Revue d’Ecologie (Terre et Vie), Vol. 70 (suppt 12 « Espèces invasives »), 2015 : 162-171

FIRST STEPS IN STUDYING CAT MOVEMENT BEHAVIOUR THROUGH VHF TRACKING AT A MAJOR WORLD BREEDING SITE FOR THE MEDITERRANEAN ENDEMIC YELKOUAN SHEARWATER

1* 2 3 4,5 Elsa BONNAUD , Gerald BERGER , Diane ZARZOSO-LACOSTE , Karen BOURGEOIS , 1,6 6 Pauline PALMAS & Éric VIDAL

1 Écologie, Systématique & Évolution, UMR-CNRS 8079, Université Paris Sud 11. 91405 Orsay Cedex, France 2 Association Développement de la Recherche en Écologie Appliquée aux zones Méditerranéennes (DREAM), 1275 A Chemin du seuil. 13760 Saint-Cannat, France 3 UMR CNRS 6553 ECOBIO, Université de Rennes 1, Campus de Beaulieu. 35042 Rennes Cedex, France 4 School of Biological Sciences, Auckland University, Private Bag 92019. Auckland 1142, Auckland, New Zealand 5 Institut Méditerranéen de Biodiversité et d’Écologie marine et continentale (IMBE), Aix-Marseille Université, UMR, CNRS, IRD UAPV, Europôle Méditerranéen de l’Arbois, Avenue Philibert, BP 80. 13545 Aix-en-Provence Cedex 04, France 6 Institut Méditerranéen de Biodiversité et d’Écologie marine et continentale (IMBE), Aix-Marseille Université, UMR, CNRS, IRD UAPV, Centre IRD Nouméa - BP A5, 98848 Nouméa Cedex, Nouvelle-Calédonie *Corresponding author: [email protected]

RÉSUMÉ.— Premières données de l’étude par radio-pistage VHF des déplacements des chats dans un site majeur de reproduction du Puffin Yelkouan, endémique de Méditerranée.— Le chat représente un des prédateurs invasifs les plus menaçants pour les espèces natives des îles et particulièrement pour les oiseaux marins adultes qui sont fortement vulnérables à la prédation. Les populations du Puffin Yelkouan, espèce endémique du bassin méditerranéen, sont réparties en quelques grandes colonies de reproduction et sont en voie de déclin, notamment du fait de l’impact des chats harets et errants. Dans une étude précédemment publiée, l’impact des chats introduits sur la population de puffins de l’île du Levant a été évalué au travers du régime alimentaire du chat sur une période de deux ans. Cette étude a mis en évidence que les chats consommaient trois proies principales : le Lapin, le Rat noir et le Puffin Yelkouan, et qu’un pic de prédation de puffins était observé dès leur arrivée sur les colonies (période de prospection). Nous avons donc cherché à compléter ce travail par une étude préliminaire visant à étudier les patrons de mouvement de quatre chats (trois chats harets et un chat domestique) via un suivi VHF (very high frequency), dans le but d’analyser leurs comportements individuels et leurs domaines vitaux sur l’île du Levant, qui est l’un des principaux sites de reproduction du Puffin Yelkouan. Nos résultats montrent que deux des trois chats harets ont été détectés dans les colonies de puffins durant les périodes de prospection et de reproduction de cet oiseau marin alors que le chat domestique n’a jamais été détecté dans ces colonies. Cela suggère que les patrons de déplacement des quatre chats suivis puissent être liés à la présence des puffins dès que ces oiseaux marins arrivent à la colonie. Les chats suivis montrent des domaines vitaux de taille relativement réduite, plus larges pour les chats harets qui couvrent de plus grandes distances, et avec des territoires chevauchants, que pour le chat domestique. Ces résultats préliminaires de patrons de déplacements, couplés aux résultats précédents de prédation du chat sur le Puffin Yelkouan, nous indiquent que l’impact du chat haret se doit d’être limité. Cet objectif doit être atteint en effectuant une stratégie de gestion efficace qui tiendrait compte des patrons de déplacements des chats afin d’éviter l’épuisement d’un des sites de reproduction les plus importants pour cette espèce de puffin endémique de Méditerranée.

SUMMARY.— Cats are considered one of the most harmful invasive predators of island native species, particularly adult shearwaters, which are highly vulnerable to predation. Populations of Yelkouan shearwater, an endemic species of the Mediterranean basin with only a few large breeding colonies, are predicted to decline in response to feral or free-roaming cats. In a previous study, the impact of introduced cats on the Yelkouan shearwater population of Le Levant Island was assessed through the analysis of cat diet over a two-year period. The study showed that cats prey upon three staple species: rabbits, rats, and shearwaters, with a peak of predation on shearwaters immediately upon their arrival at colonies (prospecting period). Here, we supplement this previous work by conducting a preliminary study on the movement patterns of four free-roaming cats (three feral and one domestic) using very high frequency (VHF) tracking to analyse individual behaviour and home ranges on Le Levant Island, one of the Yelkouan shearwater’s major breeding sites. Our results show that two of the three feral cats were recorded inside and in close vicinity to the shearwater colonies, mainly during the prospecting period, while the domestic cat was never recorded inside the colonies. This suggests that some feral cats could show movement behavioural patterns linked to the shearwater presence as soon as these seabirds arrive at the colonies. The monitored domestic cat also showed a relatively small home range, while feral cats covered larger distances

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and with overlapping territories. Based on these preliminary results of cat movement behaviour, in addition to the previous results of cat predation, it is evident that cat impact must be reduced. This may be achieved through accurate management strategy that takes cat movement behaviour into account to avoid exhausting one of the most important breeding sites for this Mediterranean endemic species.

______The introduction of mammalian predators has led to a strong decrease in native island species (Courchamp et al., 2003; Blackburn et al., 2004). Due to predation impact, some of these preyed species are now extinct, while others show a severe decline in their population dynamics. Such relict populations place a burden on conservation owing to their small size, their restricted home range, with fewer individuals for reproduction, and risk of genetic depression possibly leading to an Allee effect, and consequently, to species extinction (Brashares et al., 2010; Kuparinen et al., 2014). Domestic cats are one of the most common introduced predators on islands worldwide, being responsible for numerous extinctions of native species. Domestic cats were brought by humans to numerous islands as pets and for their ability to catch rodents. However, cats often escape, thus constituting feral populations independent of humans that reproduce and predate in the wild (Turner & Bateson, 2001). Although the presence of domestic cats can be harmful to native species on account of their ability to catch prey without eating it, and to increase feral population when not sterilized, it is generally considered less detrimental than feral populations (Medina et al., 2014). This alien carnivore has already threatened or driven to extinction 175 vertebrate taxa (25 reptiles, 123 birds, and 27 mammals) on at least 120 islands (Medina et al., 2011). The great success of establishment of this invasive species is mainly due to its ease in spreading and reproducing in the wild, its generalist and opportunistic feeding habits, and its behavioural plasticity (e.g., Doherty et al., 2014; Turner & Bateson, 2001). This predator is able to feed on a large variety of introduced and native prey, including reptiles, birds, mammals, and invertebrates (Bonnaud et al., 2011 and references therein). For several decades, the issue posed by introduced cats on islands in terms of biodiversity loss and species endangerment has been a high research and conservation priority, especially regarding long-lived seabirds that are highly vulnerable to cat predation (Rauzon et al., 2008; Veitch et al., 2011; Oppel et al., 2011a). Cat predation has mainly been inferred from diet studies involving scat and/or stomach analyses, however recent work has concomitantly studied cat diet and movement behaviour to better understand cat feeding habits (e.g., Hervías et al., 2014). Understanding individual movement and behavioural patterns can be achieved through remote tracking devices such as very high frequency (VHF) tracking, global positioning systems (GPS), or geolocators (Harper, 2007; Recio et al., 2010; McGregor et al., 2014). Such tracking is necessary when deciphering predator-prey interactions and examining consistency between individual differences in feeding behaviour (Wolf & Weissing, 2012). This is especially true for territorial and generalist top-predators due to the possible behavioural specialization of some individuals towards a selected resource (Pettorelli et al., 2011). To date, little is known about cat individual behaviour, obtained, for example, through the analysis of diet and movement patterns of particular individuals. This could nevertheless reveal possible differences in their foraging behaviour, thus leading to different impacts on targeted prey (Hervías et al., 2014). Such studies are needed in conservation biology, especially when focusing on patchy and seasonally variable prey resources like seabird colonies. This prey group, especially long-lived shearwaters or petrels, is particularly vulnerable to predation on adults (Cuthbert & Davis, 2002; Oppel et al., 2011b). Consequently, cat predation on adult prey population dynamics largely depends on the period of seabird breeding cycle that will be affected. In addition, quantifying the proportion of individuals in the predator’s population that prey upon such a resource is crucial in order to evaluate the

163 impact on such long-live prey populations, and thus conduct appropriate conservation managements. Several Puffinus species, especially those belonging to the Manx shearwater P. puffinus worldwide ‘complex’, are seriously threatened by introduced predators (Cuthbert et al., 2001; Keitt et al., 2002; Martı́nez-Gómez & Jacobsen, 2004; Bonnaud et al., 2012). The Yelkouan shearwater P. yelkouan is strictly endemic to the Mediterranean basin. This seabird is considered to be endangered (Bourgeois & Vidal, 2008) and classified as vulnerable and declining in the 2012 red list of the International Union for Conservation of Nature and Natural Resources. Global Yelkouan shearwater breeding populations were estimated between 11 300 and 54 500 pairs, but probably with overestimated censuses restricted to a few Mediterranean breeding locations (Bourgeois & Vidal, 2008). As the Mediterranean basin has been colonized by humans for several millennia (Vigne et al., 2004), most islands where shearwaters breed are already invaded by introduced predators (Bourgeois & Vidal, 2008). However, Mediterranean islands are poorly studied in relation to the ecology of the introduced cat (see Clevenger, 1995) despite this efficient predator inducing a strong and long-term impact on native and endemic species. The Hyères archipelago harbours one of the world’s major breeding sites for Yelkouan shearwater, and Le Levant Island houses the largest colony among this archipelago (Bourgeois & Vidal, 2008). In this archipelago, Bonnaud et al. (2009, 2012) showed that cat predation is particularly focused on introduced mammals (i.e., rabbits Oryctolagus cuniculus and ship rats Rattus rattus) as well as shearwaters P. yelkouan, principally during their prospecting and mating periods. In the present study, we discuss the previously published data on cat diet according to a preliminary study using VHF tracking on a few cat individuals. We aimed to study (i) the home range and movements of cats in relation to the presence (or absence) of shearwaters in their colonies and (ii) the possible existence of cat behaviours that specifically reach shearwater colonies and prey upon this food resource. These preliminary results are promising in terms of evaluating with more accuracy cat behaviour response in the presence of seabird populations and conducting targeted conservation measures on numerous inhabited Mediterranean islands, whether small or large.

MATERIAL AND METHODS

STUDY SITE This study was conducted on Le Levant Island (1080 ha) located in the north-western Mediterranean Sea (43°N, 6°E) (Fig. 1). Ninety percent of Le Levant area is under military administration, while the remaining ten percent is occupied by civilians. The island lies 9 km from the mainland, with a maximum elevation of 140 m a.s.l. The climate is sub-humid, temperate Mediterranean with an average annual rainfall of 582 mm and average annual temperature of 16.5°C (Le Levant Island Meteorological Office, 1997–2007). The island is siliceous and mainly covered with typical maquis shrub with sparse sclerophyllous oaks Quercus ilex and Aleppo pines Pinus halepensis. Le Levant Island hosts five introduced mammals, namely cats, ship rats, rabbits, mice Mus musculus, and hedgehogs Erinaceus europaeus. An approximate Yelkouan shearwater population ranging from 800 to 1300 breeding pairs was estimated in the last census conducted in 2007 during the Life Nature project (ref. LIFE03NAT/F000105). Like most seabirds, Yelkouan shearwaters have low reproductive rates; they begin breeding when about 6 years old, although the first breeding attempts generally fail and they subsequently produce only one egg annually (e.g., Brooke, 2004). They arrive at breeding sites in late October or early November, which corresponds to the prospecting period. Egg-laying is from mid-March to early April, hatching in May, and fledging in July and early August.

CAT MOVEMENT BEHAVIOUR ACCORDING TO SHEARWATER BREEDING CYCLE Cats were captured during September 2007 in live-traps baited with fish. Eight live traps were placed randomly along the island paths, not in the vicinity of shearwater colonies, and moved every 3 days if no cats were captured. Two more live traps were placed inside the military camp to catch domestic cats. The captured cats were lightly anaesthetized with an intramuscular injection of Ketamine (10 mg kg-1 body-weight). Each cat was fitted with a leather collar with a two-stage radio transmitter weighing 110g and under the maximum 5 % of body weight (no cat weighing under 2500 g was captured). Cats were subsequently located using a Telonics TR4 receiver and handheld antenna RA-14K (Klar et al., 2008). The

164 handling of cats was undertaken with respect to the directives of the Department of Conservation’s Animal Ethics Committee. Cats were then radio-tracked according to the shearwater breeding cycle to determine (i) cat activity patterns (cat presence on the birds’ arrival and during the breeding period); (ii) individual cat movements wandering inside shearwater colonies or not; (iii) the extent of cat home ranges. A total of six monitoring sessions were conducted over three periods of two consecutive nights: two in early October 2007 just before the shearwaters return to their colonies or at the early stage in the prospecting period, two in December 2007 during the shearwater prospecting period, and another two in March 2008 between the shearwater mating and laying periods. For each cat, at least six locations per night with one position every hour were calculated by triangulation, and each cat location was determined as the barycentre of the triangle obtained. Data were edited using Arcgis (version 10, Environmental Systems Research Institute, Inc., Redlands, California).

Figure 1.— Le Levant island, south east of France. Location of 200 cat scats found on Le Levant Island paths during the 2- year study, conducted simultaneously with this cat tracking study, (black dots represent scats containing shearwater remains, white dots scats with other prey remains, and shaded areas Shearwater colonies). MC denotes the location of the military camp and stars the locations of the cats when trapped. Adapted from Bonnaud et al. 2012

To obtain the total home-range size for each cat, we used the 100 % minimum convex polygon method (MCP). To assess cat territory occupancy with greater accuracy, we used the 95 % fixed-kernel estimator (Vander Wal & Rodgers, 2012) as well as the core areas generated using 50 % of locations after outlier removal: locations were calculated with only two points and those outside island limits (Edwards et al., 2002; Horne & Garton, 2006a). These calculations were performed using R statistical software with adehabitat to calculate the smoothing factor value (h) (Calenge, 2006). As h is a key factor in kernel calculation, its value was calculated and chosen based on the Akaike Information Criterion (AIC) for each cat kernel estimator to avoid biasing kernel results (Horne & Garton, 2006b). A specific source code (S. Benhamou, pers. comm.) was then used to take into account the island coast as a natural barrier (Benhamou & Cornélis, 2010). Finally, the minimum Distance Covered by each cat per Night (DCN), i.e., the sum of each distance separating the six locations recorded per night, and the Lowest Distance to Shearwater colony per Night (LDSN), i.e., the mean over the six nights of the shortest distance separating a cat and the closest shearwater colony, were calculated using Argis10 distance toolbox. One-way analyses of variance (ANOVAs) were performed after (i) data were log-transformed to fit the normal distribution and (ii) variance homogeneity verified (Sokal & Rohlf 1995; Zuur et al., 2010), with the aim to test significant differences between the distances covered by each cat.

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RESULTS

CAT MOVEMENT BEHAVIOUR ACCORDING TO SHEARWATER BREEDING CYCLE Four cats were caught and fitted with radio-collars in early September: one in the military camp was designated as free-roaming domestic as it was seen every day around the military camp before the study (one female), while three in the wild were designated as feral (two males, one female). These 4 cats were captured over a period of 4 days, corresponding to 1 cat caught per 10 night traps, for a total of 40 night traps. Although trapping location can be linked to the habitat used by cats, due to the small area of the island, we assumed that each cat could easily reach several shearwater colonies (Fig.1). No signal was recorded for the feral male 2 in December 2007 and March 2008, or for the feral male 1 in March 2008 (Tab. I), so they were assumed to be dead. The two females (one domestic, one feral) survived throughout the study period (Fig. 2a). The smoothing factor (h) was different for each cat and each season. The feral female exhibited a wider total home range and core area (kernel 95 % = 232.9 ha and kernel 50 % = 40.8 ha, respectively) than the domestic female (kernel 95 % = 53.6 ha and kernel 50 % = 10.8 ha, respectively) (Tab. I, Fig. 2b). The domestic cat covered a nightly distance ranging from 300 to 2800 m and the ferals from 400 to 3700 m (Tab. II). However, the mean nightly distances (DCN) of the domestic and feral cats were not significantly different (one-way ANOVA, P = 0.775). The feral female and feral male 2 were recorded several times inside shearwater colonies (in early October during the beginning of prospecting period: 1 night out of 6; in December during the prospecting period: 3 nights out of 6) and they were found significantly closer to the colonies (one-way ANOVA comparing mean LDSN: feral female vs. domestic female, P < 0.001; feral female vs. feral male 1, P < 0.001; feral male 2 vs. domestic female, P < 0.001; feral male 2 vs. feral male 1 P < 0.001).

TABLE I Home range and core area (ha) of cats on Le Levant island based on the minimum convex polygon (MCP) and fixed kernel methods (kernel 95 %: home range and kernel 50 %: core area, h: smoothing parameter) according to the sampling sessions. Each sampling session consists of six sessions of night monitoring with six cat locations (one per hour), i.e., 36 cat locations

October 2007 December 2007 March 2008 Total Kernel Kernel Kernel Kernel Kernel Kernel Kernel Kernel MCP h MCP h MCP h MCP h 95 50 95 50 95 50 95 50 Dom. cat, female 48.0 97.1 20.1 121.8 19.6 36.3 9.6 73.8 10.6 22.5 5.2 61.0 49.6 53.6 10.8 75.4 Feral cat, female 50.2 87.9 26.5 132.4 27.0 49.6 11.7 111.7 93.1 202.1 57.0 205.0 181.8 232.9 40.8 243.1 Feral cat, male 1 44.9 86.1 26.3 122.3 23.1 41.3 9.4 83.0 - - - - 63.8 120.9 32.6 141.2 Feral cat, male 2 63.2 135.3 37.0 207.0 ------63,2 - - -

TABLE II Distances (m) covered by cats per night (DCN) and lowest distances to shearwater colonies per night (LDSN), according to sampling sessions calculated over the six cat locations per night (one per hour) during the six nightly sessions (i.e., 36 cat locations). Ni: number of nights when cats were recorded at least once inside shearwater colonies

October-session December-session March-session Total DCN LDSN DCN LDSN DCN LDSN DCN LDSN Mean Mean Mean Mean Mean Mean Mean Mean Min Max Ni Min Max Ni Min Max Ni Min Max ± SD ± SD ± SD ± SD ± SD ± SD ± SD ± SD Dom. cat, 1838 ± 471 ± 1456 500 ± 703 ± 689 ± 1364 ± 548 ± 987 2828 0 1232 1833 0 324 1147 0 324 2828 female 627 136 ± 248 102 365 76 646 142 Feral cat, 1209 ± 123 ± 815 ± 16 ± 1580 ± 67 ± 1308 ± 72 ± 732 1545 1 667 952 3 624 3683 0 624 3683 female 318 160 127 22 1224 46 731 106 Feral cat, 1261 ± 332 ± 1366 581 ± 1202 ± 445 ± 812 1779 0 1112 1631 0 - - - - - 812 1779 male 1 321 80 ± 194 57 264 146 Feral cat, 1386 ± 80 ± 1386 ± 80 ± 415 2748 1 ------415 2748 male 2 1059 93 1059 93

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(October) (December)

(March) (a)

Feral cat, male 1 Feral cat, male 2

Feral cat, female Domestic cat, female (b)

Figure 2.— (a) Locations of VHF-tracked cats for each sampling session (October 2007, December 2007, and March 2008). Locations of the domestic cat are represented by triangles, feral female by stars, feral male 1 by squares, and feral male 2 by circles. (b) Home range (minimum convex polygon delimited by the grey line and Kernel 95 %: grey area) and core area (Kernel 50 %: dark area) of each cat for the three sampling sessions pooled and taking into account the island coast as a natural barrier (grey dotted line). Shaded areas represent shearwater colonies.

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DISCUSSION

CAT MOVEMENT BEHAVIOUR ACCORDING TO SHEARWATER BREEDING CYCLE The four monitored cats stayed in particular island areas without showing strict territorial behaviour as suggested by their overlapping home ranges. Bonnaud et al. (2012) estimated the cat population from 5 to 20 individuals, the upper value being most probable due to the relative facility of trapping the cats in this study as well as the island area. We thus consider that the four monitored cats provide an indication of the different cat movement behaviours found on this island, but we are well aware that increasing the number of cats monitored and using GPS telemetry would be two major improvements of such a study. The domestic cat always stayed around the military camp with a narrow home range and core area, staying close to human-food resources. Conversely, male and female feral cats showed movement behaviour incorporating wider home ranges and core areas. This is in accordance with results obtained from the comparison of ranging behaviours of confined and unconfined domestic cats on an oceanic island (Hervías et al,. 2014), but further studies comparing feral and domestic cat behaviour are needed in order to better deal with cat presence on inhabited and larger islands. The need for food resources and shelter with reduced availability in the wild could explain the larger territories of feral cats compared to individuals living closest to man (Turner & Bateson, 2001). Feral cats seemed to restrict their erratic behaviour to late autumn, centred on a particular habitat, either the military camp or shearwater colonies. However, the feral female greatly increased its home range in late winter, probably to find another core area, demonstrating that cats can cover long distances and have access to different shearwater colonies, mainly those located along the northern and southern coasts on the eastern side of the island. Moreover, as soon as the shearwaters arrived at the colonies in October, two of the three feral cats were recorded inside the colonies and often stayed in their vicinity; the mean lowest distance to the shearwater colonies per night over 6 and 18 nights for feral cat male 2 and feral female, respectively, was less than 100 m, unlike the domestic and feral cat male 1 that never visited the colonies, staying at a distance of at least 450 to 550 m away. These preliminary results give an initial but partial suggestion that domestic and feral cats might present different movement and predatory behaviours, leading to a differential risk in shearwater predation, with only some feral cats reaching the shearwater colonies to prey specifically upon this food resource. However, due to the low number of cats monitored, this experiment needs to be repeated with a greater number of cat individuals, both domestic and feral, and, if possible, on several islands to confirm this possible difference in cat behaviour in terms of this patchy food resource. Increasing the number of cats monitored in future studies will allow us to discuss the potential differences between domestic and feral cats, due to the different trophic requirements. It will also be possible to analyse the different movement patterns and habitat use of feral cats linked to their sex, seasonal changes, and the potential variation of prey food resources.

CAT DIET AND IMPACT ON SEABIRD POPULATION BASED ON PREVIOUS STUDIES The present study aims to couple the movement behaviour (territories, activities, etc.) of invasive predator species and their diet in order to better estimate their impact on targeted species, especially those with a seasonal presence. Contrary to the results on cat movement behaviour that gave a preliminary idea of different individual behaviours, the previous study on cat diet and impact at the same study site provided trophic behaviour results among the cat population (Bonnaud et al., 2012). Bonnaud et al. (2012) used cat scat analysis collected in the field over a two-year period (September 2006 to August 2008) according to the shearwater breeding cycle, showing a heavy cat predation upon both introduced mammals and shearwaters over the year. Three staple prey items constituted 99 % of the ingested biomass (39.8 % for rabbits, 30.9 % for shearwaters, and 28.4 % for rats; predation upon other birds was excluded due to their very low frequency of occurrence in this cat diet).

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Studying cat diet according to shearwater breeding cycle revealed striking seasonal patterns in cat predation upon this seabird, this predation occurring as soon as the birds arrived at the colonies during the prospecting period. At this time, the individuals exhibit greatest vocal activity and vulnerability, especially regarding prospectors, which are probably easier to catch due to their wandering behaviour outside burrows while looking for a mate; colonies are also at their fullest occupancy (Ristow, 1998; Brooke, 2004). These results are in concordance with those of movement behaviour, revealing that some cats are recorded inside shearwater colonies as soon as the birds arrive. However, these preliminary radiotracking results indicate that not all feral cats are perhaps tempted by this food resource. To quantify cat impact on shearwaters, Bonnaud et al. (2012) also ran shearwater population modelling based on cat diet, different cat population predation scenarios (low and high population estimates of shearwater and cat populations), and different cat predation rates on prospectors suspected of being more vulnerable to cat predation depending on the time spent on the ground, as they are not protected from cat predation outside burrows (for more details, see Bonnaud et al., 2012). Assuming a closed population of Yelkouan shearwater, the demographic models showed that the persistence of this population is only possible in the absence of cat predation. The more conservative cat predation scenario (high shearwater population size estimate of 1300 breeding pairs and only 5 cats) estimated a shearwater population survival of over 50 years. The possible high cat predation on prospectors, seldom considered in similar studies, provides clues to better understand cat predatory behaviour. Firstly, we found that cat predation on prospectors can postpone cat impact on the breeding population due to this “hidden” predation, which is not seen when considering breeding bird survival (Bonnaud et al., 2009). Secondly, this predation on prospectors can accelerate the decrease in global shearwater population by strongly affecting the global growth rate of such a population structured into age classes (for more details, see Bonnaud et al., 2012). Consequently, these results combined with those on cat movement behaviour can help conservation managers to develop more accurate cat management strategies, which need to be implemented as soon as the birds arrive at colonies due to the evidence of (i) the presence of some cats inside or in close vicinity to shearwater colonies during the prospecting period and (ii) the consequences of cat predation upon prospectors.

CONSERVATION PERSPECTIVES In the previously published study, we found that the shearwater population on Le Levant Island is under high cat predation pressure and at high risk of breeding population extinction, which may be accelerated due to possible strong cat predation upon prospectors. The present study added an interesting aspect to cat management perspectives regarding the potential presence of different cat behaviours according to their habitat use and their presence or absence inside shearwater colonies. As suggested by this preliminary VHF study, not all cats reached the shearwater colonies. However, for those that reached the shearwater colonies (two feral cats), they wandered inside and in close vicinity to the colonies as soon as the birds had arrived. Thus, deciphering predator diet on an individual scale represents a promising strategy to be used in a more exhaustive investigation into biotic interactions, as this aspect has often been neglected. This could be achieved by combining standard (e.g., scat dissection) and molecular methods (e.g., DNA microsatellites) of predator diet analysis, coupled with a more accurate method of individual movement behaviour analysis providing both temporal and spatial resolution data (e.g., GPS). We also recommend replicating such telemetry studies coupled with the study of cat diet on an individual scale on other islands facing the same conservation issues. The use of more cat individuals and GPS instead of VHF is required in order to obtain more patterns of individual behaviour with more frequent position data (Martin et al., 2013; Matthews et al., 2013). This will in turn enhance the accuracy of results and help in managing the feral cat population (Bridges et

169 al., 2015) by eradicating it or at least controlling numbers by first targeting cats that wander inside colonies. This control strategy can be an efficient way to boost native prey species, particularly on large inhabited islands where eradication is often difficult or even impossible (Veitch et al., 2011). During such management, we also recommend monitoring mesopredator populations (e.g., rodents) to avoid potential mesopredator release response to top-predator (here, cats) control or eradication (Bonnaud et al., 2010; Read & Scoleri, 2015). Conservation actions could be strongly facilitated by focussing on specific and delimited areas harbouring both threatened species and cats that wander preferentially in such areas.

ACKNOWLEDGEMENTS

We are very grateful to the DGA of Le Levant Island as well as the director and managers of Port-Cros National Park for granting permission for this research. We also thank everyone at the IMBE lab for their valuable help. Funds and support were provided by the Port-Cros National Park and the French National Research Agency (ALIENS project), the Conseil Régional PACA (particularly the fellowship to E.B.), and a European Life project grant to LPO PACA.

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