domain: phys sde

text:
Nargisse Khiara

Fabien Onimus

Stéphanie Jublot-Leclerc

Jourdan Thomas

Thomas Pardoen

Jean-Pierre Raskin

Yves Bréchet

In-situ irradiation creep experiment in a TEM revealing fast dislocation glide induced by irradiation in pure copper

Keywords: In-situ transmission electron microscopy (TEM), ion irradiation, copper, Irradiation creep

In-situ straining experiments were performed on pure copper to investigate dislocation motion under heavy ion irradiation at high stress levels. The unpinning of dislocations from irradiation defects followed by glide was observed under irradiation at stress level just below the critical stress for dislocation glide without irradiation. This phenomenon was unraveled for the first time in copper. The dislocation dynamics recorded in-situ was statistically analyzed using digital image processing to determine the pinning lifetime. Quantitative analysis of pinning lifetimes have been performed, suggesting that a cascade related mechanism is operative to explain the fast dislocation glide under irradiation. This work provides a new insight on the irradiation creep deformation at high stress level.

Introduction

Irradiation creep is a viscoplastic deformation mechanism occurring in components inside nuclear reactors subjected to constant load and long term irradiation. Structural materials such as austenitic steels and zirconium alloys undergo irradiation creep that must be properly characterized and predicted for safe operation. Usually, irradiation creep experiments are conducted on bulk material samples within test reactors under combined fast neutron flux and applied mechanical load [START_REF] Aitkhozhin | Radiation-induced creep of copper, aluminium and their alloys[END_REF][START_REF] Garnier | Irradiation creep of SA 304L and CW 316 stainless steels: Mechanical behaviour and microstructural aspects. Part I: Experimental results[END_REF]. However, such experiments suffer from major drawbacks such as long irradiation periods, activation of the test material, and no option for in-situ characterization of the elementary physical mechanisms at the origin of the macroscopic behavior. Therefore, although the macroscopic phenomenology for irradiation creep is relatively well rationalized, the underlying microscopic mechanisms are still unclear. Many theoretical mechanisms have been proposed for this phenomenon, all involving atomic displacements and creation of vacancies and interstitials under fast neutron irradiation. However, these models differ on the detailed processes explaining the point defect redistribution under stress.

Yet, due to limited experimental evidence, a controversy exists for selecting the relevant controlling mechanisms [START_REF] Garner | Irradiation creep mechanisms: an experimental perspective[END_REF][START_REF] Matthews | Irradiation creep models-An overview[END_REF].

An alternative to in-reactor irradiation creep experiments is to couple small scale mechanical testing and ion irradiation to accelerate the experiments while taking into account the much lower penetration of ions compared to neutrons. Jung et al. [START_REF] Jung | An apparatus for applying tensile, compressive and cyclic stresses on foil specimens during light ion irradiation[END_REF] and Xu et al. [START_REF] Xu | In situ proton irradiation creep of ferritic-martensitic steel T91[END_REF] developed specific experimental apparatus to deform materials under high-energy light ion flux with specimen thickness of the order of 100 𝜇𝑚. However, light ions do not simulate well the neutron damage and induce the activation of the matter [START_REF] Tai | Irradiation-induced creep in nanostructured Cu alloys[END_REF]. An alternative approach is to use heavy ions, which better simulate the neutron damage and do not activate the matter, but have an even more limited penetration than light ions. Despite the efforts of few research teams to use miniaturized specimen coupled with heavy ion beams [START_REF] Tai | Irradiation-induced creep in nanostructured Cu alloys[END_REF][START_REF] Tai | Temperature dependence of irradiation-induced creep in dilute nanostructured Cu-W alloys[END_REF][START_REF] Özerinç | In situ creep measurements on micropillar samples during heavy ion irradiation[END_REF][START_REF] Özerinç | Direct measurements of irradiationinduced creep in micropillars of amorphous Cu56Ti38Ag6, Zr52Ni48, Si, and SiO2[END_REF][START_REF] Lapouge | Creep behavior of submicron copper films under irradiation[END_REF][START_REF] Lapouge | A novel on chip test method to characterize the creep behavior of metallic layers under heavy ion irradiation[END_REF][START_REF] Jawaharram | High temperature irradiation induced creep in Ag nanopillars measured via in situ transmission electron microscopy[END_REF][START_REF] Jawaharram | Irradiation induced creep in nanocrystalline high entropy alloys[END_REF][START_REF] Dillon | Irradiation-induced creep in metallic nanolaminates characterized by In situ TEM pillar nanocompression[END_REF], the access to the underlying irradiation creep mechanisms remains indirect. The in-situ imaging of crystal defects under both straining and heavy ion irradiation inside a TEM can allow for a direct access to the active mechanisms [START_REF] Legros | In situ mechanical TEM: Seeing and measuring under stress with electrons[END_REF]. However, until recently, TEM in-situ straining experiments have mainly focused on post irradiation deformation, to determine for instance the nature of the interactions between dislocations and irradiation induced defects [START_REF] Robach | In-situ transmission electron microscopy observations and molecular dynamics simulations of dislocation-defect interactions in ion-irradiated copper[END_REF][START_REF] Robach | Dynamic observations and atomistic simulations of dislocation-defect interactions in rapidly quenched copper and gold[END_REF][START_REF] Zinkle | Observation and analysis of defect cluster production and interactions with dislocations[END_REF][START_REF] Schäublin | Dislocation defect interaction in irradiated Cu[END_REF][START_REF] Matsukawa | Dynamic observation of the collapse process of a stacking fault tetrahedron by moving dislocations[END_REF][START_REF] Kiener | In situ nanocompression testing of irradiated copper[END_REF].

A new methodology was developed by Gaumé et al. [START_REF] Gaumé | In-situ observation of an irradiation creep deformation mechanism in zirconium alloys[END_REF] for in-situ deformation experiments inside a TEM under krypton ion irradiation to study high stress irradiation creep on a recrystallized zirconium alloy at room temperature and at 450°C. The authors observed that, at high stresses, irradiation induces the release and glide of dislocations pinned on irradiation defects. The objective of the present work is to conduct in-situ TEM irradiation experiments on pure copper to provide quantitative data on dislocation dynamics, to determine if the conclusions of Gaumé et al. [START_REF] Gaumé | In-situ observation of an irradiation creep deformation mechanism in zirconium alloys[END_REF] also apply to copper, and to critically assess the existing models. Copper is often considered as a model material for metals with FCC crystal structure such as austenitic steels, thus motivating this choice.

Experimental methods

Materials

Pure polycrystalline annealed OFHC copper (99.95%) specimens were cut out from thin sheets of 25 mm x 25 mm with a thickness of 0.3 mm. The specimens were mechanically polished down to 0.13 mm, then the center of the specimen was jet electropolished to be transparent to electrons using the commercial electrolyte D2 from Struers, at a temperature of -1°C and at a voltage of 8 V.

Test methods 2.2.1. Irradiation conditions

The in-situ deformation experiments under irradiation were carried out using the 2 MV ARAMIS ion implanter [START_REF] Cottereau | Aramis: an accelerator for research on astrophysics, microanalysis and implantation in solids[END_REF][START_REF] Chauvin | Optics calculations and beam line design for the JANNuS facility in Orsay[END_REF] at the JANNuS-Orsay/SCALP platform (IJCLab, formerly CSNSM) [START_REF] Serruys | JANNUS: A multi-irradiation platform for experimental validation at the scale of the atomistic modelling[END_REF][START_REF] Bacri | SCALP, a platform dedicated to material modifications and characterization under ion beam[END_REF], which is part of the French EMIR accelerator network. The implanter is directly connected to the Transmission Electron Microscope (TEM), thus allowing for in-situ observations under irradiation. The irradiations were performed with 2 MeV Cu + ions and with fluxes in the range from 1.2 × 10 10 up to 1 × 10 11 ions/cm²/s (i.e. 9.6 × 10 -5 to 8.0 × 10 -4 dpa/s), although most experiments were performed at 5 × 10 10 ions/cm²/s (4.0 × 10 -4 dpa/s) and at 8 × 10 10 ions/cm²/s (6.4 × 10 -4 dpa/s). The final fluences ranged in between 7.3 × 10 12 ions/cm² (5.84 × 10 -2 dpa) and 1.3 × 10 14 ions/cm² (1.04 dpa).

In-situ TEM loading

The specimens were installed on a GATAN single tilt deformation holder in a 200 kV TECNAI TEM, as represented in Figure 1. The deformation of the specimen is produced by applying a controlled displacement of one of the crossheads. In total, nine deformation experiments were performed. All the experiments were carried out at room temperature. The methodology is similar to the one conducted by Gaumé et al. [START_REF] Gaumé | In-situ observation of an irradiation creep deformation mechanism in zirconium alloys[END_REF]. First, the crosshead is displaced until the first dislocation motion occurs in the area of observation. Then, by either decreasing the crosshead displacement or by waiting until the stress relaxes sufficiently, the dislocations stop or their velocity reduces significantly. In the following, the total cross-head displacement (from the beginning of the experiment) is noted δ and dδ represents the variation in crosshead displacement. The dislocation motions are recorded using a GATAN wide angle camera during successive sequences, each sequence consisting on a first step of a few tens of seconds during which the ion beam is off, followed by a second step of a few tens of seconds during which the ion beam is switched on, followed by a third step of a few tens of seconds with the ion beam turned off again. Between two sequences, the crosshead displacement can be increased and the ion flux can be modified.

Image analysis procedures

One of the main objectives of the experiments is to observe the dislocation motion successively during or after ion irradiation. In order to estimate the jump length or dislocation mean free path, an image analysis procedure presented in Figure 2 has been set up using the image processing program Fiji [START_REF] Schindelin | Fiji: an open-source platform for biologicalimage analysis[END_REF].

A line (in red in Figure 2) is first defined along the dislocation glide direction covering the whole distance travelled by the dislocation during the sequence of images. The kymograph of this frame sequence along this line is then computed. A kymograph [START_REF] Dhonukshe | Contribution of microtubule growth polarity and flux to spindle assembly and functioning in plant cells[END_REF][START_REF] Zhou | Direct visualization of the movement of the monomeric axonal transport motor unc-104 along neuronal processes in livingcaenorhabditis elegans[END_REF] represents the intensity of the pixels along a defined line for successive frames. Given that the dislocations have a black contrast, the kymograph gives the position of the dislocation along the glide direction as a function of the frame number. The kymograph is then segmented manually in order to only select the dislocation path, and the skeleton [START_REF] Arganda-Carreras | 3D reconstruction of histological sections: Application to mammary gland tissue[END_REF] of the image is extracted. A skeletonization consists in the reduction of binary objects into one pixel wide objects. The white curve generated is then exported, and the position and the frame are converted respectively in nanometers and seconds. Nonetheless, since the glide plane is not the image plane, it is important to note that the distances thereby obtained are projected distances. Therefore, the images are first corrected using a geometrical transformation before the image procedure presented is implemented. The details of this geometrical transformation can be found in the Supplementary data. For instance, the results of this procedure performed on six dislocations during an irradiation step are provided in Figure 3. Another useful tool often used to analyze TEM image sequences is the image subtraction. This tool allows the determination of the variation on an image between two frames, and thus, in our case, of the dislocation displacements.

First irradiation step

Prior to any irradiation, the crosshead is displaced until the first dislocation glides are observed. The local stress is thus equal to the critical stress for dislocation motion. For instance, in Figure 4.e,f, the dislocations glide after two successive increments in crosshead displacement. If the crosshead displacement is not increased further, the dislocations stop moving because of the stress relaxation. The local stress in thus now just below the critical stress for dislocation motion. After a few tens of seconds, the ion beam is turned on, while the crosshead displacement is held constant. During this first ion irradiation step, the dislocations remain immobile (Figure 4.d,h). After the irradiation, the crosshead displacement must be significantly increased to induce again the glide of these dislocations.

The dislocations are now pinned by irradiation defects and their motion occurs by glide through these defects. This suggests that irradiation induces the unpinning of dislocations from irradiation defects. A film showing this sequence is provided as Supplementary Data (Video 1). This dislocation pinning phenomenon is consistent with previous post-irradiation straining experiments [START_REF] Briceño | Effect of ion irradiationproduced defects on the mobility of dislocations in 304 stainless steel[END_REF][START_REF] Robach | Dynamic observations and atomistic simulations of dislocation-defect interactions in rapidly quenched copper and gold[END_REF][START_REF] Long | Deformation mechanism study of a hot rolled Zr-2.5 Nb alloy by transmission electron microscopy. I. Dislocation microstructures in as-received state and at different plastic strains[END_REF][START_REF] Kaoumi | Insights into the plastic behavior of irradiated Ni-based alloy through in-situ TEM experiments: Formation and evolution of defect-free channels[END_REF]. The dislocations observed in this sequence present a V-shape and the motion of the tip of the V-shape is along the [110] axis. Moreover, the extinctions of the dislocations are consistent with a 1 2

[110]

Burgers vector and the slip traces are consistent with the glide planes (11 ̅ 1) and (1 ̅ 11). This indicates that a segment of each dislocation cross-slipped between two glide planes (11 ̅ 1) and (1 ̅ 11). More details on this analysis can be found in the Supplementary Data.

Further irradiation steps

Before the subsequent irradiation step, the crosshead displacement is significantly increased to reach the new critical stress for dislocation motion. The dislocations presented in the Figure 4 start to slightly glide along short distances. If the crosshead displacement is held constant, they stop moving, because of progressive stress-relaxation. The local applied stress is now just below the new critical stress needed to make the dislocations move through irradiation defects. As earlier, after a few tens of seconds, the ion beam is turned on, with the crosshead displacement kept constant. The dislocations start to slightly glide under irradiation. When the ion beam is turned off, they stop moving. This suggests that irradiation induces the unpinning of dislocations from irradiation defects. A film showing this sequence is provided as Supplementary Data (Video 1). It is also observed that other dislocations glide much faster under irradiation. These dislocations are newly generated from cracks and grain boundaries. They are less jogged, compared to initial dislocations that have absorbed point defects under irradiation, and are therefore less pinned. As for the initial dislocations, these newly generated dislocations are also immobile when the ion beam is off and start to glide under ion irradiation (Figure 5). Furthermore, under irradiation, the motion of these dislocations is not continuous and smooth. The dislocations move through series of jumps, probably from obstacle to obstacle. When following on the frame sequence the endpoints of dislocations, straight lines can be drawn suggesting that, during jumps, the motion occurs by glide. This resembles the pinning-unpinning motion under irradiation observed by Gaumé et al [START_REF] Gaumé | In-situ observation of an irradiation creep deformation mechanism in zirconium alloys[END_REF]. Films showing this sequence along with other sequences are provided as Supplementary Data (Video 2 and 3). The irradiation induced glide described above was observed even at high fluences. The deformation experiments were performed until the contrast did not permit any clear detection of dislocation motion due to the high density of irradiation defects or until the local stress decreases due to stress redistribution within the thin foil, for instance because of crack opening.

The effect of the flux was also investigated. For a flux of 1.2 × 10 10 ions/cm²/s, very little motion was observed under irradiation, in comparison with higher fluxes, suggesting that for weak fluxes, the hardening effect of irradiation prevails over the irradiation induced motion of dislocations. For fluxes of 3 × 10 10 , 5 × 10 10 and 8 × 10 10 ions/cm²/s, no significant differences depending on the flux was observed.

Statistical study of dislocation dynamics

One experiment conducted on one area of a chosen specimen has been selected for more quantitative analysis. The crystallographic orientation of the grain is determined using electron diffraction data. The glide planes are determined based on the movements of the endpoints of the dislocations. The specimen thickness can be deduced from the orientation of the glide plane and from the projected distance between the two slip traces of the dislocation (more details can be found in the Supplementary Data). Three types of dislocations are observed in the sequence: screw dislocations gliding in the plane (11 ̅ 1 ̅ ) with a Burgers vector 1 2 [1 ̅ 01 ̅ ], screw dislocations gliding in the plane (111 ̅ ) with a Burgers vector equal to 1 2 [1 ̅ 01 ̅ ], and dislocations gliding in the plane (111 ̅ ). The screw nature of the two first dislocation types were determined based on the cross-slip observed from the plane (11 ̅ 1 ̅ ) to the plane (111 ̅ ). The nature and Burgers vector of the last dislocation family could not be ascertained. The sample thickness in the area of observation is found to be around 80 to 100 nm. The tensile loading direction is close to the [100] axis. In the following, the origin of time is taken to be the start of the experiment (from the first crosshead displacement, before any irradiation), although the sequence only concerns the irradiation steps from the 4 th irradiation step (after a cumulated dose of 1.22 × 10 13 ions/cm²).

Estimation of the dislocation jumps frequency

Image subtractions are performed between frames taken one second apart to measure the number of dislocation jumps per second. If a dislocation appears with a contrast on the image subtraction, it means that the dislocation has moved and this is counted as one instantaneous jump per second. If ten dislocations appear with a contrast on the image subtraction, this is counted as ten instantaneous jumps per second. This measure will be referred to, in the following, as the dislocation jump frequency.

The number of dislocation jumps measured for each image subtraction can also be added up all along the experiment. This will be referred to as the cumulated number of dislocation jumps. As shown in Figure 6, the cumulated number of dislocation jumps increases sharply when the ion beam is on. The increase in dislocation motion under irradiation is relatively linear with time, and the slope, i.e. the dislocation motion frequency, is around one jump per second, compared with 0.1 jump per second when the ion beam is off.

Sensitivity to stress

As stated earlier, if the local stress is far below the critical stress for dislocation motion, no irradiation induced glide is observed. Just below this critical stress, dislocations start to glide under irradiation. This shows that there is a high sensitivity to the applied stress. This is further illustrated when the stress relaxes. Figure 7 represents the dislocation motion frequency over five irradiation steps. This figure shows that when there is no increase of crosshead displacement in between irradiation steps (i.e the crosshead displacement δ remains constant), the frequency of dislocation jumps under irradiation progressively drops. This is most likely due to stress relaxation. A slight increase in crosshead displacement (in between the 13 th and 14 th irradiation step) induces a significant increase of the jump frequency under irradiation (in green). This shows that the jump frequency under irradiation exhibits a significant sensitivity to the local stress. Consequently, the irradiation induced unpinning mechanism observed is highly sensitive to the applied stress. This figure also shows that this slight increase in crosshead displacement does not significantly affect the jump frequency when the ion beam is off (in grey). Note that the stress level is not directly measured, but the sensitivity to the crosshead displacement reflects the sensitivity to stress.

Estimation of the dislocation mean free path

Under irradiation, the dislocation motion is not smooth, but occurs by series of jumps, probably associated to the glide from obstacle to obstacle. In order to estimate the jump length or dislocation mean free path, the image analysis procedure presented in section 2.3 was used. Figure 8.a and Figure 8.b represent, respectively, the mean distances travelled during 8 irradiation steps and the histogram of the jump distances (i.e. distance travelled by a dislocation during a jump). The travelled distances remain approximatively the same for the different steps (with a slight decrease with increasing step number). The majority of displacements are under 50 nm and the mean displacement magnitude is of 45 nm. In between two jumps, the dislocation stops moving, as seen in Figure 3, probably pinned on defects, and this waiting time is analyzed.

Post irradiation microstructural examination

Post-mortem analyses were performed on the sample studied in section 3.1.2 in order to characterize the irradiation defects. A weak beam image of the sample is presented in Figure 9. A high density of very small defects with a diameter of less than 3 nm is observed. The defects appear to be mainly small loops, although few SFT are also observed. However, considering the high density and small size of the defects as well as the possible overlap between these defects, the loop and SFT populations can be confused. The mean defect diameter 𝑑 is estimated to be around 2 nm. Given that the mean defect density 𝑁 is found around 10 23 m -3 , the average defect planar spacing 𝑙 can then be evaluated using:

𝑙 = 1 √𝑁𝑑 . (2)
The average defect planar spacing is around 70 nm, which is in the same order of magnitude as the dislocation mean free path (45 nm) previously estimated.

Figure 9 -TEM micrograph of the sample at the end of the experiment observed in weak beam dark field conditions with a g=200. The green and red dashed circles overlay respectively small loops and small SFT.

Discussion 4.1. Estimation of the mean pinning lifetime

In the sequence described above (section 3.1.2), the dislocation mean free path is equal to 45 nm, and the estimated average loop spacing in the glide plane is equal to 70 nm. Moreover, the dislocation lengths are estimated to be in the range 80 to 150 nm, with a mean dislocation length of around 100 nm (after a projection of the image on the appropriate glide plane). One can thus assume that there is, on average, one obstacle per dislocation. Therefore, the fact that the dislocation mean free path corresponds to the average distance between obstacles in the glide plane suggests that the "stop and go" motion observed is due to a pinning-unpinning of dislocations from irradiation-induced defects.

As a first approximation, we consider here that every recorded dislocation jump results from the unpinning of one dislocation from one irradiation defect. The total dislocation length observed in the sequence is equal to 2 µm. Taking a mean separation distance between pinning points of 50 nm, there are thus 40 pinning points in total in the sequence. We have recorded on average 0.1 dislocation jumps per second when the ion beam is off and 1 jump per second when the ion beam is on. This suggests a mean pinning lifetime of 40 seconds under irradiation and 400 seconds without irradiation (thermally activated glide). The mean pinning lifetime calculated using these average values is very close to the mean measured dislocation pinning lifetime of 42 seconds measured using the detailed histograms (Figure 8.d). This, again, supports the mechanism proposed which consists of the pinning-unpinning of dislocations from obstacles.

Possible irradiation creep mechanisms

In order to ensure that the unpinning event is indeed triggered by irradiation and does not result from ion beam heating effects, an estimation of the specimen heating during ion irradiation is computed and detailed in Supplementary Data. The ion irradiation induced increase in temperature appears to be low (less than 1 K). Moreover, the surface power lost in the specimen during ion irradiation is orders of magnitude lower than the surface power lost by the electron beam, which does not contribute to significant heating in conductive metals. Additional experiments, reported in Supplementary data, have been conducted to investigate the respective contributions of electron and ion beam heating. These calculations and experiments confirm that the phenomenon under investigation here is due to irradiation effects and does not result from ion beam heating effects.

Irradiation has two distinct effects. First, it induces the generation of defects such as SFT and loops that act as obstacles to dislocation motion causing a retarded creep. The second effect enhances creep; this is the irradiation creep. In our experiments, both effects are observed: an irradiation induced hardening and a glide based irradiation creep mechanism. The glide mechanisms can either be climb assisted glide resulting from homogeneous generation and diffusion of point defects [START_REF] Woo | Effects of an anisotropic dislocation structure on irradiation creep due to stress induced preferred absorption of point defects[END_REF][START_REF] Woo | Irradiation creep due to elastodiffusion[END_REF][START_REF] Bullough | Sink strengths for thin film surfaces and grain boundaries[END_REF], or direct cascade induced unpinning of dislocations. This last mechanism can result from dissolution of pinning points by interaction with the cascade or by the local production of point defect due to the cascade leading to dislocation climb over the obstacle [START_REF] Voskoboinikov | nteraction of collision cascades with an isolated edge dislocation in aluminium[END_REF][START_REF] Matthews | Irradiation creep models-An overview[END_REF].

There are three usual climb-assisted glide mechanisms. The first one is the dislocation bias driven climb mechanism, which results from a stronger elastic interaction between edge dislocations and interstitials than between edge dislocations and vacancies. There are two other mechanisms referred to as stress-induced preferential absorption of point defects (SIPA) mechanisms. The first one is the SIPA-I (for Stress Induced Preferential absorption due to Inhomogeneity effects) which results from polarizability effects [START_REF] Woo | Effects of an anisotropic dislocation structure on irradiation creep due to stress induced preferred absorption of point defects[END_REF][START_REF] Bullough | The stress-induced point defect-dislocation interaction and its relevance to irradiation creep[END_REF]. The second one is the SIPA-AD (for Stress Induced Preferential absorption due to Anisotropic Diffusion) which is due to elasto-diffusion and is considered to be a first order effect [START_REF] Woo | Irradiation creep due to elastodiffusion[END_REF].

Our experimental results are compared with standard irradiation creep models proposed in the literature. All the quantitative models are based on rate theory. Several strong hypothesis are made for these models [START_REF] Matthews | Irradiation creep models-An overview[END_REF][START_REF] Woo | Effects of an anisotropic dislocation structure on irradiation creep due to stress induced preferred absorption of point defects[END_REF][START_REF] Woo | Irradiation creep due to elastodiffusion[END_REF][START_REF] Bullough | Sink strengths for thin film surfaces and grain boundaries[END_REF]. The obstacles are assumed to be unshearable, although the reality is more complex since dislocation-loop reactions are known to occur [START_REF] Terentyev | Simulation of the interaction between an edge dislocation and a< 1 0 0> interstitial dislocation loop in α-iron[END_REF][START_REF] Rodary | Dislocation glide in model Ni (Al) solid solutions by molecular dynamics[END_REF][START_REF] Rodney | Molecular dynamics simulation of screw dislocations interacting with interstitial frank loops in a model FCC crystal[END_REF][START_REF] Robach | In-situ transmission electron microscopy observations and molecular dynamics simulations of dislocation-defect interactions in ion-irradiated copper[END_REF]. The production of point defects is considered spatially and temporally homogeneous, which is a quite simple assumption since the point defects are produced by stochastically generated displacement cascades. The climb duration, calculated using this model, that should correspond to the mean pinning lifetime, is found to be at least 30 times longer than in the experiments and does not show the same sensitivity to stress. More details about this analysis can be found in the Supplementary Data. Therefore, either the hypothesis of the model chosen are too simple, or another mechanism, resulting from cascade effects, may be operative. For instance, a displacement cascade induced climb mechanism can be considered [START_REF] Voskoboinikov | nteraction of collision cascades with an isolated edge dislocation in aluminium[END_REF]. However, the climb based mechanisms only concern edge dislocations, and do not apply to the screw dislocations observed in our experiments. Very recently, Khiara et al. [START_REF] Khiara | A novel displacement cascade driven irradiation creep mechanism in α-zirconium: A molecular dynamics study[END_REF] showed through molecular dynamics simulations performed on pure zirconium and for stresses just below the yield stress, the release of screw dislocations pinned on small loops by displacement cascades generated by irradiation. After the unpinning of the dislocation, it was observed that the freed dislocation is now straight and that the loop is partially destructed. For stresses just below the critical stress for dislocation unpinning, the pinning lifetimes estimated based on molecular dynamics calculations are in the same range as the experimental lifetimes. This new mechanism, if extended to copper, can provide an explanation to the unpinning of screw dislocations observed during our experiment.

Conclusion

In-situ straining experiments were performed under heavy ion irradiation on pure copper under high stress levels. The release, assisted by irradiation, of dislocations pinned on small loops has been revealed under high applied stresses, resembling the phenomenon observed by Gaumé et al. [START_REF] Gaumé | In-situ observation of an irradiation creep deformation mechanism in zirconium alloys[END_REF] for a zirconium alloy. This phenomenon, observed for the first time for pure copper, provides a new insight on the irradiation creep deformation mechanism at high stresses. The main findings of this work are the following:

• The dislocations glide under ion beam irradiation by series of jumps, indicating the release from one pinning point, glide to the next one before being pinned again. This is supported by the fact that the dislocation mean free path, around 45 nm, is of the same order as the distance between irradiation defects. • The underlying irradiation creep mechanism (with the ion beam switched on) is very sensitive to stress, and occurs for stresses just below the critical stress for dislocation motion (with the ion beam switched off). • The magnitude of the flux affects the dislocation activity, since for a flux of 1.2 × 10 10 ions/cm²/s very few motions were observed in comparison to higher fluxes (above 3 × 10 10 ions/cm²/s) • The mean pinning lifetime of dislocations was estimated to be around 40 seconds.

• Beam heating effects on the phenomena have been discarded.

• The traditional irradiation creep mechanisms and models that can explain the observations are the climb followed glide mechanisms, due either to dislocation bias, SIPA-I or SIPA-AD. The pinning lifetime estimated using the rate theory in the conditions of the experiments for these three mechanisms underestimate the experimental value by a factor of at least 30. Moreover, for the models for which the pinning lifetimes are the lowest, these pinning lifetimes do not show the same sensitivity to stress as in the experiments. This discrepancy between theory and experiment suggests that another mechanism, presumably cascade related, is operative to explain the much faster creep rates.

Figure 1 -

 1 Figure 1 -(a) JANNuS-Orsay SCALP modified Tecnai G20 TEM allowing for ion beam irradiation with an angle between the electron and ion beams of 68° [27] -(b) Head of the deformation GATAN TEM single tilt sample holder.

Figure 2 -

 2 Figure2-Dislocation motion analysis using the image processing program Fiji[START_REF] Schindelin | Fiji: an open-source platform for biologicalimage analysis[END_REF].

Figure 3 -

 3 Figure 3 -Evolution of the magnitude of the displacement of several dislocations during an irradiation step.

Figure 4 -

 4 Figure 4 -TEM images showing ion beam-on and beam-off dislocation motion under applied stress: (a) and (b) two successive strain increments -(c) no strain increment -(d) a first irradiation step; (e) (f) (g) and (h): images subtractions showing in white the dislocation positions at the initial time step respectively after a first load increment, a second load increment, a waiting step of 47 seconds, and an irradiation step of 65 seconds. δ and dδ represent respectively the total cross-head displacement (since the beginning of the experiment) and the variation in crosshead displacement.

Figure 5 -

 5 Figure 5 -TEM micrographs under applied stress (a) before irradiation, (b) at the end of a first irradiation step, (c) 50 seconds after the end of the first irradiation, (d) at the end of a second irradiation step, (e) 27 seconds after the end of the second irradiation; (f)-(j): image subtractions showing the initial dislocation positions in white. The arrow points at moving dislocations tracked during the irradiation phases. Bright Field TEM, g = 200, close to zone axis [012 ̅ ]. The crosshead displacement is held constant (δ = 105 µm).

Figure 6 -

 6 Figure 6 -Evolution with the time of (a) the cumulated number of dislocation jumps; (b) the dose and crosshead displacement δ. The origin of times is taken to be the beginning of the experiment.

Figure 7 -

 7 Figure 7 -(a) Dislocation jumps frequency during steps when the ion beam is off (in grey) and the ion beam is on (in green); (b) variation of the dose and crosshead displacement as a function of the time. The blue dotted line on the first graph represents the increase in crosshead displacement.

Figure 8 .

 8 c and Figure 8.d represent respectively the mean pinning lifetimes (or waiting time between two jumps) during eight irradiation steps and the histogram of mean pinning lifetimes. The mean pinning lifetime slightly increases with the irradiation dose, suggesting that the pinning ability, or obstacle strength, of irradiation defects increases with irradiation dose. The mean pinning lifetime is equal to 42 seconds.

Figure 8 -

 8 Figure 8 -(a) Mean dislocation displacement magnitude and (c) mean pinning lifetime during different irradiation steps. Distribution of (b) dislocation displacement magnitudes and (d) pinning lifetimes during irradiation from the 4 th to the 18 th irradiation step.

Acknowledgments

These experiments were supported by the French EMIR&A accelerators network. The authors are grateful to the JANNuS-Orsay/SCALP technical staff at IJCLab (formerly CSNSM) for ARAMIS ion irradiations, and technical help at the TEM during in-situ experiments, especially Cédric Baumier, Jérôme Bourçois, Sandrine Picard, and Cyril Bachelet.


Samara Danel
email: samara.danel@gmail.com

Jules Chiffard-Carricaburu

Francesco Bonadonna

Anna P Nesterova

Jules Chiffard

Exclusion in the field: wild brown skuas find hidden food in the absence of visual information

Keywords: avian cognition, Charadriiformes, cups task, exclusion performance, inferential reasoning

Inferential reasoning by exclusion allows responding adaptively to various environmental stimuli when confronted with inconsistent or partial information. In the experimental context, this mechanism allows selecting correctly between an empty option and a potentially rewarded one. Recently, the increasing reports of this capacity in phylogenetically distant species have led to the assumption that reasoning by exclusion is the result of convergent evolution. Within one largely unstudied avian order, i.e. the Charadriiformes, brown skuas (Catharacta antarctica ssp lonnbergi) are highly flexible and opportunistic predators.

Behavioural flexibility, along with specific aspects of skuas' feeding ecology, may act as influencing factors in their ability to show exclusion performance. Our study aims to test whether skuas are able to make choice by exclusion in a visual two-way object-choice task.

Twenty-six wild birds were presented with two opaque cups, one covering a food reward. Three conditions were used: 'full information' (showing the content of both cups), 'exclusion' (showing the content of the empty cup), and 'control' (not showing any content).

Skuas preferentially selected the rewarded cup in the full information and exclusion condition. The use of olfactory cues was excluded by results in the control condition. Our study highlights the cognitive potential of this predatory seabird and opens new investigations for testing further its cognition in the wild.

Introduction

The capacity of nonhuman animals to choose via exclusion has recently received considerable interest (e.g. [START_REF] Gazes | Monkeys choose, but do not learn, through exclusion[END_REF][START_REF] Nawroth | Exclusion performance in dwarf goats (Capra aegagrus hircus) and sheep (Ovis orientalis aries)[END_REF][START_REF] Petit | Inferences about food location in three cercopithecine species: an insight into the socioecological cognition of primates[END_REF][START_REF] Subias | Inference by exclusion in the Red-tailed Black Cockatoo (Calyptorhynchus banksii)[END_REF]). This cognitive mechanism, which refers to 'the ability to understand that if there are only two possibilities and if it is not A, it must be B' [START_REF] O'hara | Reasoning by exclusion in the kea (Nestor notabilis)[END_REF], is typically explored through the use of a two-way object-choice paradigm i.e. the cups task [START_REF] Call | Inferences about the location of food in the great apes (Pan paniscus, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus)[END_REF][START_REF] Mikolasch | Is caching the key to exclusion in corvids? The case of carrion crows (Corvus corone corone)[END_REF][START_REF] Pepperberg | Reasoning by inference: Further studies on exclusion in grey parrots (Psittacus erithacus)[END_REF][START_REF] Plotnik | Thinking with their trunks: elephants use smell but not sound to locate food and exclude nonrewarding alternatives[END_REF][START_REF] Subias | Inference by exclusion in the Red-tailed Black Cockatoo (Calyptorhynchus banksii)[END_REF]). In the visual version of this test, a food reward is hidden underneath one of two identical opaque cups [START_REF] Darmaillacq | Cognition animale: perception, raisonnement et représentations[END_REF]). In the crucial phase, the experimenter lifts the empty cup only. The subject is able to choose via exclusion if it excludes the presented empty cup and selects the potentially rewarded, non-lifted one.

Successful performance on this task can be achieved through the use of low-and/or highlevel cognitive strategies [START_REF] Call | Inferences about the location of food in the great apes (Pan paniscus, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus)[END_REF][START_REF] Penn | Causal cognition in human and nonhuman animals: a comparative, critical review[END_REF][START_REF] Watanabe | Animal logics: decisions in the absence of human language[END_REF].

Subjects may solve the task by learning to avoid the empty cup, without acquiring any knowledge about the other rewarded container (i.e. low-level strategy). Other individuals, however, may be able of inferring the logical presence of the food under the non-lifted container (i.e. high-level strategy). Although both mechanisms are not mutually exclusive, e.g. one subject can follow a mixed strategy (e.g. [START_REF] Jelbert | Reasoning by exclusion in New Caledonian crows (Corvus moneduloides) cannot be explained by avoidance of empty containers[END_REF], choosing by avoiding the incorrect option is less cognitively challenging than inferential reasoning per se [START_REF] Mikolasch | Is caching the key to exclusion in corvids? The case of carrion crows (Corvus corone corone)[END_REF], and may be present in more individuals.

The ability to choose by exclusion has been found in various animals such as elephants (Plotnick et al. 2014), dogs [START_REF] Aust | Inferential reasoning by exclusion in pigeons, dogs, and humans[END_REF][START_REF] Erdőhegyi | Dog-logic: inferential reasoning in a twoway choice task and its restricted use[END_REF], goats [START_REF] Nawroth | Exclusion performance in dwarf goats (Capra aegagrus hircus) and sheep (Ovis orientalis aries)[END_REF], or non-human primates [START_REF] Hill | Inferential reasoning by exclusion in great apes, lesser apes, and spider monkeys[END_REF][START_REF] Sabbatini | Inferences about the location of food in capuchin monkeys (Cebus apella) in two sensory modalities[END_REF][START_REF] Schmitt | Inferential reasoning and modality dependent discrimination learning in olive baboons (Papio hamadryas anubis)[END_REF]. In birds, mainly in Corvids and Parrots, some species pass the cups task (e.g. carrion crows Corvus corone, [START_REF] Mikolasch | Is caching the key to exclusion in corvids? The case of carrion crows (Corvus corone corone)[END_REF]Clark's nutcracker, Nucifraga Columbiana, Tornick and Gibson 2013;red-tailed black cockatoo Calyptorhynchus banksii, Subias et al. 2019), whereas others demonstrate marked difficulties (e.g. Eurasian Jays, Garrulus glandarius, [START_REF] Shaw | Exclusion in corvids: The performance of foodcaching Eurasian jays (Garrulus glandarius)[END_REF]jackdaws, Corvus monedula, Schloegl 2011;keas, Nestor notabilis, Schloegl et al. 2009, but see O'Hara et al. 2016).

Whilst some researchers have supported hypotheses about ecological factors promoting the emergence of exclusion in specific animal orders (e.g., 'the adapted intelligence hypothesis' in food-caching corvids e.g. [START_REF] Mikolasch | Is caching the key to exclusion in corvids? The case of carrion crows (Corvus corone corone)[END_REF][START_REF] Schloegl | What you see is what you get? Exclusion performances in ravens and keas[END_REF][START_REF] Schloegl | What you see is what you get-reloaded: can jackdaws (Corvus monedula) find hidden food through exclusion?[END_REF], but see [START_REF] Shaw | Exclusion in corvids: The performance of foodcaching Eurasian jays (Garrulus glandarius)[END_REF]'social complexity' in primates e.g. [START_REF] Petit | Inferences about food location in three cercopithecine species: an insight into the socioecological cognition of primates[END_REF], others have proposed this ability to be a fundamental cognitive capacity ('general intelligence'; [START_REF] Pepperberg | Reasoning by inference: Further studies on exclusion in grey parrots (Psittacus erithacus)[END_REF], or, lately, to be of higher importance in flexible foraging species [START_REF] Nawroth | Exclusion performance in dwarf goats (Capra aegagrus hircus) and sheep (Ovis orientalis aries)[END_REF].

A good way to further understand the possible drivers favouring the emergence and evolution of exclusion is to expand the spectrum of species studied with this paradigm in species possessing traits associated with this ability [START_REF] Marsh | Inference by Exclusion in Lion-Tailed Macaques (Macaca silenus), a Hamadryas Baboon (Papio hamadryas), Capuchins (Sapajus apella), and Squirrel Monkeys (Saimiri sciureus)[END_REF][START_REF] Mikolasch | African grey parrots (Psittacus erithacus) use inference by exclusion to find hidden food[END_REF][START_REF] Schloegl | What you see is what you get? Exclusion performances in ravens and keas[END_REF]. Moreover, experiments with free-living animal populations are crucial to understand how exclusion performanceand cognitive processes more generallyoperates under natural conditions [START_REF] Byrne | Cognition in the wild: exploring animal minds with observational evidence[END_REF][START_REF] Cauchoix | Cognition in the field: comparison of reversal learning performance in captive and wild passerines[END_REF][START_REF] Völter | Causal and inferential reasoning in animals[END_REF].

However, as testing individuals in the wild is often prohibitively challenging [START_REF] Shaw | Wild psychometrics: evidence for 'general' cognitive performance in wild New Zealand robins, Petroica longipes[END_REF], we still lack studies about the use of exclusion in wild individuals. For instance, species can show high neophobia (preventing their willingness to participate in the task), be non-territorial (struggling with locating individuals when several trials need to be administrated), or live in groups (making difficult their isolation from conspecifics; [START_REF] Shaw | Wild psychometrics: evidence for 'general' cognitive performance in wild New Zealand robins, Petroica longipes[END_REF]. To our knowledge, only two captive exclusion experiments have been conducted with wild birds brought into captivity (New Caledonian crows, Corvus moneduloides [START_REF] Jelbert | Reasoning by exclusion in New Caledonian crows (Corvus moneduloides) cannot be explained by avoidance of empty containers[END_REF]Clark's nutcrackers: Tornick and Gibson 2013).

This study aims to assess exclusion performance in a new avian model: the wild brown skua (Catharacta antarctica ssp lonnbergi). This large gull-like seabird frequents Antarctic Peninsula and Subantarctic islands of Atlantic, Indian and Pacific Oceans [START_REF] Carneiro | Foraging behaviour and habitat use by brown skuas Stercorarius lonnbergi breeding at South Georgia[END_REF][START_REF] Carneiro | Flexibility in foraging strategies of Brown Skuas in response to local and seasonal dietary constraints[END_REF][START_REF] Furness | The Brown Skua (Catharacta antarctica)[END_REF]. Skuas are top marine predators that display a wide range of feeding techniques i.e. aerial and terrestrial hunting, cannibalism, fishing, kleptoparasitism, and scavenging [START_REF] Moncorps | Diet of the Brown Skua Catharacta skua lonnbergi on the Kerguelen archipelago: comparisons between techniques and between islands[END_REF][START_REF] Mougeot | Predation on burrowing petrels by the brown skua (Catharacta skua lonnbergi) at Mayes Island, Kerguelen[END_REF][START_REF] Schreiber | Biology of marine birds[END_REF].

When spatial or seasonal fluctuations in prey availability occur, these birds exhibit high degree of flexibility by adjusting their movements and switching to different resources [START_REF] Burton | Breeding biology of the brown skua, Catharacta skua lonnbergi (Mathews), at Signy Island, South Orkney Islands[END_REF][START_REF] Carneiro | Flexibility in foraging strategies of Brown Skuas in response to local and seasonal dietary constraints[END_REF]. Moreover, skuas can be locally highly selective in their prey intakes and show preferences for hunting specific seabird species (e.g. blue petrel, Halobaena caerulea), while others seem to be avoided (e.g. diving petrels, Pelecanoides urinatrix; [START_REF] Mougeot | Predation on burrowing petrels by the brown skua (Catharacta skua lonnbergi) at Mayes Island, Kerguelen[END_REF]. Both previous characteristics, behavioural flexibility and selectivity, have been suggested as explanatory factors for exclusion skills in animals [START_REF] Nawroth | Exclusion performance in dwarf goats (Capra aegagrus hircus) and sheep (Ovis orientalis aries)[END_REF]. Indeed, some highly flexible species may show higher aversion to losses through the avoidance of the empty container [START_REF] Nawroth | Exclusion performance in dwarf goats (Capra aegagrus hircus) and sheep (Ovis orientalis aries)[END_REF]).

Regarding one species' feeding ecology may also provide significant insights on the occurrence of exclusion performance under natural conditions [START_REF] Völter | Causal and inferential reasoning in animals[END_REF]). In the Kerguelen archipelago, some populations (e.g. skuas at Cape Ratmanoff) predate and scavenge mainly on king penguin chicks and eggs (Aptenodytes patagonicus; [START_REF] Furness | The Skuas[END_REF].

In this environment, a penguin egg covered by its incubating parent's fold of skin is often hardly visible (FB, JCC, AN: personal observations). In this context, being able to locate prey without directly perceiving it could be highly adaptive for skuas, which may have favoured the appearance of exclusion performance in the wild [START_REF] Marsh | Inference by Exclusion in Lion-Tailed Macaques (Macaca silenus), a Hamadryas Baboon (Papio hamadryas), Capuchins (Sapajus apella), and Squirrel Monkeys (Saimiri sciureus)[END_REF].

Noteworthy, similarly to other birds endemic to isolated islands (e.g. kea, Nestor notabilis, Gadjon et al. 2006;wild robin, Petroica longipes, Garland et al. 2014), the skua is highly explorative and shows very low neophobia [START_REF] Furness | The Brown Skua (Catharacta antarctica)[END_REF]. It is also highly territorial during the breeding season, forming breeding pairs and establishing a territory that is fiercely defended against conspecific intruders or potential predators [START_REF] Hahn | Feeding territoriality and the reproductive consequences in brown skuas Catharacta antarctica lonnbergi[END_REF][START_REF] Trivelpiece | Feeding territories of Brown Skuas (Catharacta lonnbergi)[END_REF]. Such aspects of this species' feeding ecology, along with complex foraging flexibility and territoriality, make the skua a promising model for exclusion experiments in the field (Gadjon et al. 2006).

In this study, we aimed to assess whether wild brown skuas were capable to use exclusion to find hidden food in a visual two-way object choice task. According to the species' feeding ecology, we predict skuas to be able to solve the task.

Methods

Study population

Between Feb 2010and Mar 2012, 26 wild brown skuas were tested at Cape Ratmanoff, Courbet Peninsula, Kerguelen Island (48°25-50°00S; 68°27'-70°35E). All birds had a minimum age of 1 year (i.e. colour pattern, uniformity, distinct wing flashes of the body plumage, [START_REF] Furness | The Brown Skua (Catharacta antarctica)[END_REF]. Eighteen subjects were individually identified based on leg bands coloured by Darvik©, 2 based on temporary coloured dye sprays (Porcimark©, Langeskov, Denmark, 22 Feb 2010-3 Mar 2010), and 6 based on specific aspects of their appearance (i.e. body plumage, presence/absence of scars on the feet' webbing, 2 Mar 2012 -10 Mar 2012). In both field seasons, trials were performed between 7 a.m. and 5 p.m. and each subject was tested separately (a second experimenter prevented conspecifics from entering the test area, by approaching and making the intruders leave within approximately 5 meters radius of the setup). Prior to the current study, no skua had ever been tested in any exclusion task.

Experimental setup and Materials

Two identical opaque plastic cups (13 cm in height and 11 cm in diameter) with a piece of string forming a loop on the top (braided sisal cord, 3 mm in diameter and 6 cm in length) were set up on the ground, 25 cm apart (Fig. 1). Both cups and string loops were coated with fish flesh to avoid birds to use any smell-related clue. Depending on the procedure (see below), two (training phase) or one container (test phase) held a motivating food reward (a fresh fish piece of Salmo trutta, ca. 3 g). At the onset of each trial during all 3 conditions, an opaque blind (a flat wooden board, 30 x 40 cm) was placed vertically between the subject and the cups (3 cm before the containers) to prevent subjects from seeing under which cup the experimenter hid the food reward. The free-ranging 26 subjects were initially familiarized with the experimental apparatus.

When a skua was sufficiently close to the apparatus (≤ 1 meter), the experimenter showed two food rewards and placed one under each cup. Skuas were then allowed to touch the cups.

Once a subject successfully retrieved the rewards by lifting the cups (4 times within 2 min), we considered that it was familiarized with the apparatus.

Test phase.

Three conditions were presented to all birds in the following order: full information, exclusion, and control. In all conditions, the position of the reward was pseudo-randomized and balanced equally on the left/right side in each condition (i.e. Control: left = 93/right = 94; Exclusion: left = 255/right = 263; Full information: left = 124/right = 126). The subject's choice was considered as correct when the bird lifted the cup and retrieved the reward. The order in which birds participated every day was randomly assigned through the subjects' voluntary participation. If a bird left the test area during the test and did not return within 1 minute, the trial was performed again from the beginning later in the day. During the whole procedure, the experimenter stayed approximately 0.5 m behind the cups, looking straight ahead.

Full information condition. In this condition, we wanted the birds to learn that they would have to make a choice. For that purpose, one reward was hidden under one cup by the experimenter behind the blind. Then, the experimenter removed the blind and both cups were simultaneously lifted so the subject could locate visually which cup contained the food. After 3 sec, both containers were put down in same places and the skua could choose between one of them (see S1 in supplementary material). If the subject lifted the rewarded cup, it was allowed to consume the food. Conversely, if the subject chose the unrewarded cup, the experimenter stopped the trial to prevent the bird from obtaining the food. Each subject received 10 trials in total (60 sec each, max. 10 per day, between 10 sec-1 min apart).

Exclusion condition. Same procedure as in Full information condition was followed, except that after baiting, only the empty cup was lifted while the experimenter simultaneously touched the rewarded cup (see S1 in supplementary material). This was made to prevent the reliance on enhancement effects (e.g. the tendency to choose the cup that is manipulated last, [START_REF] Mikolasch | Is caching the key to exclusion in corvids? The case of carrion crows (Corvus corone corone)[END_REF][START_REF] Nawroth | Exclusion performance in dwarf goats (Capra aegagrus hircus) and sheep (Ovis orientalis aries)[END_REF]. Each subject received 20 trials in total (60 sec each, max. 20 per day, between 10 sec-1 min apart).

Control condition. We assessed whether the subjects' choices were influenced by the use of olfactory or any other uncontrolled cues. Here, one cup still contained the reward, but subjects had no information about food position i.e. the experimenter did not lift the cups but touched them simultaneously. Each subject received 10 trials in total (60 sec each, max. 10 per day, between 10sec-1 min apart).

Analysis

All trials were coded live. When the weather allowed (no strong winds, nor flying sand), we also recorded trials using a video camera mounted on a tripod. Inter-observer reliability was not calculated as the subjects' choice in each trial was unambiguous (binary response: success or failure). We modelled birds' success probability at trials as a random variable following a binomial distribution. To compare birds' performance depending on test condition and to assess the possible effects of lateralization and learning, the probability to find the reward was modelled as a function of test condition (full information, exclusion, control), position of the food (right/left), and trial number (or ID_trial), in a generalized linear mixed-effects model (GLMM). To account for individual differences in probability to pass the test, we set individual identity as a random effect with a Gaussian distribution in the error structure of the model. We implemented generalized linear mixed-effects model trees (Fokkema et al. 2019) to identify relevant clusters and effects. The algorithm finds subgroups (or terminal nodes) that differ in their success rate, while taking into account the nested structure of the dataset (individuals). The algorithm evaluates the stability of model parameters (i.e. stability of the intercept coefficient) at each node. The node becomes terminal (stable) if p > 0.05 for all splitting variable of the model.

As position of the food (right or left cup) was not available for all testing sequences, we implemented two trees. Model tree I was based on a reduced dataset with only individuals for whom we had full information on the position of food (left/right) within the 3 conditions (n = 19). This first model was implemented with test condition and food position effects in interaction. Model tree II was implemented on the complete dataset including all the individuals (N = 26). We back transformed coefficients from the logit scale to get success probabilities on the response scale. We refer to these transformed coefficients as probability of success (Ps). Ps is reported with 95% confidence intervals. All the analyses were carried out with R version 3.6 (R Core Team, 2019), and glmertree package (Fokkema et al. 2019) was used to perform the analysis.

Results

Model tree I: test conditions and food position

With the model accounting for food position, we identified a difference in success probability between each of the 3 experimental conditions (control, exclusion and full information), as well as an effect of food position on success during control tests. Results are shown in Fig. 2.

< Fig. 2 about here >

There was a difference in performance between the control condition and the exclusion/full information condition (p < 0.001; Node 1). Food position had an effect on success probability but only within the control condition (p = 0.023; Node 2): birds' success probability was Ps = 0.43 [95% IC: 0.32 -0.54] when food was on the left (Node 3), and Ps = 0.62 [95% IC: 0.47 -0.75] when the reward was located on the right (Node 4). Thus, there was a mean side bias

Discussion

Overall, our results demonstrate that free-living skuas are capable of showing exclusion performance. Subjects succeeded above the level expected by chance in the full information and exclusion conditions, and their performances confirm that they did not use olfactory cues (smell of the hidden food reward) in the control condition. This experiment is the first to reveal exclusion skills in Charadriiformes within a natural setting.

When no information was available (control condition), most birds failed in locating the baited cup above chance. This result is not surprising when we refer to the species' biological and behavioural features. Skuas possess small olfactory bulbs and display no reaction when faced to striking odour changes of a same food item [START_REF] Furness | The Brown Skua (Catharacta antarctica)[END_REF]. Noteworthy, a mean side bias emerged over trials in the control condition with a mean preference to select the right cup in tested individuals. At the individual level, four subjects chose constantly the same side (three skuas selected the right one, one the left one). This behaviour is not uncommon when subjects are confronted with two-way object-choice tasks (e.g. Danel et al. 2018;[START_REF] Gagne | Performance on patterned string problems by common marmosets (Callithrix jacchus)[END_REF][START_REF] Ketchaisri | The use of a human's location and social cues by Asian elephants in an object-choice task[END_REF], and represents a useful strategy when environmental stimuli are unpredictable, such as in our control condition (Jacobs and Osvath 2015; [START_REF] Ketchaisri | The use of a human's location and social cues by Asian elephants in an object-choice task[END_REF][START_REF] Plotnik | Visual cues given by humans are not sufficient for Asian elephants (Elephas maximus) to nd hidden food[END_REF][START_REF] Tebbich | Non-tool-using rooks, Corvus frugilegus, solve the trap-tube problem[END_REF]). In contrast, skuas were able to derive relevant information from the full information and exclusion conditions without displaying any side bias, probably because both conditions were predictable.

Similarly to other avian species (e.g., African grey parrots, [START_REF] Mikolasch | African grey parrots (Psittacus erithacus) use inference by exclusion to find hidden food[END_REF][START_REF] Pepperberg | Reasoning by inference: Further studies on exclusion in grey parrots (Psittacus erithacus)[END_REF], Schloegl et al. 2012;carrion crows, Mikolasch et al. 2012;Clark's nutcrackers, Tornick and Gibson 2013;Eurasian jays, Shaw et al. 2013;jackdaws, Schloegl et al. 2011;keas, Schloegl et al. 2009;ravens, Schloegl et al. 2009; red-tailed black cockatoos, [START_REF] Subias | Inference by exclusion in the Red-tailed Black Cockatoo (Calyptorhynchus banksii)[END_REF], skuas chose significantly more the rewarded cup over the unrewarded one in the full information condition (success probability Ps = 0.72 [95% IC: 0.64 -0.80]). Our results indicate that this species was able to rely on direct visual information for solving the task. We also suggest that skuas showed spontaneous object permanence [START_REF] Piaget | The construction of reality in the child[END_REF], i.e. the understanding that hidden objects do not cease to exist [START_REF] Pepperberg | Reasoning by inference: Further studies on exclusion in grey parrots (Psittacus erithacus)[END_REF], as they successfully selected the opaque container covering the food reward.

Crucially, this species was also able to use indirect information (i.e. the view of the empty cup) to choose substantially more the baited cup over the unbaited one in the critical exclusion condition (success probability Ps = 0.64 [95% IC: 0.53 -0.74]). Unlike other captive avian species (e.g., [START_REF] Schloegl | What you see is what you get? Exclusion performances in ravens and keas[END_REF], skuas in this experiment did not require extensive training and all birds were naive to experimental testing. In this case, the subjects may not have had the time to learn the causal cues (low-level explanation i.e. avoidance of the empty cup) required to solve the task [START_REF] Nawroth | Exclusion performance in dwarf goats (Capra aegagrus hircus) and sheep (Ovis orientalis aries)[END_REF][START_REF] Shaw | Exclusion in corvids: The performance of foodcaching Eurasian jays (Garrulus glandarius)[END_REF][START_REF] Schloegl | What you see is what you get-reloaded: can jackdaws (Corvus monedula) find hidden food through exclusion?[END_REF][START_REF] Schloegl | What you see is what you get? Exclusion performances in ravens and keas[END_REF]. Moreover, exclusion performance might not normally require elementary associative learning when a limited number of tests is proposed [START_REF] Darmaillacq | Cognition animale: perception, raisonnement et représentations[END_REF][START_REF] Shaw | Exclusion in corvids: The performance of foodcaching Eurasian jays (Garrulus glandarius)[END_REF]. However, we found a progressive amelioration in subjects' performance over trials within the exclusion condition. We therefore cannot rule out the possibility that subjects have used a non-costly cognitive strategy through the avoidance of the empty cup in the following trials [START_REF] Call | Inferences by exclusion in the great apes: the effect of age and species[END_REF][START_REF] Paukner | Tufted capuchin monkeys (Cebus apella) spontaneously use visual but not acoustic information to find hidden food items[END_REF][START_REF] Mikolasch | Is caching the key to exclusion in corvids? The case of carrion crows (Corvus corone corone)[END_REF][START_REF] Schmitt | Inferential reasoning and modality dependent discrimination learning in olive baboons (Papio hamadryas anubis)[END_REF]. Only a further study with proper controls will clarify the use of low-level strategies in this species (e.g. see the different tests used in [START_REF] Call | Inferences by exclusion in the great apes: the effect of age and species[END_REF][START_REF] Mikolasch | African grey parrots (Psittacus erithacus) use inference by exclusion to find hidden food[END_REF], with the procedure pioneered by [START_REF] Premack | Levels of causal understanding in chimpanzees and children[END_REF]; or the follow-up conditions of the modified tubes task employed by [START_REF] Jelbert | Reasoning by exclusion in New Caledonian crows (Corvus moneduloides) cannot be explained by avoidance of empty containers[END_REF]. Future experiments may also assess whether wild skuas still show success when they are confronted with an increased number of containers, such as in the 3-cups object choice task [START_REF] Grether | An experimental study of insight in monkeys[END_REF]. This version may provide contrasting results and is considered more complex than the binary cups task [START_REF] Marsh | Inference by Exclusion in Lion-Tailed Macaques (Macaca silenus), a Hamadryas Baboon (Papio hamadryas), Capuchins (Sapajus apella), and Squirrel Monkeys (Saimiri sciureus)[END_REF].

At first glance, our results support the adaptive specialisation hypothesis (De [START_REF] Kort | An evolutionary perspective on caching by corvids[END_REF], which suggests that cognitive abilities may vary depending on specific aspects of one species' feeding ecology. Referring to this assumption, [START_REF] Nawroth | Exclusion performance in dwarf goats (Capra aegagrus hircus) and sheep (Ovis orientalis aries)[END_REF] recently found a link between highly selective feeding behaviour and exclusion performance in small ruminants. Although it would be tempting to propose similar mechanisms operating in our model species, only a comparative approach may reveal the potential role behavioural flexibility plays in exclusion. For instance, further experiments may test exclusion performance in skuas' genetically related species that possess different feeding ecologies, e.g. specialist predators such as long-tailed skuas (Stercorarius longicaudus, Sitter et al. 2011).

Naturally, predatory animals might be highly attentive and sensitive to the sound emitted by their preys [START_REF] Grant | Acoustic prey and a listening predator: interaction between calling katydids and the bat-eared fox[END_REF][START_REF] Rice | Acoustic location of prey by the marsh hawk: adaptation to concealed prey[END_REF][START_REF] Schmitt | Inferential reasoning and modality dependent discrimination learning in olive baboons (Papio hamadryas anubis)[END_REF]. Although vision seems to play a significant role in skuas' ecology, this species relies also on acoustic information during foraging [START_REF] Furness | The Skuas[END_REF]. For instance, on Mayès Island, Kerguelen archipelago, acoustic information helps specialized skuas in finding burrow-nesting birds at night [START_REF] Mougeot | Predation as a cost of sexual communication in nocturnal seabirds: an experimental approach using acoustic signals[END_REF]. Thus, in order to determine potential variations in levels of exclusion performance across sensory modalities, further experiments should assess skuas' capacity to locate hidden food when given acoustic information.

In conclusion, wild brown skuas are capable of showing exclusion performance in the visual domain. Although the role ecological traits play in the emergence of exclusion performance must be the object of further research, our results corroborate the hypothesis that this capacity has emerged in distantly related taxa through convergent evolution [START_REF] O'hara | Inference by exclusion in Goffin cockatoos (Cacatua goffini)[END_REF]. Skuas possess life history and ecological traits that are commonly linked to enhanced cognition [START_REF] Morand-Ferron | Food stealing in birds: brain or brawn?[END_REF][START_REF] Ricklefs | The cognitive face of avian life histories: the 2003 Margaret Morse Nice lecture[END_REF]) e.g. late sexual and physical maturity, longevity, high innovation rate, kelptoparasitism, and generalism [START_REF] Chester | Antarctic birds and seals. Wandering Albatross[END_REF]Furness et al. 2020;[START_REF] Spear | Kleptoparasitism by brown skuas on albatrosses and giant-petrels in the Indian ocean[END_REF]. However, to date, we are aware of only one previous experimental cognitive work showing the capacity of skuas to recognize human faces [START_REF] Lee | Antarctic skuas recognize individual humans[END_REF]. Research is needed to explore further the cognitive abilities of this promising new model system. We also hope this study will encourage future field comparative work on this capacity across taxonomic groups, so as to enhance our current view of the distribution of exclusion skills in avian phylogeny. showed on a wooden platform. In the sketch, the reward is located under the right cup. The subject is watching a human experimenter lifting the empty left cup. To solve the task, the skua must exclude the left cup (low-level explanation) and/or infer that the right cup covers the reward (high-level explanation), by raising this container accordingly.

Figure captions

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  Fig. 1 about here > Procedure Familiarization phase.

  towards the right container during this condition. At the individual scale, we could detect birds 100% lateralized (Left side: N = 1, right side: N = 3). Skuas were on average more successful in the full information condition (Ps = 0.74 [95% IC: 0.63 -0.83]) than in the exclusion condition (Ps = 0.64 [95% IC: 0.53 -0.74]) (p = 0.024; Node 5). Model tree II: test conditions and learning With the model accounting for trial number (complete dataset), we identified a difference between each of the 3 experimental conditions (control, exclusion and full information), as well as an effect of trial number (ID_trial) during exclusion tests. Results are shown in Fig. 3. < Fig. 3 about here > There was a difference in performance between the control condition and the exclusion/full information condition (p < 0.001; Node 1). Mean success probability during the control condition (Node 2) was Ps = 0.51 [95% IC: 0.43 -0.59]. The model tree procedure identified a difference between the exclusion and full information condition (p = 0.021; Node 3). During the exclusion condition, there was a difference between the first 12 trials and the last 8 trials (p = 0.021; Node 4), with an average success probability Ps = 0.58 [95% IC: 0.49 -0.67] during the first 12 trials (Node 5), and an average success probability Ps = 0.71 [95% IC: 0.62 -0.79] during the last 8 trials (Node 6). Thus, there was a learning effect emerging over the course of the exclusion condition. During the full information condition (Node 7), mean success probability was Ps = 0.72 [95% IC: 0.64 -0.80].

Fig. 1

 1 Fig. 1 Schematic representation of the conditions and the experimental setup for a typical

Fig. 2

 2 Fig. 2 Model tree I: test conditions and food position. The Tree shows skuas' estimated mean

Fig. 3

 3 Fig. 3 Model tree II: test conditions and learning. The Tree shows skuas' estimated mean

Fig. 1

 1 Fig. 1

Fig. 2

 2 Fig. 2

Fig. 3

 3 Fig. 3

Acknowledgments of support

This work was financed by the French Polar Institute (IPEV): project ETHOTAAF 354 to FB. We are very grateful to Yoanna Marescot for helping with the experiments on the field.