to be philopatric (i.e. individuals tend to return to breed where they had been reared as chicks) and to demonstrate nest-site fidelity (i.e. adult breeders use the same burrow for several years) like most Procellariids (Warham 1990, Villard et al. 2006). In New Caledonia, the species is considered to have an extended breeding cycle throughout the year (including the austral winter), with breeding pairs and colonies exhibiting an asynchronous phenology (Ravache et al. 2020), which is atypical among Procellariids that generally have a synchronous breeding behaviour (Brooke 2004).

## Vocal activity assessment

The call count method was used to approximate changes in the Tahiti Petrel population size in the Koniambo massif between the early pre-mining (2004–2007) and intensive mining (2017–2018) activity periods. Due to multiple reasons (logistics, mine access, weather conditions), study periods were not strictly identical from one year to another (2004: January to February; 2007: January to March; 2017: February to May; and 2018: January to March). However, this did not compromise data reliability since i) Tahiti Petrels have an extended breeding cycle (which lasts for about five months) (Villard et al. 2006), and ii) they have been proved to exhibit an asynchronous phenology (i.e. throughout the year) (Ravache et al. 2020), meaning this species does not show any particular peak of vocal activity.

Tahiti Petrel calls were counted at 114 call count stations distributed among nine different sectors of the Koniambo massif (Figure 1). A call count consisted of counting all Tahiti Petrel calls for 10 minutes. At the beginning of the study (2004 and 2007), 95 call count stations were set up across eight different sectors. In 2017–2018, 15 of these stations could not be reached again or located precisely, and 15 new stations had to be set up as close as possible to the old ones. Four new call count stations were created for the Coco sector, which was not surveyed in 2004. Eleven of the call count stations of the pre-mining period became unreachable because of mining impact and could not be re-surveyed. A total of 159 and 128 call counts were monitored and repeated during the pre- and full-mining periods, respectively. All 10-minute call counts were conducted during night-time, from 30 minutes after sunset until about 01h00 i.e. during the species’ night-time peak of vocal activity (Titmus 2017, authors’ pers. obs). Given the large area of the Koniambo massif, the 10 minutes of listening was the appropriate time to perform the maximum call counts during the peak of vocal activity of this species. For each call count, the number, as well as start and end times of calls heard from the ground and/or in flight, were recorded. The number of calls recorded during 10-minute counts was then averaged for each call count station in 2004–2007 (n = 95) and 2017–2018 (n = 103) to describe and compare vocal activities between the pre-mining and full-mining periods respectively, following the method of Borker et al. (2014) and Oppel et al. (2014b). For each year of study, the same listening effort could not be made for logistical and/or weather-related reasons. Therefore, not all call counts stations were strictly repeated for each year of the study. Analyses were first performed on the full dataset, i.e. all call counts stations combined, regardless of listening effort and time period (n = 114), but for more refined results and to avoid time-related biases, we then compared only the 25 call counts stations repeated year-to-year. From those 25 repeated call counts stations, a total of 182 call counts were collected (2004: n = 25, 2007: n = 75, 2017: n = 57, 2018: n = 25). This implied considering only data from Bilboquet, Centre, Guerioiu Sud, Manguen and Trazy sectors for annual comparisons.

Our field surveys were carried out under quite homogeneous conditions to avoid potential bias from different factors such as poor weather (Jones et al. 1990, McKown 2008) or moonlight
avoidance by nocturnal seabirds returning to their breeding sites (Mougeot and Bretagnolle 2000, Bourgeois *et al.* 2008, Ravache *et al.* 2020), which could affect colony attendance.

## Estimation of habitat change

Mining leads to the destruction of the vegetation cover and the development of bare ground. To assess the overall habitat change on the Koniambo massif due to the intensification of mining activities, we compared the Percentage of Bare Soil Cover (PBSC) between 2001 (before industrial mining) and 2016 (during full mining activity). The PBSC around each call count station was estimated using orthophotos (geometrically corrected satellite imagery of the Earth's surface) of the Koniambo massif obtained from the KNS Environment Department and using QGIS software 2.18 (QGIS Development Team 2016). A 400-m radius buffer was applied around each call count station, corresponding to the maximum detection distance estimated for Tahiti Petrel calls (700 m; authors' pers. obs) and precluding any buffer overlap. The PBSC at each station was then estimated visually using the National Vegetation Classification method (Rodwell 2006). We used eight of the 10 Domin scale values: 3 = <4%, 4 = 4-10%, 5 = 11-25%, 6 = 26-33%, 7 = 34-50%, 8 = 51-75%, 9 = 76-90%, 10 = 91-100%. Given the limited availability of bare soil percentage data, the years 2001 and 2016 were used as proxies for the pre-mining and full-mining periods, respectively.

## Location of breeding sites

In 2004 and 2007, Tahiti Petrel burrows were searched on the day following the night-time call count survey in areas with high vocal activity, and/or where landings were observed, using a burrowscope (Turbotech TTBS050 model and LCD7TFTHV (LED B/L) with dominator V3 modular 3D FPV headset model). A probability index of active breeding burrows was assigned to each breeding site based on the evidence found. The status “Certain” was assigned to areas with at least one burrow showing clear and fresh signs of digging, with one or two adults present, or containing an egg, a chick, eggshells, or regurgitation residues. The status “Possible” was assigned to areas where Tahiti Petrel vocal activity was recorded repeatedly on the ground (i.e. at least during two call counts) but where none of the previous evidence was found. All nesting areas identified in 2004 and 2007 (January to March, recorded with a GPS) were reassessed in 2017 and 2018 (January to May) through extensive and repeated ground searches.

## Statistical analysis

All statistical analyses were conducted using R software version 3.4.4 (R Core Team 2016). The non-parametric Kruskal-Wallis (K-W) test was used to compare Tahiti Petrel vocal activity between all call counts. Variation in the vocal activity of the Tahiti Petrel were then studied using a generalised linear mixed effect model assuming a Poisson distribution using the package “lm4” v.1.1-21 (Bates *et al.* 2014). The number of calls was considered the numerical response variable, and the period (i.e. “pre-mining” for 2004–2007 and “full-mining” for 2017–2018), time categories, sectors, and PBSC index were all included as categorical fixed effects. Call count stations were also included as a random effect. The usefulness of the random effect was tested by computing an ANOVA test on two full models, including the effect or not. Model assumptions were checked both graphically and statistically (following Zuur *et al.* 2010, Zuur and Ieno 2016), with the correlation matrix of the model, a Shapiro-Wilk normality test, the calculation of the overdispersion index and a Levene variance homogeneity test with the package “car” v3.0.8 (Fox and Weisberg 2019). The relative contribution of each categorical variable to variations in call counts was assessed through an analysis of deviance on the fitted model. Post hoc pairwise comparisons with Tukey p value correction were conducted with the package “emmeans” v1.4.7 (Lenth 2020).
# Results

## *Variations in Tahiti Petrel vocal activity*

Call counts were performed across five different years of field survey, for a total of 65 nights and 107 hours of listening. The average number of Tahiti Petrel calls per 10 min, all sectors and years combined, was 2.26 ± 0.24. Based on the full dataset (n = 114), the vocal activity was significantly higher during the pre-mining period compared to the full mining period (K-W $\chi^2$ = 31.9, df = 1, P = 0.001). On the basis of annual data (n = 182), the period and time categories had a significant impact on the vocal activity ($\chi^2$ = 118.245, P < 0.001 and $\chi^2$ = 111.771, P < 0.001, respectively, Table 1). Again, the overall vocal activity was significantly higher during the pre-mining compared to the full mining period (mean = 3.8 ± 8.8 calls per 10 min, n = 372 and mean = 0.6 ± 1.2, n = 53, respectively, Tukey post-hoc test: + 1.8 ± 0.2, P < 0.001, Figure 2a). Time categories also contributed to explain the vocal activity (Figure 2b), with a significantly higher “peak” of contacts between 23h00 and 00h00 (mean = 6.7 ± 14.7, n = 80) compared to 19h00–20h00 (mean = 0.6 ± 1.7, n = 25, Tukey post-hoc test: + 2.7 ± 0.3, P < 0.001), 20h00–21h00 (mean = 3.1 ± 6.3, n = 138, Tukey post-hoc test: + 1.6 ± 0.2, P < 0.001), 21h00–22h00 (mean = 1.6 ± 3.0, n = 69, Tukey post-hoc test: + 1.2 ± 0.2, P < 0.001), 22h00–23h00 (mean = 2.9 ± 9.2, n = 104, Tukey post-hoc test: + 1.4 ± 0.2, P < 0.001), and 00h00–01h00 (mean = 2.2 ± 3.9, n = 9, Tukey post-hoc test: + 2.2 ± 0.4, P < 0.001; Figure 2b). However, the vocal activity was lower at the beginning of the night (19h00–20h00) compared to the following hours (20h00–23h00, Tukey post-hoc test: -1.1 ± 0.2 to -1.5 ± 0.3, P < 0.001; Figure 2b). If a slightly higher vocal activity was recorded in two sectors (Centre and Trazy, Figure 2c) and very few contacts were recorded at one site (Guerioum Sud), these differences were not significant while accounting for the other effects. Similarly, slightly fewer vocal activities were recorded at call count stations with a PBSC index of 4 and 6 (Figure 2d), but these differences were not significant in our model. Our model had an R² of 0.58 for fixed effects only, and of 0.83 for the full model.

## *Assessment of Tahiti Petrel breeding sites*

In 2004–2007, five “certain” and 33 “possible” breeding areas were identified. In 2017–2018, no conclusive evidence of Tahiti Petrel breeding was observed, neither in the previously identified breeding areas nor at any new site despite intensive effort (34 days of search for a total of c.272 hours of field survey) (Figure 3).

# Discussion

## *Petrel vocal activity and habitat degradation*

Our results show a significant decline in the vocal activity of Tahiti Petrels in the Koniambo massif after 10 years of developing nickel-mining activities. The same general trend is observed whether comparing the full or repeated annual dataset. To the best of our knowledge, this result is the first temporal assessment of vocal activity change for the species worldwide. As suggested in previous studies (e.g. Buxton *et al.* 2013, Oppel *et al.* 2014b), vocal activity can be considered a proxy of

<table><caption>Table 1. Summary of the analysis of deviance on the fitted model</caption><thead><tr><td>Variables</td><td>Chi sq</td><td>P</td></tr></thead><tbody><tr><td>Periods</td><td>118.245</td><td>&lt;2e-16</td></tr><tr><td>Sectors</td><td>9.319</td><td>0.053</td></tr><tr><td>PBSC</td><td>8.499</td><td>0.074</td></tr><tr><td>Time categories</td><td>111.771</td><td>&lt;2e-16</td></tr></tbody></table>
<figure><img src="image_74.png" /></figure>

Figure 2. Results of the Tahiti Petrel vocal activity (call counts n = 25): (a) represents the log of vocal activity per period, (b) represents the log of vocal activity per time category, (c) represents the log of vocal activity depending on sectors and (d) represents the log of vocal activity depending on the Percentage of Bare Soil Cover (PBSC). For (a) and (b), different letters indicate a significant difference in vocal activity. No significant difference in vocal activity was detected for sectors and the PBSC index. White dots represent extreme values.

population size, since it can be positively correlated with nest density. Therefore, we suggest that the dramatic decrease in vocal activity revealed by this study could reflect a substantial and rapid decrease in the Tahiti Petrel breeding populations.

Despite standardised conditions to avoid biases we acknowledge that the listening effort was likely not the same every year as experience and detection skills may vary among investigators. Working on industrial sites also involves a lot of constraints with little control over the intervention schedule, which depends on many other parameters dictated by the industrialist. However, we are confident that we took all necessary precautions to avoid these potential biases and strongly believe the decline in vocal activity is a good proxy for a decline in population size, such as suggested by Oppel et al. (2014b).

After 10 years of mining activity, and consequently a direct modification or destruction of the habitat, our data did not reveal a significant correlation between PSBC and Tahiti Petrel vocal activity. There are two possible explanations. First, the philopatric behaviour of the Tahiti Petrel and its high nest-site fidelity imply that adult breeders return to the same burrows each year, even if they are not suitable for reproduction anymore. However, Procellariid breeders can take several years to build another burrow and nest again (Brooke 2004). A time lag could then occur between the destruction of the Tahiti Petrel habitat and when it starts nesting again. Second, the noise, light
<figure><img src="image_75.png" /><figcaption>Figure 3. Number of breeding sites depending of the breeding sites status (“possible” or “certain”) during the pre- and full-mining periods.</figcaption></figure>

or particulate pollution linked to the mining activity possibly represent additional disturbances that prevent petrels from finding alternative nesting sites and may lead to desertion of the breeding site (Reville *et al.* 1990, Saha and Padhy 2011, Smith *et al.* 2012, Duarte *et al.* 2015). Intense light pollution can also disorient Tahiti Petrels, as burrow-nesting seabirds are guided and attracted by lights when they fly at night. On the ground, this can lead to increased mortality by increasing predation risk on stranded individuals (Le Corre *et al.* 2002, Rodríguez *et al.* 2017). In addition, road building for mining operations, presence of buildings and food at mines can indirectly facilitate the spread of invasive species such as feral cats and rats (Croxall *et al.* 2012), which are both predators of the Tahiti Petrel. In New Caledonia, feral cats have been reported to prey on Tahiti Petrels on mine sites (Palmas *et al.* 2017).

Finally, population structure may also influence Procellariid vocal activity. A population of such nocturnal seabirds is generally composed of breeders (which return to their burrows each year to breed), non-breeders (young birds not engaged in reproduction but physiologically able to do so) and pre-breeders (those which have not yet reached sexual maturity) (Warham 1996). Studies on nocturnal petrels have suggested that non-breeders are responsible for the majority of call rates within the colony (Warham 1996). In contrast, breeders tend to remain silent until they enter their burrows, especially when the breeding season is well advanced (Warham 1996, Schreiber and Burger 2001). The vocal activity of the studied Tahiti Petrel population suggests that it may now be composed mainly of non-breeders with high vocal activity, especially near breeding sites, but who only occasionally return to the ground (James 1985, Warham 1990, Monteiro *et al.* 1999, Arneill *et al.* 2019). However, it is difficult to establish a link between a net population decline and a temporal change in population structure associated with the reproductive status of individuals, based solely on the temporal monitoring of vocal activity. This is why we coupled our approach to a temporal monitoring of the nesting sites.
## Evolution of petrel breeding sites

A total of 38 breeding sites (five “certain” and 33 “probable”) were identified in 2004–2007 in the Koniambo massif thanks to high vocal activity. However, in 2017–2018, no active breeding burrows, breeding pairs or chicks were found in these potential breeding sites despite the persistence of vocal activity. These results suggest that the calls that are still recorded could be due to non-breeding individuals flying around but not nesting. Although our data indicate a decrease of the breeding population of Tahiti Petrels on the Koniambo massif (based on burrows search), the remaining vocal activity suggests that, because of the philopatric behaviour of the species, a small population still visits and frequents this area but fails to find suitable nesting sites. This remaining population probably consists – in addition to non-breeders – of former breeding individuals whose burrows were destroyed or turned unsuitable by mining activities. It is likely that they are now unable to find suitable novel breeding sites in their area of hatching (Brooke 2004, Arneill et al. 2019). According to our results, we believe that the extirpation of the species from the Koniambo massif is likely to occur soon and we call for urgent conservation effort focused on offering suitable nesting sites to the few remaining individuals.

## Implications for conservation strategies

The positive relationship between the vocal activity and time categories from 20h00 to 0oh00 suggests a higher vocal activity around 23h00. This new information about the vocal activity pattern of the Tahiti Petrel should be taken into account in future acoustic studies and for more effective breeding site searches.

Assuming that adult Tahiti Petrels looking for breeding sites still regularly visit the area, providing them with new favourable and attractive breeding sites or mates would represent an efficient conservation strategy to prevent their extirpation. This means creating artificial breeding sites to provide favourable and attractive conditions for breeding (Kildaw et al. 2005, Jones and Kress 2012), including artificial burrows (Bolton et al. 2004, Carlile et al. 2012, Bourgeois et al. 2015), absence of introduced predators (Miskelly et al. 2009, Jones et al. 2011) as well as vocal and olfactory stimulation (Podolsky 1990, Parker et al. 2007, Buxton and Jones 2012b). These artificial breeding sites should ideally be located in reserves (i.e. unaltered and protected areas) or in sectors where operations have ceased and are being ecologically rehabilitated (i.e. reconstitution of soil and revegetation). These ecological refuges should be selected based on several characteristics derived from field measurements and observations: physical parameters, vegetation and substrate cover, level of disruption (light and noise pollution), etc. Such an effort could rapidly stop the decline of the Tahiti Petrel population in the Koniambo massif. For population rehabilitation, artificial breeding sites could also be created outside the mining concessions in locations offering new safe breeding areas to prospecting individuals and former breeders. However, without the urgent implementation of appropriate conservation measures, the Tahiti Petrel population of the massif will be extirpated, as this is probably also the case on many other mining massifs in New Caledonia.

## Acknowledgements

This study was carried out on the Koniambo massif was commissioned by the mining company to understand and mitigate the impacts that its activities may have on the Tahiti Petrel population, but also to develop appropriate conservation and restoration strategies. Funds and support for this study were mainly provided by Koniambo Nickel SAS, with additional support by the CNRT “Nickel et son Environnement” (Transloc-Petrel project), the DAFE-NC and by the Province Sud (PhD scholarship to A.P.). We warmly thank the many collaborators and students involved in the fieldwork, especially Andreas Ravache, Malik Oedin, Jean-Yve Meunier, Yves Papillon, Mathieu Matthivet and Edouard Bourget.
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RESEARCH ARTICLE

# Nest-site selection and its influence on breeding success in a poorly-known and declining seabird: The Tahiti petrel

*Pseudobulweria rostrata*

Angélique Pagenaud<sup>1,2*</sup>, Andreas Ravache<sup>1,2</sup>, Karen Bourgeois<sup>2</sup>, Mathieu Mathivet<sup>1</sup>, Édouard Bourguet<sup>3</sup>, Éric Vidal<sup>1,2</sup>, Martin Thibault<sup>1</sup>

1 UMR ENTROPIE, IRD, IFREMER, CNRS, University of Reunion, University of New Caledonia, Noumea, New Caledonia, 2 University of Aix-Marseille, CNRS, IRD, University of Avignon, IMBE, Centre IRD, Noumea, New Caledonia, 3 EB Expertise, Noumea, New Caledonia

* angelique.pagenaud@ird.fr

<figure><img src="image_77.png" /></figure>

## Abstract

The Tahiti petrel (*Pseudobulweria rostrata*) is a rare and declining seabird whose breeding biology and nest-site selection are poorly known. Nest-site selection is critical to seabird population fitness, and understanding the factors driving it is essential for designing effective conservation measures. Here, we measured several variables (topographical, physical and environmental) to characterize Tahiti petrel nesting habitats and burrows (i.e., width, height, depth and type: rocky cavity, dug into the soil or under a root) on Nemou Island in New Caledonia. The data were clustered using the HCPC (Hierarchical Clustering on Principal Component) method to identify principal habitat groups. This method was combined with logistic regressions to examine the influence of the variables on nest-site selection and breeding success. Our results showed that nest-site selection is linked to habitat groups (a combination of substrate and vegetation data), slope, orientation and soil depth, while breeding success is only influenced by nest characteristics (i.e., burrow type and width). Tahiti petrels prefer to nest on steep slopes in mature forests with rocky substrate and deep soil. Burrows were scattered in small sub-colonies or isolated pairs, suggesting that nest-site selection depends on habitat quality rather than conspecific density. The study also revealed that breeding success is lower in rocky cavities and increases in burrows with wide entrances. Our nest-site selection survey is the first for the genus *Pseudobulweria*, and provides critical information for designing effective conservation programs in New Caledonia and the Pacific.

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### OPEN ACCESS

**Citation:** Pagenaud A, Ravache A, Bourgeois K, Mathivet M, Bourguet É, Vidal É, et al. (2022) Nest-site selection and its influence on breeding success in a poorly-known and declining seabird: The Tahiti petrel *Pseudobulweria rostrata*. PLoS ONE 17(4): e0267408. https://doi.org/10.1371/journal.pone.0267408

**Editor:** Vitor Hugo Rodrigues Paiva, MARE – Marine and Environmental Sciences Centre, PORTUGAL

**Received:** October 4, 2021

**Accepted:** April 7, 2022

**Published:** April 27, 2022

**Copyright:** © 2022 Pagenaud et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

**Data Availability Statement:** All data associated with this paper can be found without restriction in DataVerse Repository https://doi.org/10.23708/QO40D7.

**Funding:** Funds and support for this study were mainly provided by the Province Sud (PhD scholarship to A.P.), the CNRT "Nickel et son environnement" and the DAFE-NC. The funders had no role in study design, data collection and

## Introduction

Many seabird populations are declining due to invasive alien species (cats *Felix catus*, rats *Rattus spp.* and pigs *Sus scrofa*), fisheries bycatch and various anthropogenic pressures (e.g., nest-ing habitat destruction, light and noise pollution, egg collection, disturbance and climate change) [1–3]. Over the past 60 years, these threats have contributed to an overall decline of
analysis, decision to publish, or preparation of the
manuscript.

**Competing interests:** The authors have declared
that no competing interests exist.

70% in the global seabird population [3]. Seabirds are considered bio-indicators of marine
environmental health [4] and ecosystem engineers on land [5]; therefore, limiting this decline
is essential to preserve their natural habitats. On land, conservation and restoration programs,
including predator control and nesting habitat improvement, can reduce this decline by
expanding existing colonies or restoring historical populations [6]. However, basic knowledge
about the biology, ecology and nesting habitat selection (i.e., nest-site selection) of the species
is often limited [7–9].

Johnson [10] ranked habitat selection into four category: selection of the physical or geo-
graphical range of a species (first-order selection), selection of a home range of individual or
social group (second-order selection), selection of various habitat patches within the home
range (third-order selection), and selection of specific resources within a habitat patch (fourth-
order of selection) [11]. Nest-site selection corresponds to the fourth-order selection, in which
habitat characteristics are used as specific resources. Studies based on habitat characteristics
generally examine nesting site preference by comparing the sites used with available sites [11,
12]. Thus, nest-site selection relies on several factors, including local and micro-climate [13],
conspecific social attraction [14, 15], vegetation and topography (i.e., soil type, slope, aspect,
elevation) [16–18] and predation [19]. Most seabirds are faithful to their breeding site [20],
providing fitness advantages over the years [21]. Thus, understanding the mechanisms of nest-
site selection in declining species is essential to plan adapted conservation strategies.

Monitoring breeding success is critical for determining the status of populations and their
responses to anthropogenic and environmental stressors. The selection of good-quality nest-
sites is a significant predictor of breeding success [22, 23], along with the quality of breeders
based on their breeding experience [24]. Besides, for burrow-nesting seabird species like
petrels (i.e., order Procellariidae), burrow characteristics including depth, orientation and sin-
uosity impact breeding success by modulating the birds' exposure to predation, flooding, high
temperature or collapse [17, 23]. However, the relative contribution of nest-site selection and
burrow characteristics to breeding success is still understudied in seabirds [25-27].

Previous studies have described the nest-site selection of some burrow-nesting Procellarii-
dae species [18, 28–32], but quantitative information for the genus *Pseudobulweria*, the world's
most threatened seabird genus, remains scarce [33]. *Pseudobulweria* includes one extinct spe-
cies (*Pseudobulweria rupinarum*) and four extant species. The Beck's petrel (*Pseudobulweria
becki*), the Fiji petrel (*Pseudobulweria macgillivrayi*) and the Mascarene petrel (*Pseudobulweria
aterrima*) are all considered Critically Endangered by the IUCN. The last species, the Tahiti
petrel (*Pseudobulweria rostrata*), is a cryptic species currently listed as Near-Threatened,
under suspected decline and in lack of quantitative data [34]. The Tahiti petrel faces two major
threats: predation by introduced vertebrates (mainly cats and rats) at their breeding sites and
destruction of the breeding sites by mining activity [1, 35, 36]. Minor other threats often
occur, such as light pollution induced by mining activity and cities, plastic ingestion and
bycatch at sea [2, 37]. Its world population is estimated between 10,000 and 20,000 breeding
individuals [34, 38], with colonies distributed over several tropical Pacific islands: American
Samoa, Fiji, French Polynesia and New Caledonia, for an estimated terrestrial area about 6 850
000 km². However, the exact area for occupancy is not known [34]. The current lack of basic
knowledge about the biology, ecology and population distribution of the Tahiti petrel prevents
the implementation of efficient conservation or restoration strategies that could reverse popu-
lation decline. Villard et al. (2006) and Rauzon & Rudd (2014) pioneered the monitoring of
Tahiti petrel and provided preliminary data about the vocal repertoire and the breeding biol-
ogy of the species [39, 40]. Nest site management and threat mitigation actions dedicated to
Tahiti petrel are currently confronted with an urgent need for quantitative assessments of its
population distribution, nest-site selection, and burrow characteristics [6, 41].
We investigated the Tahiti petrel nest-site selection, burrow characteristics and their influ-
ence on breeding success at a New Caledonian breeding colony. Our study specifically aimed to
determine: (i) the habitat characteristics that influence the location of the Tahiti petrel nesting
sites, and (ii) the main burrow features influencing Tahiti petrel breeding success. We moni-
tored and surveyed Tahiti petrel for the first time in a colony isolated from the most direct
human pressures present on the mainland. The set of habitats naturally available on Nemou
Island represents the diversity of breeding habitats present in New Caledonia. The Tahiti petrel
is a nocturnal and cryptic species. The data collected will help conservation managers identify
the most suitable habitats to preserve the Tahiti petrel population, especially before starting min-
ing activities at a new site. This study also intends to improve the general species knowledge.

**Materials and methods**

Ethics statement

This study was conducted with the approval and authorization of the legal authority in charge of the environment and biodiversity of the South Province of New Caledonia: Direction de l'Environnement de la Province Sud, permit n° 836-2019/ARR/DENV. The Tahiti petrel is a legally protected species under the environmental legislation of the South Province. No contact or handling of animals was made during the study.

Study site and species

This study was carried out on Nemou Island (20.38°S, 164.04°E South-West Pacific Fig 1a),
located off the eastern coast of New Caledonia (1 km from Grande Terre) (Fig 1b), between
May 2018 and July 2020. Nemou is an island of 125 hectares (Fig 1c), rising to 168 m above sea
level. It is uninhabited but occasionally visited for the day or camping activities. Invasive spe-
cies present on the island include rats (Rattus rattus and Rattus exulans) and Rusa deers (Cer-
vus timorensis russa). A feasibility study for rat eradication showed low rat population
abundance (catch rate approximatively 1.5% for a total of 556 trap nights [42]). Before this
study, the Tahiti petrel population on the island was estimated to be a few tens of breeding
pairs. No other Procellariform species are known to nest on this island. Nemou Island is under
the customary authority of Petit Borendi tribe. Customary formalities were carried out with
Radji Kainda, the island owner, to gain access to the study site. Radji Kainda himself organized
transportation to the study site.

The species is known to breed in two types of habitats: most nest-sites are located in moun-
tainous areas (up to 1,200 m) or high rocky islands (between 40 m and 200 m), while only a
few burrows are found in backshore sandy areas. Besides, on mountainous areas, Tahiti petrel
nest in forests and maquis [36] but not in the rainforest (personal observations). They also nest
on some islets in the lagoon [39]. Like many procellariid species, Tahiti petrels spend most of
their time foraging at sea, except during their breeding period, when they depend on terrestrial
environments to reproduce. New Caledonian populations breed asynchronously (i.e., breeding
cycles extend throughout the year) [43], an atypical feature in Procellariidae [38]. Villard et al.
[39] estimated incubation and chick rearing periods to last 55 and 120 days, respectively [39].
Individuals are considered philopatric, with high nest fidelity, as they return to the same breed-
ing site each year and breed in the same burrow for several years [39, 44].

Nest-site selection

Systematic ground surveys, searching for and mapping Tahiti petrel burrows, were conducted
for the first time at Nemou Island throughout 2018. Adult Tahiti petrels return on land at
<figure><img src="image_79.png" /><figcaption>**Fig 1. Map of Nemou Island.** Location of the island in the Pacific region (a) and New Caledonia (b), and location of Tahiti petrel burrows (n = 116), sub colonies (n = 13) and vegetation classification (c). The anthropized area is where some of the vegetation has been cut down to allow visitors to come ashore and camp in an artificial structure. Republished from the New Caledonia Government and South Province under a CC BY license, with permission from Déborah David and Justin Pilotaz respectively, original copyright 2011.</figcaption></figure>

https://doi.org/10.1371/journal.pone.0267408.g001

night only to breed, incubate the egg or feed the chick. They display high vocal activity at the beginning of the night. Nocturnal vocal activity monitoring preceded each survey at the beginning of the night when vocal activity peaks (i.e., between 6 pm and 9 pm) [36]. Two observers climbed the two highest ridges of the island (western ridge and eastern ridge of Nemou, Fig 1) to do call counts and listen to the Tahiti petrel activity during the first few hours of 10 nights (see S1 Fig). Positioned simultaneously on each ridge, the observers were able to identify areas of highest vocalization activity over the whole island (> 100 calls / 5 min). These areas were then visually searched to map the breeding burrows. While this approach allowed for mapping all petrel sub colonies, remote and isolated burrows may have been missed.

We recorded the coordinates of each burrow using a GPS (Garmin® GPS 64 S, mean accuracy: ± 3 m). After producing the burrows distribution map from field prospections, we recorded Tahiti petrel nest-site characteristics through a set of biotic and abiotic variables measured in 102 quadrats (8 m x 8 m) [28]. Quadrats were randomly positioned using the searching function for random locations in a polygon in Qgis (3.2, v3.2.3-Bonn, gaussian distribution) so that only half of them included Tahiti petrel nesting areas (n = 51), and all habitats on the island were represented (S1 Fig). Burrow patches in Fig 1 represent a circular buffer of 15 m radius around the GPS locations recorded on top of monitored burrows. Each
patch encompasses closely related burrows and surrounding landing/take-off areas. Within each quadrat, 14 topographical, physical and environmental variables were collected [28] (Table 1). Physical variables were measured at the centre of each quadrat, while substrate and vegetation variables were visually estimated (in percent) over the whole surface of each quadrat. We established and oriented all quadrats (North-South and East-West) using a 30 m tape measure and a compass. Elevation, orientation and slope were determined at the center of each quadrat using a GPS (GpsMap 64s), with a hand-held compass and an inclinometer. Because orientation is a circular variable, the mean orientation was calculated using the function *mean.circular* of the R package “circular” v.0.4–93 [45, 46]. Soil depth was calculated as the average of the maximum penetration of a metal bar into the soil based on 5 measurements: 4 measurements at 1 m from each corner towards the centre of the plot, and one at the centre [47]. The metal bar was pushed into the ground as far as possible until it encountered an obstruction or was buried. Distance to the coastline was determined using Qgis for each quadrat, and trunk (i.e., the main stem of a living tree) diameter was assessed using a 30 m tape measure at 1 m above the ground.

## Breeding success

Burrows were monitored every two months between May 2018 and July 2020 (14 field sessions of 5 consecutive days each) to account for breeding asynchrony. We used a "burrowscope", i.e., a miniature endoscopic camera (Adult Tortoise Camera System) connected with virtual reality glasses (Fatshark Dominator V3) and LCD screen (fat Screen 7" diagonal display LCD colour monitor). Each burrow was monitored (empty, eggs, chicks, adults). Only the burrows actively used by Tahiti petrels for breeding (i.e., burrows with eggs, chicks, adults) were included in the analyses. Active breeding burrows were considered successful or unsuccessful based on whether the egg laid led to a fledgling or not. Breeding success was determined by the percentage of eggs that gave a fledgling.

We used 16 variables describing the burrows and their environment to account for burrow characteristics as potential predictors of breeding success (Table 1). Physical, substrate and vegetation variables were measured within a 1 m radius of each burrow. Burrow dimensions were measured using a 30 m tape measure, except for the depth of the burrow, which was obtained by measuring the length of the burrowscope tube that could be inserted into the burrow. Roots and/or stones at the burrow entrance were considered as burrow entrance protection from predators (e.g., rats). Tahiti petrels used three types of burrows: 1) dug under a root or a boulder (BB), 2) dug in the soil (BS), and 3) a rocky cavity that they did not need to dig (C).

## Statistical analyses

All statistical analyses were conducted using the R software version 4.0.3 [48]. We replicated the same analytical framework on two datasets to determine the best predictors of (i) nest-site selection and (ii) breeding success of Tahiti petrels. The binary response variable of the first dataset was presence (1) and absence (0) of burrows for each of the 102 quadrats, so the unit of replication in the model is the quadrat. The second dataset consisted of 87 active burrows monitored throughout the study (May 2018 to July 2020). With this second dataset, we considered breeding success as a binary response variable (i.e., success (1), failure (0)), so the burrow is the unit of replication of the model. Variations along the North-South and East-West axes were included in the models using the cosinus and sinus of the orientation angle) [46]. Multicollinearity among explanatory variables was assessed using the function *cor* of the R package Stats v.4.0.3 [48]. Variables correlated beyond 50% (e.g., substrate and vegetation cover
<table><caption>Table 1. Variables measured in 102 quadrats (51 within and 51 outside nesting areas) and around 87 burrows (35 successful and 52 unsuccessful).</caption><thead><tr><th rowspan="3"></th><th rowspan="3">Variables</th><th rowspan="3">Units</th><th colspan="4">Nest site selection (14 variables)</th><th colspan="2">Breeding success (16 variables)</th></tr><tr><th colspan="2">Quadrats (mean ± se)</th><th colspan="2">Burrows (mean ± se)</th></tr><tr><th>Within nesting areas</th><th>Outside nesting areas</th><th>Successful</th><th>Unsuccessful</th></tr></thead><tbody><tr><td rowspan="4">Physical</td><td>orientation</td><td>degree</td><td>x</td><td>168.6 ± 19.4</td><td>158.4 ± 14.7</td><td>x</td><td>216.6 ± 19.8</td><td>165.7 ± 17.6</td></tr><tr><td>elevation</td><td>meter</td><td>x</td><td>47.5 ± 4.8</td><td>55.4 ± 5.3</td><td>x</td><td>43.5 ± 5.0</td><td>49.4 ± 4.9</td></tr><tr><td>slope</td><td>degree</td><td>x</td><td>33.1 ± 1.4</td><td>25.3 ± 1.5</td><td>x</td><td>16.5 ± 1.6</td><td>17.9 ± 1.5</td></tr><tr><td>distance to coastline</td><td>meter</td><td>x</td><td>94.0 ± 8.6</td><td>107.4 ± 11.1</td><td></td><td></td><td></td></tr><tr><td rowspan="5">Substrate</td><td>outcrop cover</td><td>percent</td><td>x</td><td>13.6 ± 2.8</td><td>3.4 ± 1.0</td><td></td><td></td><td></td></tr><tr><td>block cover</td><td>percent</td><td>x</td><td>16.0 ± 2.4</td><td>12.1 ± 2.4</td><td>x</td><td>19.7 ± 4.3</td><td>28.6 ± 4.2</td></tr><tr><td>scree cover</td><td>percent</td><td>x</td><td>23.5 ± 2.9</td><td>27.8 ± 3.3</td><td>x</td><td>15.0 ± 2.3</td><td>14.3 ± 2.4</td></tr><tr><td>loose soil</td><td>percent</td><td>x</td><td>47.2 ± 4.1</td><td>60.0 ± 4.6</td><td>x</td><td>65.0 ± 4.1</td><td>58.5 ± 4.2</td></tr><tr><td>soil depth</td><td>cm</td><td>x</td><td>24.3 ± 1.8</td><td>18.0 ± 2.2</td><td></td><td></td><td></td></tr><tr><td rowspan="5">Vegetation</td><td>tree cover</td><td>percent</td><td>x</td><td>83.9 ± 4.3</td><td>68.6 ± 5.2</td><td>x</td><td>75.3 ± 5.1</td><td>78.3 ± 4.6</td></tr><tr><td>shrub cover</td><td>percent</td><td>x</td><td>31.7 ± 2.9</td><td>45.0 ± 4.4</td><td>x</td><td>34.8 ± 3.5</td><td>38.3 ± 4.0</td></tr><tr><td>grass cover</td><td>percent</td><td>x</td><td>41.9 ± 4.5</td><td>25.0 ± 2.3</td><td>x</td><td>27.1 ± 3.0</td><td>25.8 ± 2.5</td></tr><tr><td>trunk count (diam > 20cm)</td><td>-</td><td>x</td><td>41.5 ± 6.7</td><td>27.2 ± 3.0</td><td></td><td></td><td></td></tr><tr><td>trunk count (diam < 20cm)</td><td>-</td><td>x</td><td>2.6 ± 0.3</td><td>2.3 ± 0.4</td><td></td><td></td><td></td></tr><tr><td rowspan="5">Burrow</td><td>entrance width</td><td>cm</td><td></td><td></td><td></td><td>x</td><td>19.5 ± 0.9</td><td>22.0 ± 1.0</td></tr><tr><td>entrance height</td><td>cm</td><td></td><td></td><td></td><td>x</td><td>13.7 ± 1.4</td><td>12.5 ± 0.7</td></tr><tr><td>burrow depth</td><td>cm</td><td></td><td></td><td></td><td>x</td><td>105.3 ± 6.2</td><td>119.7 ± 5.3</td></tr><tr><td>tunnel shape</td><td>no, straight, winding</td><td></td><td></td><td></td><td>x</td><td>-</td><td>-</td></tr><tr><td>entrance protection</td><td>without, with</td><td></td><td></td><td></td><td>x</td><td>-</td><td>-</td></tr><tr><td rowspan="2">Conspecifics</td><td>type</td><td>root, boulder (BB), soil (BS), rocky (C)</td><td></td><td></td><td></td><td>x</td><td>-</td><td>-</td></tr><tr><td>burrow density (3m radius)</td><td>-</td><td></td><td></td><td></td><td>x</td><td>0.5 ± 0.1</td><td>0.5 ± 0.1</td></tr></tbody></table>

x indicates variables considered predictors in nesting habitat selection and/or breeding success models of the Tahiti petrel.

https://doi.org/10.1371/journal.pone.0267408.t001

variables) were considered highly correlated with a risk of multicollinearity. Because substrate and vegetation cover variables were highly correlated, the following three-step analysis was designed: 1) a Hierarchical Clustering on Principal Component (HCPC) to cluster the multiple and non-independent habitat predictors, 2) a logistic regression including clusters assignments as categorical variables with the other available predictors and, 3) a downward stepAIC to select the best of all available and non-independent predictors [49, 50].

In step 1, an HCPC method was applied to reduce the number of vegetation and substrate cover variables to a smaller set of uncorrelated components using the "FactoMiner" v.2.3 and "Factoextra" v.1.0.7 R packages [51, 52]. The number of clusters was chosen based on the "FactoMiner" package's suggested optimal level. Using Ward's minimum variance criterion, the HCPC method clusters variables into groups based on the distance (i.e., inherent similarity) between them to minimize the total within-cluster variance. As a result, the vegetation and substrate cover variables were merged into 3 clusters for the two analyses, depending on their position along the two dimensions of a Principal Component Analysis (PCA). The first two components of the PCA extract the most important information from the dataset. The main variables associated with the PCA are indicated with their contribution (i.e., c). The PCA function scaled and normalized each variable with a zero mean and a unit standard deviation. Thus, using HCPC to develop a typology based on the first two components of the PCA
provides a more consistent and accurate classification because noise is reduced in the analysis. For each cluster, three values represented the contributions of the variables: the Overall Mean of the variable (OM), the Mean value In the Cluster (MIC) and a value-test (v.test) representing the sign and significance of the MIC-OM difference. A v.test value greater than 1.96 indicates a significant contribution of the variable to the cluster at a 95% confidence level [53].

In step 2, we modelled nest-site selection with the physical variables measured in the quadrats (i.e., orientation, slope, elevation and distance to the coastline, Table 1) and the HCPC habitat clusters using logistic regression. We then modelled breeding success using burrow characteristics (Table 1), density, orientation, elevation, slope and HCPC habitat clusters using logistic regression as well.

In step 3, after constructing full logistic regressions models, we conducted a downward stepwise predictors selection based on corrected Akaike's Information Criteria for limited sample sizes (AICc) using the R package "MASS" v.7.3-53 [53, 54]. This method determines which explicative variable removal would result in the most significant AICc reduction beginning with the full model for each response variable. These explicative variables were then removed, and the procedure repeated until no AICc reduction was possible. Models with the lowest AICc were ranked with the *aictab* function of the R package "AICmodavg" v 2.3-1 [55], using Akaike's Information Criterion for limited sample sizes (AICc), the difference between the model AICc and the lowest AICc (ΔAICc) and AICc weights (wAICc). Models with ΔAICc ≤ 2 and 95% confidence intervals not overlapping zero were selected [56] (see S1 and S2 Tables). Models with ΔAICc ≤ 2 were then averaged with the "MuMin" R package v.1.43.14 [56, 57]. To avoid any overestimation of averaged estimates, we provide estimates from the full-model averaging, meaning that parameters estimates were set to 0 when not included in a model [58].

We built and tested our logistic regressions, the best model for binary response variables, using the R package "stats" v.4.0.3 [48]. Model assumptions and predictive performances were checked statistically [59, 60]. The overdispersion index validated the absence of overdispersion in our data. Odds ratios represent the ratio of the probability of the event of interest occurring (i.e., burrow presence (1) or success (1)) to the probability of it not occurring. The pseudo R² estimated the predictive performance of our models [61, 62]. Values are shown with a mean ± standard error.

# Results

## Burrow distribution and nest-site selection

A total of 116 burrows were found across the whole island. Among these burrows, 113 showed evidence of occupation by Tahiti petrels (i.e., feather, fresh guano), of which 87 showed evidence of breeding (i.e., egg, chick or adult). Burrows were relatively distant from each other, with a burrow density of 0.03 ± 0.01 per m² (i.e., within quadrats) (Table 1) and an average distance to the nearest burrow of 7.4 ± 0.8 m. The study detected 13 sub-colonies of 9 ± 1.6 burrows, and five isolated burrows, totalising 87 breeding pairs (Fig 1).

The first two principal components given by the PCA performed on vegetation and substrate cover variables explained 28.9% and 20.0% of their total variation, respectively. The first principal component was mainly associated with grass (c = 24.2%), tree (c = 18.5%) and shrub (c = 15.7%) cover. The second principal component was mainly linked to loose soil (c = 35.8%), scree (c = 18.6%) and tree (c = 13.5%) cover (Fig 2a). The HCPC analysis classified vegetation and substrate variables into three "habitat" groups (Fig 3). Group 1 (G1) represented "open" habitats, characterized by loose soil (69%), grassy (mean in category (MIC) = 3.4) and shrubby vegetation (MIC = 3.2). In contrast, group 2 (G2) represented "closed" habitats with
<figure><img src="image_80.png" /><figcaption>Fig 2. Classification of habitat groups based on Principal Component Analysis (PCA) and Hierarchical Clustering on Principal Components (HCPC) (i.e., dimensions) applied to substrate and vegetation data in (a) the 102 quadrats characterized for the analysis of Tahiti petrel nest-site selection and (b) the 87 1-m radius plots around burrows used to identify factors influencing Tahiti petrel breeding success. Axes represent scores for every 102 quadrats and 87 1-m radius plots from variables included in the PCA. The percentage in brackets is the variation of the variables explained by each dimension. Plots were classified into three groups for both the Tahiti petrel nest-site selection and breeding success. Each group represented a specific habitat type, as described in the results section. For nesting habitat selection (a): G1 = open habitats, G2 = closed habitats and G3 = rocky habitats. For breeding success (b): GA = habitat mainly with tree and scree, GB = habitat mainly with rocky substrate and GC = habitat with low vegetation.</figcaption></figure>

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mature forest (Tree: MIC = 4.9; Trunks: MIC = 3.2) and 78% of loose soil. The last group (G3) represented "rocky" habitats with rocks (blocks, outcrops and scree) and trees (MIC = 4.3) (Fig 3).

The best model describing nest-site selection by Tahiti petrels for nesting included habitat
groups (as defined by the HCPC analysis), slope, orientation and soil depth (ΔAICc = 0.0,
wAICc = 0.63, CI = 5.03–12.64, S1 Table). Rocky and closed habitats were the best predictors
of burrow presence, with rocky habitats much more likely to have burrows (Table 2a). Steep
slopes were the model’s second major predictor, with a positive relationship between slope and
Tahiti petrel burrow presence (Table 2a). North and east orientation also had a positive effect.
Finally, Tahiti petrel burrow presence was positively influenced by soil depth (Table 2a). The
model explained 42% of the Tahiti petrel nest-site selection variation (R² = 0.42).

Burrow characteristics and factors influencing breeding success

Burrow entrances were wide (about 20.8 ± 0.6 cm) and located on steep slopes, 55.4% (i.e., 17.2° ± 1.1°) on average across the entire elevation gradient (Table 1). Breeding success was 0.32 ± 17.5 on average for the 108 breeding attempts monitored throughout the 26 months of our study. The first two principal components given by the PCA performed on vegetation and substrate cover variables explained 33.2% and 31.5% of their total variation, respectively. The first principal component was mostly correlated to trees (c = 32.5%), grass (c = 21.8%) and
<table><caption>Fig 3. V.test value (positive or negative), mean in category value (MIC) and overall mean value (OM) of habitat selection groups by Tahiti petrels, based on Hierarchical Clustering on Principal Components (HCPC). Group 1 (G1): habitat mainly composed of grass, shrubs and loose soil; Group 2 (G2): habitat mainly composed of loose soil, trees and large trunks; Group 3 (G3): habitat mainly composed of blocks, outcrops, scree and trees with large trunks.</caption><thead><tr><th>Variables</th><th>G1 = Open habitats</th><th>G2 = Closed habitats</th><th>G3 = Rocky habitats</th></tr></thead><tbody><tr><td><strong>Grass</strong></td><td></td><td></td><td></td></tr><tr><td>v.test</td><td>7.7</td><td>-3.1</td><td>-3.9</td></tr><tr><td>MIC</td><td>3.4</td><td>1.0</td><td>1.0</td></tr><tr><td>OM</td><td>1.7</td><td>1.7</td><td>1.7</td></tr><tr><td><strong>Shrubs</strong></td><td></td><td></td><td></td></tr><tr><td>v.test</td><td>5.8</td><td>-3.2</td><td>-2.2</td></tr><tr><td>MIC</td><td>3.2</td><td>1.2</td><td>1.5</td></tr><tr><td>OM</td><td>1.9</td><td>1.9</td><td>1.9</td></tr><tr><td><strong>Loose soil</strong></td><td></td><td></td><td></td></tr><tr><td>v.test</td><td>3.2</td><td>5.4</td><td>-7.9</td></tr><tr><td>MIC</td><td>69.1</td><td>78.1</td><td>24.4</td></tr><tr><td>OM</td><td>52.0</td><td>52.0</td><td>52.0</td></tr><tr><td><strong>Trees</strong></td><td></td><td></td><td></td></tr><tr><td>v.test</td><td>-7.6</td><td>4.3</td><td>2.7</td></tr><tr><td>MIC</td><td>1.6</td><td>4.9</td><td>4.3</td></tr><tr><td>OM</td><td>3.8</td><td>3.8</td><td>3.8</td></tr><tr><td><strong>Large trunks</strong></td><td></td><td></td><td></td></tr><tr><td>v.test</td><td>-4.6</td><td>2.2</td><td>2.0</td></tr><tr><td>MIC</td><td>0.5</td><td>3.2</td><td>3.0</td></tr><tr><td>OM</td><td>2.4</td><td>2.4</td><td>2.4</td></tr><tr><td><strong>Blocks</strong></td><td></td><td></td><td></td></tr><tr><td>v.test</td><td>-3.1</td><td>-3.1</td><td>5.5</td></tr><tr><td>MIC</td><td>5.1</td><td>5.7</td><td>24.8</td></tr><tr><td>OM</td><td>14.0</td><td>14.0</td><td>14.0</td></tr><tr><td><strong>Outcrops</strong></td><td></td><td></td><td></td></tr><tr><td>v.test</td><td>-2.8</td><td>-2.2</td><td>4.5</td></tr><tr><td>MIC</td><td>1.2</td><td>3.1</td><td>16.4</td></tr><tr><td>OM</td><td>8.5</td><td>8.5</td><td>8.5</td></tr><tr><td><strong>Scree</strong></td><td></td><td></td><td></td></tr><tr><td>v.test</td><td></td><td>-3.7</td><td>3.7</td></tr><tr><td>MIC</td><td></td><td>13.3</td><td>34.8</td></tr><tr><td>OM</td><td></td><td>25.6</td><td>25.6</td></tr></tbody></table>

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blocks (c = 15.9%). The second principal component was mainly linked to loose soil (c = 41.2%), blocks (c = 33.5%) and grass (c = 14.1%) (Fig 2b). The final typology of the HCPC results also classified vegetation and substrate variables into three groups, but none was significant in the final model. Group A was mainly composed of trees (MIC = 4.3), loose soil and scree (73.7% and 20.0% cover, respectively). Group B was mainly composed of a rocky substrate (60% of block cover). In contrast, the last group was made of low vegetation, including grass (MIC = 4.7), shrubs (MIC = 3.7) and loose soil.

The best model included burrow type, width and depth as predictors of Tahiti petrel breeding success (ΔAICc = 0.00, wAICc = 0.50, CI = 0.3–5.39, S2 Table). Burrow type was the first predictor of the model and showed a negative effect of rocky cavities (C) on breeding success
<table><caption>Table 2. Summary tables of logistic regressions explaining the influence of nest-site selection on burrow presence (a) and the factors influencing breeding success (b) of the Tahiti petrel at Nemou Island.</caption><thead><tr><th></th><th>Estimate</th><th>Std.error</th><th>P—value</th><th>Odds Ratios</th></tr></thead><tbody><tr><td colspan="5"><strong>a) Nest-site selection variables</strong></td></tr><tr><td>Intercept</td><td>-7.76</td><td>1.98</td><td>1.08e-04</td><td>***</td></tr><tr><td>Cos Orientation (North-South)</td><td>1.15</td><td>0.37</td><td>0.002</td><td>**</td></tr><tr><td>Sin Orientation (East-West)</td><td>1.36</td><td>0.57</td><td>0.02</td><td>*</td></tr><tr><td>Slope</td><td>0.09</td><td>0.03</td><td>0.007</td><td>**</td></tr><tr><td>G2</td><td>3.42</td><td>1.13</td><td>0.003</td><td>**</td></tr><tr><td>G3</td><td>3.85</td><td>1.08</td><td>0.0004</td><td>***</td></tr><tr><td>Soil depth</td><td>0.07</td><td>0.02</td><td>0.004</td><td>**</td></tr><tr><td>Distance to the nearest coastline</td><td>0.01</td><td>0.01</td><td>0.07</td><td>.</td></tr><tr><td colspan="5"><strong>b) Breeding success variables</strong></td></tr><tr><td>Intercept</td><td>-2.22</td><td>1.35</td><td>0.10</td><td>—</td></tr><tr><td>Burrow entrance width</td><td>0.10</td><td>0.04</td><td>0.02</td><td>*</td></tr><tr><td>Burrow depth</td><td>0.01</td><td>0.01</td><td>0.08</td><td>.</td></tr><tr><td>Burrow Type BB</td><td>-0.36</td><td>0.52</td><td>0.48</td><td>.</td></tr><tr><td>Burrow Type C</td><td>-1.75</td><td>0.71</td><td>0.01</td><td>*</td></tr></tbody></table>

Asterisks indicate significant difference as follows:

https://doi.org/10.1371/journal.pone.0267408.t002

(Table 2b), but there was no significant relationship between burrows dug under blocks or roots (BB) and breeding success. Burrow entrance width was the second determinant of the model, with a positive relationship between burrow width and breeding success. Burrow depth had a slight, yet insignificant, positive impact on breeding success (Table 2b). However, this model explained only 12% of Tahiti petrel breeding success variation (R² = 0.12).

## Discussion

We provide the first assessment of the most suitable habitats for breeding Tahiti petrels. Mature trees and rocky substrates appear as critical features in the species breeding behaviour. We show that the width of the burrow entrances is positively correlated to breeding success.

## Habitat suitability

Our results show that habitat groups, slope, orientation and soil depth influence the selection of nesting sites by Tahiti petrel. Closed and rocky habitats most significantly explained the occurrence of Tahiti petrel burrows at our study site. These habitats include mature trees (i.e., trees with large trunks) on which Tahiti petrels like to climb to take off [63]. Therefore, a plausible hypothesis is that Tahiti petrels favour closed and rocky habitats for nesting because they offer easy take-off points [18]. Indeed, adult Tahiti petrels can walk up to 50 m away from their burrow to reach a mature tree from which they can take off, and claw marks have been observed on several trunks (Pagenaud, pers. obs.). Similar behaviour was observed for Cook's petrels, which can walk up to 100 m to take off from a tree [18]. Mature trees could also represent a landmark for adult breeders to locate their burrows. Mature forests are often preferred because they provide shelter from intense weather, sun or aerial predators [64] and a more stable substrate thanks to well-developed root systems. Conversely, Tahiti petrels seem to avoid

<footnote>* p < 0.05,</footnote>

<footnote>** p < 0.01,</footnote>

<footnote>*** p < 0.001.</footnote>
areas without branches, and only one burrow (out of 113) was found in such a situation. Tahiti petrels also prefer rocky habitats for nesting, especially with scree and blocks. This type of substrate may improve the stability of the cavity [47, 65] and minimize burrow collapse due to heavy rainfall or large mammals (i.e., rusa deer on Nemou Island) [65]. Moreover, outcrops can be used as take-off platforms like trees.

Our nest-site selection models revealed that Tahiti petrels prefer northerly or easterly oriented slopes (i.e., facing the prevailing north-east winds), perhaps because they facilitate landing through drag force and take-off through lift force [44]. Finally, most Tahiti petrel burrows were found on steep slopes, a characteristic often observed in other seabirds, particularly Procellariiformes [19, 64–66]. Steep slopes generally provide better water drainage, preventing water from entering the burrow [67]. Slopes also facilitate burrow excavation [68] and ease bird take-off [65, 69]. We also found a link between soil depth and the presence of Tahiti petrel burrows. Burrowing seabirds usually select soft or deep soil, which is easier to dig in [28, 70, 71].

We recommend that this set of predictors (i.e., habitat groups, slope, orientation and soil depth) be considered in habitat modelling analyses to identify other potential breeding sites. Applications include the identification of suitable nest-sites for Tahiti petrels in mining impacted areas on Grande Terre.

In addition, the overall distribution of Tahiti petrels may be influenced by the availability of breeding sites (mature forest, rocky habitat, steep slope, soil depth), which can be sought out in other archipelagos (e.g., Fiji, American Samoa, French Polynesia). Despite Nemou's small surface area and generally low altitude, the island hosts a diverse range of habitats, which match the majority of available known habitats for Tahiti petrels (as previously cited). Thus, the diversity of habitats on the island has been assumed as representative of habitats on Grande Terre and of petrel habitats in general in the region.

## Determinants of breeding success

Breeding success was lower in rocky cavities but improved when burrow entrances were wider. However, the environment directly surrounding the burrows seems to have no influence on breeding success compared to burrow features. Surprisingly, wider burrow entrances enhance Tahiti petrel breeding success. Indeed, smaller burrow entrances generally limit access to predators and protect from weather conditions [22, 72]. However, regular monitoring of burrows revealed three dead Tahiti petrel fledgling chicks trapped in burrow entrances smaller than average (about 10 cm wide). Larger burrow entrances could then be easier to pass through, with less risk of chicks getting trapped when they come out to exercise their wings daily or to fledge. A wider entrance could also make it easier for adults' access to the nest chamber [73]. Besides, a wide entrance enhances burrow ventilation and thermoregulation, improving chick survival [74].

As confirmed by previous observations, substrate type can significantly affect breeding success in seabirds [75]. Tahiti petrels prefer digging their burrows near rocky substrates, increasing roof stability during and after building the burrow. However, natural rocky cavities or crevices, where the nest chamber is made of stones or rocks (i.e., not dug by Tahiti petrels), could increase the probability of egg breaking. This feature has been reported for other procellariiform species such as the Macaronesian shearwater (*Puffinus lherminieri baroli*) or Cory's shearwater (*Calonectris diomedea*) [16, 76].

Overall, this study provides the first data on the breeding success rate for the species. This breeding success rate was relatively low (32%) compared to other Procellariids which typically have breeding success rates near or above 50% [77–81]. Breeding success can be affected at any stage (i.e., egg, chick or fledgling) by invasive predators such as rats, pigs or feral cats [82, 83].
However, the limited abundance of invasive predators on Nemou Island is unlikely to impact Tahiti petrel breeding success. A camera trap survey could provide additional data on the potential effects of rats or other nocturnal predators on the Tahiti petrel demography. Nest desertion during bad weather could also apply (e.g., storms) [84].

Further studies on the breeding biology of the species are needed to enhance our understanding of these interactions and identify potential additional factors impacting breeding success. The predictive performance of our breeding success model could be improved by including other predictors of the Tahiti petrels' breeder quality. Indeed, age, body size, foraging performance, experience and intra-specific competition could directly affect hatching success, fledging success and breeding success [85-87].

Finally, long-term monitoring of Tahiti petrel breeding success would provide additional data on the interannual variability of hatching, fledging and breeding success and could help identify the factors driving this variability [88, 89].

## Tahiti petrel population distribution

Our results indicate that Tahiti petrel burrows are isolated from each other with a burrow density of only 0.03 ± 0.01 per m². These results suggest that the species prefers to nest in low density within a large area. In contrast, most burrowing Procellariidae nest in crowded colonies like the wedge-tailed shearwater or Cory's shearwater [16, 90, 91]. Also, Tahiti petrels tend to nest in small sub-colonies or isolated pairs (i.e., one burrow). This patchy distribution suggests that nest-site selection is based on habitat quality rather than conspecific density [92]. Consequently, preserving nesting habitat is a critical factor for conservation.

In addition, this uneven distribution means that searching for burrows in large areas requires more effort and experience. This information can help observers to improve breeding ground survey strategies: they can identify smaller evidence (e.g., feather, down, guano) rather than searching for a colony.

Out of all the sites identified in New Caledonia (n = 34), our survey revealed that Nemou Island was the only accessible site where a large breeding population of Tahiti petrel (between 100 and 200 pairs) can be studied. This population of Tahiti petrels is the largest recorded to date in New Caledonia.

## Implications for conservation and future research

We believe that Nemou Island should be considered a refuge for Tahiti petrels in New Caledonia. The island is free from mining activities or other Procellariform species (e.g., the wedge-tailed shearwater *Ardenna pacifica*) that could compete with nesting sites and burrows. It is also very little impacted by human activities or introduced rodents and is free from feral cats. Therefore, Nemou Island is a valuable study site for conserving this poorly known, rare and declining species. Further actions should include raising public awareness of the species, increasing protection of nesting habitats, and eradicating rodents from Nemou Island.

Our study provides the first data about the Tahiti petrel nest-site selection and burrow characteristics. These data are essential for conservation purposes and can be used to (i) develop predictive habitat models that identify and map potential nesting areas in New Caledonia or other islands in the Pacific, (ii) facilitate the search for nesting sites in the vast mountain range of the main island of New Caledonia, (iii) and improve population recovery [93, 94].

## Supporting information

S1 Fig. Map of the quadrats distribution on Nemou Island. The map shows (i) the position of quadrats whitin and outisde Tahiti petrel (TP) nesting habitat, (ii) the location of the edge
of the TP sub colonies, and (iii) the vegetation classification. Quadrats are shown to scale. The
black section (Anthropized area) is the area where vegetation was cut out to set up a camping
area.

(PDF)

**S1 Table. Predictive habitat models selection for Tahiti petrel based upon corrected Akaike's Information Criterion.**

(PDF)

**S2 Table. Models statistics used to predict the influence of burrow characteristics and environmental variables on the Tahiti petrel breeding success.**

(PDF)

**Acknowledgments**

We warmly thank the customary of Nemou island, Radji Kainda, for his cooperation during the three years of the study and the many collaborators and students involved in the fieldwork, especially Mathilde Alphand, Cynthia Lagourgue, Nicolas Silva, Wilfried Weiss, Bernard Pelle-tier and Mégane Paul. We also thank Romain Barrière, who provided valuable data for the Nemou Island map.

**Author Contributions**

**Conceptualization:** Angélique Pagenaud, Karen Bourgeois, Éric Vidal.

**Data curation:** Angélique Pagenaud, Andreas Ravache, Martin Thibault.

**Formal analysis:** Angélique Pagenaud, Andreas Ravache, Martin Thibault.

**Funding acquisition:** Éric Vidal.

**Investigation:** Angélique Pagenaud, Andreas Ravache, Mathieu Mathivet, Édouard Bourguet, Martin Thibault.

**Methodology:** Angélique Pagenaud, Martin Thibault.

**Project administration:** Angélique Pagenaud, Martin Thibault.

**Resources:** Éric Vidal.

**Software:** Angélique Pagenaud, Andreas Ravache, Martin Thibault.

**Supervision:** Éric Vidal, Martin Thibault.

**Validation:** Angélique Pagenaud, Andreas Ravache, Karen Bourgeois, Mathieu Mathivet, Éric Vidal, Martin Thibault.

**Visualization:** Angélique Pagenaud, Andreas Ravache, Karen Bourgeois, Édouard Bourguet, Éric Vidal, Martin Thibault.

**Writing – original draft:** Angélique Pagenaud.

**Writing – review & editing:** Angélique Pagenaud, Andreas Ravache, Karen Bourgeois, Éric Vidal, Martin Thibault.

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94. Bolton M, Medeiros R, Hothersall B, Campos A. The use of artificial breeding chambers as a conservation measure for cavity-nesting procellariiform seabirds: A case study of the Madeiran storm petrel (*Oceanodroma castro*). Biol Conserv. 2004; 116: 73–80. https://doi.org/10.1016/S0006-3207(03)00178-2
<figure><img src="image_81.png" /></figure>
<figure><img src="image_82.png" /></figure>

PROJET ASAP

(Artificial Sites for Tahiti Petrel) :

Mise en place et suivi scientifique de colonies
artificielles de reproduction destinées aux pétrels
de Tahiti *Pseudobulweria rostrata* sur le site de
Koniambo Nickel SAS (KNS)

Rapport final (seconde version)
**Nota** : Ce document correspond à une seconde version du rapport final du programme ASAP (Artificial Sites for Tahiti Petrel) - Mise en place et suivi scientifique de colonies artificielles de reproduction destinées aux pétrels de Tahiti Pseudobulweria rostrata sur le site de Koniambo Nickel SAS (KNS), réalisé dans le cadre de la Convention 310182/00 IRD/KNS.

**Réalisation des missions de terrain et/ou examen des photos/vidéos/enregistrements sonores :**

Angélique Pagenaud (ingénieure d'étude IRD et doctorante), Mathieu Mathivet (ingénieur d'étude IRD puis chargé de projet Bird Conservation New Caledonia), Tao Joly (Stagiaire IRD), Wilfried Weiss (Doctorant IRD), Aurore Pupjapujane (stagiaire IRD), Théo Verdier (stagiaire IRD).

**Rédaction du document** : Angélique Pagenaud, Mathieu Mathivet & Eric Vidal

**Remerciements**

Il nous est particulièrement agréable de remercier ici l'ensemble des équipes de KNS avec qui cette première phase de mise en place et d'expérimentation des colonies artificielles a été conduite avec succès, et particulièrement Sarah Eurisouké, Elodie Bouyé et Ludovic Moto. Nous souhaitons également remercier chaleureusement les équipes techniques de l'IRD qui se sont mobilisées pour élaborer et construire les dispositifs de repasse vocale automatisées, notamment Pierre Desfontaines, Angélo Di-Matteo et Benoit Adrian. Enfin, nous remercions les collègues du programme LIFE+ Pétrels de la SEOR (Réunion) et du Department of Planning & Environment de Nouvelle-Galles du Sud (Australie) qui ont partagé leur expérience et prodigué de précieux conseils, respectivement Jérôme Dubos et Nicholas Carlile.
# 1. Introduction

Du fait de la situation particulière des populations de pétrels de Tahiti rencontrées en sites miniers, notamment sur le massif du Koniambo (grande rareté des individus reproducteurs mais présence et persistance d'oiseaux prospecteurs fréquentant très régulièrement les sites) (Pagenaud et al. 2019; Pagenaud et al. 2021), nous avons proposé à KNS d'inclure dans sa stratégie et ses actions de conservation et de restauration des populations de pétrels de Tahiti du massif, la mise en place et le suivi de sites artificiels de reproduction couplés à des dispositifs de dynamisation de la reproduction (protection anti-prédateurs introduits, terriers artificiels, simulation vocale et olfactive). Dans cette approche assez globale et totalement novatrice pour la Nouvelle-Calédonie et pour l'espèce ciblée, il ne s'agissait pas uniquement de créer des conditions favorables à d'éventuelles futures opérations de translocation (cf programme Transloc-Pétrels financé par le CNRT ; Vidal et al. 2019) mais surtout de tenter de fixer des individus encore présents sur les sites miniers (individus prospecteurs ou anciens reproducteurs) mais qui à l'heure actuelle semblent ne pas s'y reproduire (probablement car les conditions y étaient devenues trop dégradées). L'objectif à moyen et long terme est de permettre le maintien durable de populations reproductrices de pétrels au sein même des emprises minières, au niveau de sites optimisés et sécurisés afin d'en favoriser la recolonisation des habitats une fois achevées les phases d'exploitation et de revégétalisation des sites. Ce programme expérimental, pionnier en la matière et à valeur d'exemple et de test, a également vocation à entrainer d'autres exploitants miniers à développer des stratégies similaires au niveau de leurs sites respectifs. Bien évidemment, cette stratégie de conservation et de restauration des colonies de pétrels au sein des emprises minières ne sera pas suffisante pour assurer la conservation durable des populations de pétrels à l'échelle de la Nouvelle-Calédonie et d'autres actions, hors sites miniers devront être entreprises en parallèle, notamment au niveau des principales colonies connues (îlots Némou, Mato, Uo, roches de la Ouaième) ou sur les sites identifiés qui hébergent des petite populations éparses. Nous nous employons à cela également, en partenariat avec nos partenaires de Bird Conservation NC et les différentes institutions impliquées (Provinces en particulier).

Le présent programme collaboratif entre l'IRD et KNS fait suite aux éléments présentés dans le plan d'action et le cahier des charges rédigés par notre équipe et issus du programme TRANSLOC-PETRELS financé par le CNRT et la DAFE et réalisé en partenariat avec KNS. Ce programme s'appuie sur les synthèses et analyses bibliographiques réalisées dans le cadre de ce travail précédant mais également sur les investigations de terrain complémentaires et les nombreux échanges et discussions que nous avons eus avec les équipes de KNS sur la thématique de la conservation / restauration des populations de pétrels au sein des emprises minières. Dans le cadre du programme TRANSLOC-PETRELS, les analyses bibliographiques réalisées et la synthèse des données existantes avaient clairement permis de conclure que les conditions n'étaient pas réunies en Nouvelle-Calédonie pour envisager pour l'heure, comme action compensatoire aux impacts miniers sur les populations de pétrels, la mise sur pied d'opérations de
translocations sensu stricto de pétrels au sein de l'espace minier (i.e. sur le modèle des opérations développées ces dernières années en Nouvelle-Zélande, Australie ou Hawaii), qu'il s'agisse des populations de pétrels de Tahiti ou des populations de Pétrels de Gould.

Trois raisons majeures à cela :

-i / Un déficit encore marqué de connaissances fines indispensables sur la biologie, voire la physiologie de ces espèces, notamment lors du cycle de reproduction, qui ne permettrait pas d'envisager d'entreprendre sereinement les lourdes et complexes opérations de manipulation et déplacement des poussins (notamment le maintien en captivité et le nourrissage manuel).

-ii / Le fait qu'aucune colonie d'effectifs suffisants ne soit actuellement connue en site minier pour faire éventuellement l'objet d'une opération de translocation et fournir des individus (poussins) en nombre suffisant. La biologie et l'écologie très particulière des oiseaux, notamment du Pétrel de Tahiti, font de toute façon de cette espèce, une espèce dont la conservation peut difficilement s'appuyer majoritairement sur des approches de type « translocation ».

-iii / La rareté et le manque de personnels formés et de structures ad-hoc (structures associatives ou bureaux d'étude) pour conduire ce genre de programme qui nécessitent à la fois des interventions humaines complexes avec un niveau de technicité élevé et la mobilisation de personnels nombreux lors de phases-clés des protocoles de translocation.

Suite aux prospections de terrain réalisées en 2017 et 2018 en lien étroit avec les services de KNS, nous avons proposé que soient installés des dispositifs expérimentaux destinés à accueillir des colonies artificielles au niveau de deux sites du massif du Koniambo, conçus, cadrés et suivis scientifiquement par notre équipe. Le suivi scientifique est nécessaire dans le cadre de ce type de démarches encore expérimental, à la fois afin de mesurer l'efficacité du procédé au travers du suivi de différents indicateurs et également d'en augmenter la pertinence, en réajustant si nécessaire, les différents compartiments.

## 2. Matériels et Méthodes

Cinq étapes ont été réalisées afin de mettre en place des sites artificiels destinés à accueillir des colonies de pétrels :

(1) Sélection des sites artificiels (2019-2021)

(2) Mise en place des terriers artificiels (2021)

(3) Installation des repasses sonores automatisées (2021)

(4) Déploiement des pièges létaux anti-chats (trapinators) et anti-rats (Good nature A24) (2021-2022)

(5) Mise en place de suivis et indicateurs de suivis (2022)
## 2.1. Sélections des sites pour l'installation des dispositifs expérimentaux

Les caractéristiques suivantes ont été recherchées au sein des sites disponibles proposés par KNS et non concernés par l'exploitation : (i) Des sites en cours de réhabilitation écologique et/ou (ii) des sites isolés et préservés au sein des emprises minières, et présentant des caractéristiques favorables : conditions écologiques aussi proches que possibles des habitats fréquentés par les pétrels de Tahiti, en particulier pente marquée, couverture végétale arbustive importante, fréquentation humaine réduite en vue d'accueillir de futurs sites artificiels de reproduction ; un accès relativement aisé en véhicule, à la fois pour la mise en place des dispositifs et des actions et pour leur suivi technique et scientifique ; perturbations directes liées à l'activité minière réduites autant que possible (bruit, pollution lumineuse, infrastructures) et donc éloignées du site choisi ou masquées par le relief ; une orientation aisée vers la mer est souhaitée pour favoriser les déplacements des oiseaux.

Des visites de terrain ont été réalisées en partenariat avec les services de KNS pour identifier et caractériser ces sites potentiels sur le massif du Koniambo et des fiches typologiques descriptives ont été produites. Au final, deux sites ont été sélectionnés pour accueillir chacun une expérimentation complète, un sur le secteur de Confiance et un autre sur le secteur de Taléa. Le secteur de Confiance présentait l'avantage d'avoir une couverture végétale de type paraforestière « naturelle » un peu ouverte, non dégradée, avec des parties rocheuses et composée d'arbres matures et d'une forte pente. Le secteur de Taléa avait l'avantage d'être dans une réserve botanique, avec une couverture végétale de type paraforestière « naturelle » et éloignée de toutes sources de pollution. Ce secteur était aussi en forte pente, ce qui est une caractéristique primordiale dans la sélection de l'habitat pour les pétrels de Tahiti (Figure 2.1) (Pagenaud et al. 2022).
<figure><img src="image_83.png" /><figcaption>**Figure 2.1** : A gauche : Colonie artificielle de Confiance avec l'enceinte n°1 en 1er plan puis les terriers enfouis et l'enceinte n°2 au 2ème plan. Haut à droite : Terriers artificiels sur la colonie de Taléa en train d'être mis en place et enfouis. Bas à gauche : Les mêmes terriers artificiels enfouis sur la colonie de Taléa.</figcaption></figure>

## 2.2. Mise en place de terriers artificiels pour favoriser la reproduction au niveau de sites expérimentaux

Des nids artificiels (nichoirs ou terriers artificiels) ont déjà été utilisés avec succès ailleurs dans le monde pour l'étude et la conservation de plusieurs espèces d'oiseaux marins qui nichent dans des terriers ou des cavités (e.g. Bourgeois et al. 2015). Ces dispositifs peuvent être utilisés pour compléter l'habitat de reproduction disponible (Priddel et Carlile 1995, Lalas 1999, De León et Mínguez 2003, Bried et al. 2009, Madeiros et al. 2012, Sherley et al. 2012), augmenter le succès de reproduction et la survie adulte (Byrd et al. 1983, Priddel et Carlile 1995, De León et Mínguez 2003, Bolton et al. 2004, Libois et al. 2012, Sherley et al. 2012), et établir (c'est notre cas ici) de nouveaux sites de reproduction lorsqu'ils sont couplés à une translocation des poussins et/ou une attraction vocale ainsi que le contrôle d'autres perturbations (Podolsky 1990, Miskelly et Taylor 2004, Priddel et al. 2006, Bester et al. 2007, Miskelly et al. 2009, Carlile et al. 2012, Gummer et al. 2015), et également parfois rendre l'étude du comportement et de la biologie de reproduction plus facile et moins dérangeant en facilitant l'accès aux oiseaux (Wilson 1986, 1993, Bolton 1995, Gaston 1996, Gummer et al. 2015).

Les taux d'occupation des terriers artificiels peuvent largement varier d'un site à un autre en raison de variations dans la qualité/optimalité de l'habitat (Bolton, 1995 ; Priddel et Carlile, 1995), dans l'activité vocale conspecifique (Podolsky 1990), dans la disponibilité naturelle des terriers (De León et Mínguez 2003) et dans la pression de prédation ou de dérangement (Libois et al., 2012). L'occupation des terriers artificiels peut
également être accélérée et augmentée lorsque ceux-ci sont utilisés pour remplacer directement les terriers naturels (De León et Mínguez 2003). Étant donné que les caractéristiques des terriers naturels (dimensions, forme, environnement environnant comme la pente, la végétation et le couvert du substrat, l'altitude et la densité des conifères) influent sur leur occupation et le succès de reproduction des oiseaux (p. ex. Ramos et al. 1997 ; Bourgeois et Vidal 2007), il est probable que des caractéristiques semblables auraient aussi une incidence sur la qualité et l'attractivité des terriers artificiels. Dans ce cadre, un important travail de mesure et de caractérisation de terriers naturels fréquentés par les pétrels de Tahiti a été réalisé en parallèle de ce projet, permettant de disposer d'informations uniques sur les caractéristiques qui sont recherchées pour les sites de reproduction (Tableau 2.1). Ces données proviennent de la mesure de 116 terriers sur l'îlot Némou (au large de Thio), 27 terriers sur l'îlot Mato (Grand Lagon Sud) et 3 terriers sur le site de Tiébaghi.

<table><caption>**Tableau 2.1** : N : Nombre de terriers de pétrels de Tahiti mesurés ; L : Largeur moyenne des terriers ± écart-type (cm) ; Lmax : Largeur maximale ; Lmin : Largeur minimale (cm) ; H : Hauteur moyenne des terriers ± écart-type (cm) ; Hmax : Hauteur maximale (cm) ; Hmin : Hauteur minimale (cm) ; P : profondeur moyenne des terrier s± écart-type (cm) ; Pmax : Profondeur maximale (cm) ; Pmin : Profondeur minimale (cm) ; Tunnel : Absence/Présence d'un tunnel d'entrée ; Sinuosité : tunnel droit ou coudé</caption><thead><tr><th>N</th><th>L</th><th>Lmax</th><th>Lmin</th><th>H</th><th>Hmax</th><th>H min</th><th>P</th><th>Pmax</th><th>Pmin</th><th>Tunnel</th><th>Sinuosité</th></tr></thead><tbody><tr><td>129</td><td>19.3±6.2</td><td>50</td><td>9</td><td>12.3±4.2</td><td>26</td><td>6</td><td>108.4±37.1</td><td>230</td><td>40</td><td>8% sans 92 %avec</td><td>54% coude 46% droit</td></tr></tbody></table>

<figure><img src="image_84.png" /><figcaption>**Figure 2.2** : Design des terriers artificiels qui ont été mis en place sur les colonies artificielles du massif de Koniambo, avec un tunnel droit et un tunnel coudé. CA = couvercle amovible.</figcaption></figure>
Ainsi, 20 terriers artificiels ont été installés (avril 2021) sur les deux sites expérimentaux de Confiance et de Taléa (Figure 2.2 et 2.3). Pour ce projet, le choix des matériaux s'est porté sur du bois contreplaqué marin qui possède plusieurs avantages : il assez résistant dans le temps et aux conditions climatiques tropicales, il ne dégage que peu d'odeur, ne nécessite que peu d'entretien et l'assemblage est rapide et facile sur le terrain.
Les terriers installés sont composés de 4 plaques de contreplaqué marin de 400mm x 220mm (longueur x hauteur) de 18mm d'épaisseur, représentant les 4 côtés de la chambre principale. Ces plaques sont fixées entre elles par des vis auto-perforantes. Le toit est constitué par une plaque de contreplaqué marin de 18mm d'épaisseur et de dimensions 450mm x 450mm, de façon à dépasser des bords du terrier et éviter les infiltrations d'eau à l'intérieur de la chambre. Le toit est simplement posé sur le terrier pour faciliter le suivi d'occupation et limiter le potentiel dérangement sur des oiseaux nichant à l'intérieur lors des ouvertures et fermetures répétées. Sur le terrain, le dispositif est stabilisé par de grosses roches et de la terre.

<figure><img src="image_85.png" /><figcaption>**Figure 2.3** : Début de mise en place des terriers artificiels sur la colonie de Confiance (gauche) et de Taléa (droite), Massif de Koniambo.</figcaption></figure>

<figure><img src="image_86.png" /></figure>

La face avant du terrier a été percée à 60 mm du bas avec une scie cloche (ou scie trépan), afin d'emboiter un tube en PVC de 160mm de diamètre représentant le tunnel d'accès. Ce tube a été découpé au ¾ dans sa longueur de façon à ce que les pétrels soient toujours en contact avec le sol lorsqu'ils circulent à l'intérieur.
Sur les sites d'implantation des colonies artificielles, les emplacements de tous les terriers ont été repérés et géo-référencés. Les colonies étant situées sur des terrains à fortes pentes (>30°), les emplacements choisis ont été terrassés afin de pouvoir stabiliser le terrier et enterrer le tunnel (1, 2). Une couche de quelques centimètres de scorie fine a été étalée sur la surface du terrassement (3) puis recouverte d'une couche de terre (4). Cette opération a pour but de drainer de l'eau de pluie pouvant potentiellement entrer dans la chambre. Les terriers sont assemblés sur place, le tunnel est placé (5) puis fixé et étanchéifié avec du mastic (6). Un surplus de terre et de litière est ajouté à l'intérieur de la chambre du terrier. La chambre et le tunnel sont ensuite recouverts de terre et de litière de façon à être les plus dissimulés possible dans la végétation (7). Les terriers ont été installés soit dans le sens de la pente, soit perpendiculairement à la pente. Ils ont été montés puis enfouis sous de la litière pour imiter au maximum les terriers naturels et limiter l'exposition au soleil. Les terriers ont été disposés de façon dispersée (environ 2/3 m de distance entre eux), tout en restant groupés près de l'implantation prévue des enceintes sonores.

<figure><img src="image_87.png" /></figure>

## 2.3. Systèmes d'attraction sonore et olfactive

Afin d'accélérer le processus de colonisation des terriers artificiels, des systèmes d'attraction du pétrel de Tahiti ont été développés : (i) Un système d'attraction sonore automatisé pour diffuser des chants pré-enregistrés à des horaires définis et (ii) un système d'attraction olfactive qui consiste à disposer de la terre odorante prélevée dans des terriers naturels de pétrels de Tahiti.
## Systèmes d'attraction sonore

Ce type de dispositif de dynamisation artificielle, indispensable dans le cadre de telles opérations, consiste à diffuser régulièrement de nuit via des haut-parleurs des cris enregistrés (cris au sol ou cris en vol) de l'espèce cible dont on cherche à stimuler la reproduction (e.g Gummer et al. 2015 ; Young et al. 2018). Ils doivent être disposés dans des habitats favorables et à proximité des terriers artificiels, de façon à ce que les oiseaux, notamment ceux survolant la zone, puissent les entendre. Le positionnement exact des haut-parleurs est parfois contraint par la longueur de câble disponible ou l'éloignement aux panneaux solaires nécessaires à l'alimentation du système en énergie. L'attraction acoustique est généralement déployée sur des temps longs (plusieurs années), en complément des autres dispositifs artificiels. Cette méthode ne peut cependant porter des fruits et être efficace par rapport aux objectifs visés que si des oiseaux fréquentent et survolent régulièrement la zone concernée ; d'où l'importance de réaliser des points d'écoute nocturne pour s'assurer de la présence d'oiseaux prospecteurs en vol, ce qui a été confirmé sur les sites du massif du Koniambo (Pagenaud et al. 2019 ; Pagenaud et al. 2022). Cette méthode vise tout autant à encourager la fixation d'oiseaux prospecteurs à la recherche d'un site de reproduction que de favoriser le retour d'individus transloqués au stade poussin quelques années plus tôt. Les dispositifs d'attraction acoustique doivent être régulièrement contrôlés, notamment de nuit, de façon à s'assurer qu'ils sont effectifs et fonctionnels et que le volume sonore est suffisant pour que la diffusion des sons soit efficace et entendue des oiseaux prospecteurs, sans pour autant être trop forts pour des oiseaux qui seraient posés au sol à proximité des enceintes. Si des oiseaux s'installent et entament une reproduction, le volume sonore peut être légèrement abaissé de façon à ne pas occasionner de gêne aux oiseaux (e.g Gummer et al., 2015 ; Young et al., 2018). Enfin, étant donné le cycle de reproduction asynchrone des pétrels de Tahiti, avec la possibilité d'avoir des oiseaux en reproduction active à n'importe quelle période de l'année, les dispositifs d'attraction acoustiques devront demeurer en fonctionnement tout au long du cycle annuel.

Le dispositif d'attraction sonore est divisé en trois éléments : une boite énergie, une boite son et un système de diffusion (Figure 2.4). N'existant pas dans le commerce, ce système a été conçu en impliquant le service d'électronique du centre IRD sur la base de recommandations en provenance de collègues de la Réunion (Programme Life+ Pétrels).

La boite « énergie » était composée d' (i) un panneau solaire poly-cristallin de 160ww, (ii) une batterie 85Ah 12V et un régulateur de charge. Le panneau solaire a été fixé sur une structure en cornières aluminium de 40 cm x 40 cm x 3 cm, puis toute la structure a été fixée par des boulons/écrous/rondelles et inclinée à 22°Nord (maximum d'ensoleillement en Nouvelle-Calédonie).

Le câble énergie a été gainé puis enterré afin de résister aux intempéries, ainsi que pour prévenir les potentiels dégâts causés par les rongeurs. Deux sangles ont été fixées de part et d'autre de la structure afin de limiter les mouvements liés au vent (Figure 2.5).
<figure><img src="image_88.png" /><figcaption>**Figure 2.4** : Schéma du dispositif d'attraction sonore comprenant la boite « énergie », la boite « son » et le système de diffusion.</figcaption></figure>

<figure><img src="image_89.png" /><figcaption>**Figure 2.5** : Panneau solaire fixé à l'aide d'une structure métallique implantée dans le sol et de deux sangles de chaque côté.</figcaption></figure>
La boite « son », également conçue en interne à l'IRD selon des recommandations issues du programme LIFE+ Pétrels conduit à la Réunion, est composée : (i) d'un programmateur horaire 12V, (ii) d'un amplificateur 12V, (iii) d'un lecteur MP3. Ces trois dispositifs ont été placés dans une valise étanche (pelicase 1400) qui a été percée et équipée d'un presse-étoupe pour faire entrer le câble alimentation depuis la boite « énergie » et qui se branche sur le programmateur horaire. Deux sorties via des connecteurs étanches relient l'amplificateur au système de diffusion.

Le système de diffusion est composé de deux enceintes marines IP5x encastrées dans des caissons en contreplaqué marin de 18mm d'épaisseur. Le côté du caisson est percé pour faire passer le câble d'enceinte et étanchéifié avec du mastic une fois en place. Les deux brins du câble sont fixés directement sur les enceintes par des petites cosses puis reliés à la boîte « son » par les connecteurs étanches (Figure 2.6).

<figure><img src="image_90.png" /><figcaption>**Figure 2.6** : Boite « son » et enceinte mise en place sur la colonie de Taléa, massif du Koniambo.</figcaption></figure>

La bande son choisie correspond à une compilation de 15 minutes de chants de pétrels de Tahiti enregistrés précédemment par notre équipe via des enregistreurs « Song Meter Mini BAT » puis sélectionnés et assemblés sur le logiciel « Audacity ». Les chants enregistrés proviennent de plusieurs localisations et de plusieurs individus différents. Le dispositif de repasse vocale est configuré pour diffuser des séquences de chants tout au long de la nuit de façon à pouvoir être détectés par des pétrels de Tahiti ayant un cycle d'activité vocale généralement actif entre 18h et 2h du matin (Pagenaud 2017, Pagenaud et al. 2021). Suivant la période de l'année, l'heure de début de fonctionnement a été retardé ou avancé selon l'heure du coucher de soleil : début de la diffusion à partir de 18h entre mai et octobre environ, et 19h entre novembre et avril. Le programme de fonctionnement est le suivant :
<table><thead><tr><td>Heure de début de fonctionnement</td><td>Durée de fonctionnement</td></tr></thead><tbody><tr><td>18h00</td><td>15 min</td></tr><tr><td>18h30</td><td>15 min</td></tr><tr><td>19h00</td><td>15 min</td></tr><tr><td>19h30</td><td>15 min</td></tr><tr><td>20h00</td><td>15 min</td></tr><tr><td>20h30</td><td>15 min</td></tr><tr><td>21h00</td><td>15 min</td></tr><tr><td>21h30</td><td>15 min</td></tr><tr><td>22h00</td><td>15 min</td></tr><tr><td>22h30</td><td>15 min</td></tr><tr><td>23h00</td><td>15 min</td></tr><tr><td>23h30</td><td>15 min</td></tr><tr><td>00h00</td><td>15 min</td></tr><tr><td>00h30</td><td>15 min</td></tr><tr><td>01h00</td><td>15 min</td></tr><tr><td>01h30</td><td>15 min</td></tr><tr><td>02h00</td><td>15 min</td></tr><tr><td>03h00</td><td>15 min</td></tr></tbody></table>

## Attraction olfactive

Les pétrels de Tahiti étant connus, comme la plupart des pétrels, pour avoir un odorat développé leur permettant notamment de retrouver précisément leur terrier après de longs trajets en mer, une attraction olfactive a donc été ajoutée à l'attraction sonore. Pour cela, du sol a été récupéré dans des terriers naturels et récemment actifs sur l'île de Némou. Ce sol, imprégné d'odeur de pétrel de Tahiti, a été réparti dans les tunnels et le toit des terriers des deux colonies et au niveau des enceintes sonores. De plus, des morceaux de tissus ont été placés plusieurs jours dans des terriers naturels actifs de pétrels de Tahiti sur l'île de Némou, de façon à s'imprégner de l'odeur des pétrels, puis ont été installés à proximité des terriers artificiels pour servir de leurres olfactifs.

## 2.4. Contrôle des prédateurs introduits

Le succès d'installation et de reproduction des pétrels de Tahiti dans ces colonies artificielles est dépendant de l'élimination des prédateurs introduits de la zone, notamment des rongeurs et des chats harets (e.g. Palmas et al, 2017). Ces espèces peuvent avoir des impacts très importants sur les populations de pétrels de Tahiti. De par leur biologie et leur comportement, les pétrels sont particulièrement sensibles à l'impact des prédateurs introduits, particulièrement les rats invasifs (Rattus sp) qui peuvent impacter les stades œufs et jeunes poussins et les chats harets (Felis s. catus) qui prédatent les adultes et les grands poussins (Jones et al. 2008, Faulquier et al. 2009, Medina et al. 2011). Dans le cas du Pétrel de Tahiti, les études de terrain que nous avons initiées montrent un comportement assez particulier des poussins qui passent beaucoup de temps à l'extérieur des terriers (parfois plusieurs heures chaque nuit) même à un stade encore peu développé, les rendant particulièrement vulnérables à la prédation (Pagenaud et al. 2022). Dans le cadre de la mise en place de colonies artificielles et de dispositifs artificiels de dynamisation de colonies, la lutte contre les prédateurs introduits
constitue une action complémentaire absolument indispensable pour créer des conditions attractives et sécures pour les oiseaux qui chercheraient à se reproduire et augmenter leurs chances de survie en limitant les risques de mortalité aux différents stades démographiques (adultes, poussins, œufs).

**Rongeurs introduits** – Entre 14 et 15 pièges à piston « Goodnature A24 » ont été disposés sur chaque colonie artificielle. Ces pièges ont l'avantage de pouvoir éliminer jusqu'à 24 rongeurs sans recharge avec un appât conçu pour rester attractif jusqu'à 6 mois. Ils sont fixés par deux vis sur des arbres penchés de préférence, permettant une meilleure accessibilité par les rongeurs et une meilleure conservation de l'appât (qui coulera moins, Figure 2.7). Les nombres de frappes et de rongeurs retrouvés sont répertoriés à chaque passage sur les colonies. Les rongeurs tués peuvent être enlevés par des chats harets ou d'autres rongeurs.

<figure><img src="image_91.png" /><figcaption>**Figure 2.7 :** Pièges à piston Goodnature A24 fixé sur un arbre.</figcaption></figure>

**Chats harets** – Dans le cadre la mise en œuvre expérimentale de tels dispositifs sur une espèce et des sites atypiques à savoir le pétrel de Tahiti en contexte minier, nous avons initialement considéré, en accord avec KNS, que dans un premier temps l'opération pouvait être lancée sans être associée à l'installation d'un dispositif onéreux de clôture anti-prédateurs, au moins dans l'attente d'indices indiquant que l'attraction sociale fonctionnait et attirait les pétrels. Nous avons donc choisi de lui substituer (temporairement) une lutte active par piégeage au niveau de la zone expérimentale et de sa périphérie. La phase d'évaluation du bon fonctionnement et de l'efficacité des différents modules du dispositif sur l'espèce cible (terriers artificiels, attraction acoustique, lutte active contre les prédateurs), devait ensuite permettre de prendre une décision éclairée plus pérenne, sur l'intérêt ou pas de maintenir le dispositif et de le compléter par une clôture. Ainsi, pour la lutte contre les chats harets, des pièges létaux de type « Trapinator » ont été sélectionnés car nécessitant peu de passages réguliers de la part d'une équipe. Six pièges « Trapinator » ont ainsi été déployés sur chaque colonie artificielle. Ils sont fixés sur des arbres par des vis, à environ 30cm du sol. Ces pièges sont appâtés
avec de la sardine à l'huile qui semble être l'appât le plus attractif pour les chats harets en Nouvelle-Calédonie (observations personnelles). Les sardines sont placées dans un sac ziploc percé afin de diffuser de l'odeur sans sécher trop rapidement. Les appâts peuvent ainsi rester attractifs pendant une dizaine de jours et sont renouvelés à chaque passage de l'équipe (Figure 2.8).

Il faut cependant garder à l'esprit, que l'approche temporairement choisie au début de l'opération (pas de construction d'exclos), le temps de tester l'efficacité des autres modules du dispositif sur le Pétrel de Tahiti en contexte minier, est pour ainsi dire à contre-courant puisque l'option qui est quasi-systématiquement mise en œuvre dans ce genre d'opération consiste à protéger l'ensemble du site expérimental par une clôture anti-prédateurs (« fencing »). La construction d'une clôture présente en effet le double avantage d'une grande efficacité (le niveau d'abondance des prédateurs est réduit à zéro) et d'un caractère relativement permanent de l'efficacité de l'action (moyennant cependant une maintenance et un entretien du dispositif de façon à ce qu'il conserve son « étanchéité » aux prédateurs au fil des années). L'inconvénient majeur de ce genre de dispositif est bien entendu son coût (financier et logistique) initial élevé lors de la mise en place, notamment lorsque le dispositif doit être étanche et résistants à plusieurs types de prédateurs introduits (clôture solide pour résister aux cerfs et cochons, enfoncée dans le sol pour éviter que les rats ou les chats passent dessous et suffisamment haute et recourbée en son sommet pour empêcher les chats et les chiens, voire les cerfs de passer par-dessus et enfin à maille fine pour ne pas que les rongeurs puissent passer à travers), à fortiori lorsque ce type de dispositif est installé dans des sites pentus, rocheux et végétalisés comme le sont les sites de prédilection pour la reproduction des pétrels de Tahiti.

En outre, il est indispensable que les clôtures soient éloignées de plusieurs dizaines de mètres de la zone d'attraction des pétrels (zone de terriers artificiels et de diffusion acoustique) afin d'éviter que les oiseaux percutent le grillage lorsqu'ils sont attirés par les dispositifs d'attraction, ou bien lorsqu'ils reviennent ou

<figure><img src="image_92.png" /><figcaption>**Figure 2.8** : Piège létaux de type « Trapinator » fixé sur un arbre (droit). Le piège ouvert (gauche) montre l'appât composé de sardine à l'huile dans un ziplock perçé.</figcaption></figure>
repartent de leurs terriers artificiels. La surface des sites à clôturer est donc généralement relativement
conséquente (1ha environ minimum, voire bien plus), occasionnant des coûts élevés (Young et al., 2018).

Du fait des résultats positifs (attraction de pétrels) rapidement obtenus dans le cadre du projet et de l'impact
toujours fortement négatif des chats harets sur les populations de pétrel de Tahiti (voir résultats ci-dessous),
un exclos anti-chats a finalement été construit sur le site de Confiance en 2022. Le coût élevé ainsi que l'impact
non négligeable sur la flore lors du défrichage et de la construction a conduit à réaliser cette opération
uniquement sur Confiance dans un premier temps, et non dans la réserve botanique de la Taléa, en attendant
de voir pouvoir tester son bon fonctionnement et réunir assez de fonds. L'exclos a été construit de façon à être
suffisamment éloigné de la zone d'attraction des pétrels et possède un capot roulé vers l'extérieur qui empêche
les chats harets d'escalader et de s'introduire dans la colonie (Figure 2.9). La réalisation s'est déroulée en trois
phases : (i) le défrichement de la végétation dans un rayon de 30 à 50 m autour de la colonie sur une bande
d'environ 2 à 3m de large permettant d'édifier la clôture dans de bonnes conditions, (ii) la mise en place des
poteaux et le séchage des éléments cimentés, et (iii) la mise en place d'un grillage doublé d'une hauteur de 2
m minimum au-dessus du sol (grillage à cerf + grillage à poule), assorti d'un capot roulé, destiné à empêcher
le franchissement de la clôture par les chats. Les opérations de défrichement ont été réalisées en Décembre
2021 mais un élagage complémentaire a été nécessaire au niveau de quelques points qui pouvaient fragiliser
l'étanchéité du système aux chats.

<figure><img src="image_93.png" /><figcaption>**Figure 2.9** : Exclos anti-chats réalisé tout autour de la colonie de Confiance (à 30m de distance des terriers) sur le massif du Koniambo. Un capot roulé est visible sur le haut de l'exclos, la porte d'entrée de la colonie est verrouillée par un cadenas et bien fermée pour empêcher l'introduction des chats.</figcaption></figure>

Concernant le site artificiel de Taléa, le système ne pourra pas être remis en fonctionnement tant qu'un exclos
ne sera pas érigé. Les terriers artificiels ont été laissés en place sur le site mais les pièges photos ont été
transposés sur la colonie de Confiance. La lutte contre la présence de rats et de chats a cependant continué sur
ce site avec la recharge des pièges létaux.
## 2.5. Suivis et indicateurs de suivi

Le suivi des colonies artificielles est réalisé par différentes méthodes : (i) un suivi indirect par caméra à déclenchement automatique (« Camtrap ») permettant de photographier la présence et le comportement de pétrels de Tahiti ou de prédateurs introduits, (ii) un suivi indirect par enregistreurs sonores permettant de capter une potentielle réponse à la repasse acoustique mise en place, (iii) un suivi direct sur le terrain lors du passage des équipes toutes les 6 à 8 semaines afin de réaliser une maintenance des différents dispositifs et de rechercher la présence de traces de pétrels de Tahiti.

Sur les sites expérimentaux, 23 camtraps ont été déployées devant les terriers et les enceintes. Ces caméras sont programmées pour prendre 3 photographies à 1 seconde d'écart dès qu'un mouvement est détecté dans le champ visuel de la caméra. Ces caméras permettent de suivre la présence de pétrels de Tahiti sur les colonies, la fréquentation des terriers artificiels, ainsi que la présence d'autres espèces (notamment les prédateurs introduits). Elles permettent également d'observer le comportement des pétrels de Tahiti face aux systèmes d'attraction sonore et olfactive mis en place.

En complément, un enregistreur sonore de type « Song Meter Mini BAT » a été installé sur chaque colonie. Les enregistrements ont été programmés pour démarrer une minute avant la fin de la repasse vocale et s'arrêter une minute après le début du nouveau chant enregistré, soit pendant 17 minutes. Ceci permet d'enregistrer les potentiels chants de réponse des individus. Le programme d'enregistrement mis en place est le suivant :

<table><thead><tr><td>Heure de début d'enregistrement</td><td>Durée d'enregistrement</td></tr></thead><tbody><tr><td>18h14</td><td>17 min</td></tr><tr><td>18h44</td><td>17 min</td></tr><tr><td>19h14</td><td>17 min</td></tr><tr><td>19h44</td><td>17 min</td></tr><tr><td>20h14</td><td>17 min</td></tr><tr><td>20h44</td><td>17 min</td></tr><tr><td>21h14</td><td>17 min</td></tr><tr><td>21h44</td><td>17 min</td></tr><tr><td>22h14</td><td>17 min</td></tr><tr><td>22h44</td><td>17 min</td></tr><tr><td>23h14</td><td>17 min</td></tr><tr><td>23h44</td><td>17 min</td></tr><tr><td>00h14</td><td>17 min</td></tr><tr><td>00h44</td><td>17 min</td></tr><tr><td>01h14</td><td>17 min</td></tr><tr><td>01h44</td><td>17 min</td></tr><tr><td>02h14</td><td>60 min</td></tr><tr><td>03h14</td><td>60 min</td></tr></tbody></table>

Plusieurs indicateurs simples ont été identifiés et définis permettant d'évaluer l'efficacité des dispositifs qui rappelons-le, constituant une opération réellement expérimentale, du fait du contexte particulier dans laquelle ce projet se situe (originalité de travailler en contexte minier et espèce-cible peu connue et n'ayant à ce jour jamais fait l'objet d'une telle expérimentation).