M Le Corre

Ocean control of the breeding regime of the sooty tern in the southwest Indian Ocean

Keywords: Seabirds, Sterna fuscata, breeding phenology, SST, Chlorophyll, Mozambique Channel, Seychelles Archipelago, southwest Indian Ocean

Food availability, which is often seasonal, is regarded as a key factor in the breeding success of seabirds. In oceanic tropical areas, the resources are mostly patchy and ephemeral at the surface, and the seasonality is less marked than at higher latitudes. Such a situation influences greatly the breeding strategies of the oceanic seabird species. We conducted a comparative study of the breeding phenology of the sooty tern (Sterna fuscata) in relation to the local and regional oceanographic conditions around the four major colonies (Europa, Juan de Nova, Lys and Bird Islands) of the southwest Indian Ocean. Over the 1997-2003 period, around all the studied locations, the sea-surface temperature (SST) and the chlorophyll concentration in the Mozambique Channel and the Seychelles area showed clear seasonal differences related to the southern climate and the monsoon phenomena. The breeding activity is synchronized at each studied colony, but the timings are very different. Seasonal reproduction occurs in austral winter at Europa and Bird Island and in austral summer at Juan de Nova; at Lys Island the reproduction is non-seasonal. For the seasonal colonies, there is a large monthly change in SST just before the beginning of reproduction, which is a proxy indicating the annual phytoplankton bloom. This variation is accompanied by the development of oceanic features such as fronts that favour aggregation of prey, and may also play an important role in the presence of schools of surface tuna, which are very important for the foraging success of sooty terns. Conversely, around Lys Island the seasonal variations of the marine environment do not lead to pronounced development of oceanic structures, and consequently, the longer-lasting phytoplankton bloom could explain the non-seasonal breeding regime there. Further studies will help discern the advantages and disadvantages of seasonal and non-seasonal reproduction regime in response to unpredictable fluctuations of the marine environment.

Introduction

Since breeding is energetically costly, seabirds should attempt to initiate breeding at a time that ensures optimal overlap between nutritional requirements and food availability [START_REF] Lack | Ecological adaptations for breeding in birds[END_REF][START_REF] Frederiksen | Scale-dependent climate signals drive breeding phenology of three seabirds species[END_REF]. Because individuals are tied to their colony while breeding, the distribution and abundance of food within their foraging range are the key factors affecting reproductive success [START_REF] Chastel | Body condition and seabird reproductive performance: A study of three petrel species[END_REF], colony size [START_REF] Cairns | Population regulation of seabird colonies[END_REF][START_REF] Parrish | Effects of colony location and nekton abundance on the at-sea distribution of four seabird species[END_REF][START_REF] Ainley | Prey resources, competition, and geographic structure Kittiwakes colonies in Prince William Sound[END_REF] and breeding season (Le Corre 2001). Hence, seabirds may be able to rely on proximal environmental factors of their marine habitat, which will allow them to initiate reproduction at the best time to coincide with the largest availability of prey and ultimately optimise their breeding success.

In long-lived species like seabirds, the breeding regime is mostly seasonal and related to the seasonal availability of prey [START_REF] Ainley | Seabirds of the Farallon Islands[END_REF][START_REF] Hamer | Breeding biology, lifes histories, and life history-environment interactions in seabirds[END_REF]. At polar and temperate latitudes, the synchrony between food supply and breeding schedules is often forced by the well marked seasonality of the environment [START_REF] Schreiber | Nesting chronology of the eastern brown pelican[END_REF]. Several studies have already demonstrated correlation between spring temperature and breeding phenology, when prey are sufficiently abundant to support high predation rate and to limit competition [START_REF] Martin | Food as limit on breeding birds: a life-history perpective[END_REF][START_REF] Lindberg | Effects of spring environment on nesting phenology and clutch size of Black Brant[END_REF][START_REF] Ainley | Seabirds of the Farallon Islands[END_REF]. Availability of prey, consequently, is regarded as a nearly indispensable condition for individuals to succeed in reproduction.

At high latitudes, the photoperiod plays a critical role in the decision to initiate and terminate breeding because it is a highly reliable factor, which is not affected by local environmental conditions [START_REF] Schreiber | Nesting chronology of the eastern brown pelican[END_REF][START_REF] Wingfield | Natural regulation of reproductive cycles[END_REF]. The length of the day is correlated with specific seasons and associated with specific foraging conditions (i.e., availability of prey). Hence, the photoperiod could be considered as the ultimate factor driving reproduction, with local environmental factors like the temperature and food availability being important proximal cues to adjust the timing of reproduction [START_REF] Schreiber | Climate and weather effects on seabirds[END_REF].

In tropical zones the marine environment has, for a long time, been believed to have little seasonality and little productivity compared to higher latitudes [START_REF] Ashmole | Seabird ecology and the marine environment[END_REF]. This widely held notion is largely not supported anymore [START_REF] Shea | Temporal variation in growth performance in six species of tropical, pelagic seabirds[END_REF][START_REF] Le Corre | Seasonal and interannual variation in the feeding ecology of a tropical oceanic seabird, the red-tailed tropicbird Phaethon rubricauda[END_REF], although seasonality is less marked than at higher latitudes. Similarly, the annual changes in photoperiod are small at low latitudes. Interestingly, many bird species show annual reproductive cycles that often resemble the seasonal reproductive strategies of temperate birds [START_REF] Wikelski | Reproductive seasonality of seven neotropical passerine species[END_REF][START_REF] Beebe | A seasonally breeding tropical bird lacks absolute photorefractoriness in the wild, despite high photoperiodic sensitivity[END_REF]. The causes, however, are not very well understood. In tropical seabird species the relation between initiation of reproduction, hormonal secretion and photoperiod is weak, and other local environmental factors like sea surface temperature (SST) could trigger breeding activity [START_REF] Lormée | Reproductive endocrinology of tropical seabirds : sex-specific pattern in LH, steroids, and prolactin secretion in relation to parental care[END_REF]. Especially, in places where huge seasonal changes of the marine environment like monsoon or seasonal upwelling occur, seabird populations show breeding patterns similar to those that exist at higher latitudes [START_REF] Harrison | Hawaiian seabird feeding ecology[END_REF][START_REF] Le Corre | Breeding seasons of seabirds of Europa Island (southern Mozambique Channel) in relation to seasonal changes in the marine environment[END_REF].

In the southwest tropical Indian Ocean, although some seabird species have nonseasonal breeding periods [START_REF] Diamond | Subannual breeding and moult cycles in the Bridled tern (Sterna anaethetus) in the Seychelles[END_REF], for most of them breeding activity is seasonal. At the Seychelles Archipelago [START_REF] Feare | The biology of the Sooty tern Sterna fuscata in the Seychelles and the effects of experimental removal of its eggs[END_REF][START_REF] Feare | Breeding schedules and feeding strategies of Seychelles seabirds[END_REF][START_REF] Ramos | Influence of local and large-scale weather events and timing of breeding on tropical roseate tern reproductive parameters[END_REF], at Aldabra Atoll [START_REF] Diamond | The biology of terns nesting at Aldabra Atoll, Indian Ocean, with particular reference to breeding seasonality[END_REF] and at Europa Island in the southern Mozambique Channel [START_REF] Le Corre | Breeding seasons of seabirds of Europa Island (southern Mozambique Channel) in relation to seasonal changes in the marine environment[END_REF][START_REF] Le Corre | Seasonal and interannual variation in the feeding ecology of a tropical oceanic seabird, the red-tailed tropicbird Phaethon rubricauda[END_REF], most of the seabirds breed seasonally. The breeding seasons depend on various environmental factors being favourable for reproduction. However, no study has clearly identified the changes of the marine environment that generate these favourable conditions for seabirds, although Le Corre (2001) noted, at Europa Island, a match between the presence of cold subtropical waters in the vicinity of the island and the breeding periods of some species, among which is the widespread and superabundant sooty tern (Sterna fuscata).

We study the timing of reproduction of the sooty tern in 4 major colonies of the southwest Indian Ocean: three in the Mozambique Channel and one in the Seychelles Archipelago (Figure 1a). From south to north, we considered Europa (22°20'S, 40°22'E), Juan de Nova (17°03'S, 42°44'E), Lys (Glorieuses Archipelago 11°31'S, 47°22'E) and Bird Islands (Seychelles Archipelago, 3°53'S, 55°12'E). These islands are oceanic and are the regular breeding locations of several hundreds of thousands of breeding pairs of sooty terns [START_REF] Feare | The biology of the Sooty tern Sterna fuscata in the Seychelles and the effects of experimental removal of its eggs[END_REF][START_REF] Rocamora | Seychelles. In Africa and associated islands[END_REF], Le Corre & Jaquemet 2005). Within this latitudinal range, breeding seasons are very different, with austral winter reproduction on Europa (Le Corre 2001) and Bird Island [START_REF] Feare | The biology of the Sooty tern Sterna fuscata in the Seychelles and the effects of experimental removal of its eggs[END_REF][START_REF] Feare | Breeding schedules and feeding strategies of Seychelles seabirds[END_REF][START_REF] Rocamora | Seychelles. In Africa and associated islands[END_REF] and summer reproduction on Juan de Nova [START_REF] Le Corre | Breeding seasons of seabirds of Europa Island (southern Mozambique Channel) in relation to seasonal changes in the marine environment[END_REF]Jaquemet 2005). Finally, at Lys Island, sooty terns do not breed seasonally [START_REF] Le Corre | Assessment of seabird community of the Mozambique Channel and its potential use as indicator of tuna abundance[END_REF]. These 4 islands are in the same hemisphere and not very distant from one another, so why are there such differences in the breeding season of the sooty tern between these sites?

In several cases, the prey abundance around breeding locations is correlated with the reproductive biology of seabirds (i.e. [START_REF] Anderson | Brown pelican: influence of food supply on reproduction[END_REF][START_REF] Cruz | Effect of El-Nino Southern Oscillation conditions on nestling growth rate in the dark-rumped petrel[END_REF]. Similarly, the abundance and availability of prey have often been related to fluctuation of SST driven by climate variability [START_REF] Hyrenbach | Ocean warming and seabird communities of the California Current System: response at multiple temporal scales[END_REF][START_REF] Peck | Sea-surface temperature constraints wedge-tailed shearwater foraging success within breeding seasons[END_REF], such as the seasonal cycle [START_REF] Spear | Response of seabirds to thermal boundaries in the tropical Pacific: the thermocline versus the Equatorial Front[END_REF]. In tropical areas, surface enrichment in chlorophyll leads to a higher abundance of zooplankton and flying fish (Piontkovki and Williams 1995), which are largely preyed on by tropical birds. Since our studied colonies are very large and the breeding activity is highly synchronized within locations, the food requirements by colonies are enormous, while the foraging is temporally and spatially restricted for the breeding adults. We hypothesize that, except at Lys Island, there should be cyclic and predictable oceanic forcing around the breeding sites that largely increase the availability of food and lead to favourable reproductive conditions. Hence, the aim of this study is to investigate which environmental factors could drive the breeding regime observed in the different colonies of sooty terns in an area where the seasonal photoperiodicity is weak. For this purpose, we examine the temporal relation between the breeding phenology of the birds and the variation of the local marine environment. We focus on the SST and the chlorophyll concentration around the colonies to test whether these parameters could indicate changes of the marine environment that lead to an increase in the food availability for birds. Finally, we try to relate the local conditions observed around the colonies to the general oceanic forcing in the southwest Indian Ocean that may be responsible for the favourable changes for seabirds and could explain the differences in the regimes of reproduction.

Currents and upwelling in study area

We focus our analysis on the tropical southwest Indian Ocean and more precisely on the Mozambique Channel and the Seychelles area. Oceanographic conditions in both of these regions are highly influenced by the South Equatorial Current (SEC), itself seasonally varying in location and in intensity in response to the monsoon phenomenon [START_REF] Piton | Les conditions favorables à la présence de thons de surface dans les parages de Madagascar[END_REF][START_REF] Donguy | Seasonal variations of sea-surface salinity and temperature in the tropical Indian Ocean[END_REF]. After it reaches the East Madagascar coast at 15-20°S, the SEC divides, with one component, the North Madagascar Current, flowing around the northern end of Madagascar [START_REF] Swallow | The boundary currents east and north of Madagascar 1.Geostrophic currents and transports[END_REF] and then branching when reaching the Comoro Archipelago. The southern branch of this feeds the flow southward through the Mozambique Channel, whilst the northern branch, known as the East African Coastal Current, feeds the Equatorial Counter-Current or, in austral winter, reaches the Somalia Upwelling [START_REF] Donguy | Une année d'observations de surface dans la zone de mousson de la partie occidentale de l'océan Indien[END_REF][START_REF] Marsac | Etude des relations entre l'hydroclimat et la pêche thonière hauturiere tropicale dans l'océan Indien occidental[END_REF].

Hence, the circulation of the Mozambique Channel is mainly derived from the SEC and has a dominant southward flow, along the African coast. The Davie Ridge (17°S), however, separates the channel into two distinct parts with very different circulation systems. In the northern part the circulation is dominated by a large anticyclonic gyre [START_REF] Donguy | The Mozambique Channel revisited[END_REF][START_REF] Schouten | Eddies and variability in the Mozambique Channel[END_REF] that is limited to the south by the narrows of the channel. A number of deep anticyclonic eddies have been found along the Mozambican coast [START_REF] De Ruijter | Observations of the flow in the Mozambique Channel[END_REF]. They appear to be generated by the pinching off of the instabilities of an eastward current that penetrates temporally through the gap at 17˚S [START_REF] Ridderinkhof | Moored current observations in the Mozambique Channel[END_REF]. A mooring across this constriction has revealed 4 to 5 eddies per year [START_REF] Ridderinkhof | Moored current observations in the Mozambique Channel[END_REF]. [START_REF] Schouten | Eddies and variability in the Mozambique Channel[END_REF] picked up a similar frequency for the anticyclonic eddies propagating poleward along the Mozambican coast. [START_REF] Quartly | Eddies in the southern Mozambique Channel[END_REF] showed that these features often have a pronounced ocean colour signature as they entrain productive waters from the coast and advect them into the centre of the channel. In addition the southern part of the channel, in austral winter, is penetrated by subtropical upwelled waters [START_REF] Stequert | La pêche de surface des thonidés tropicaux dans l[END_REF]) and cyclonic eddies generated by the East Madagascar Current (Di [START_REF] Marco | Volume transport and property distributions of the Mozambique Channel[END_REF][START_REF] Quartly | Eddies in the southern Mozambique Channel[END_REF].

The Seychelles Archipelago consists of numerous islands and banks that strew the current system constituted by the SEC and the Equatorial Counter-Current. These currents are present all year round, but their boundary, which is a divergence zone, varies in latitude between 6-7°S in austral summer and 9-10°S in winter [START_REF] Piton | Les conditions favorables à la présence de thons de surface dans les parages de Madagascar[END_REF]. To the northwest of the Seychelles, in austral winter, the southwest monsoon promotes the upwelling off Somalia and induces a transport of coastal African waters towards the Arabian Sea. These productive upwelled waters from the East African coast generate a large increase in the chlorophyll concentration that extends up to north of the Seychelles.

Data and analysis

Sooty tern breeding seasons

We estimated laying dates on Europa (1995,2002,2003), Juan de Nova (2002, 2003) and Lys (2003, 2004) Islands by determining the age of randomly selected chicks from measurements of wing length using the growth curve of [START_REF] Feare | The biology of the Sooty tern Sterna fuscata in the Seychelles and the effects of experimental removal of its eggs[END_REF] and assuming a mean incubation period of 28 days [START_REF] Schreiber | Sooty Tern (Sterna fuscata)[END_REF]. Seychelles data were from [START_REF] Feare | The biology of the Sooty tern Sterna fuscata in the Seychelles and the effects of experimental removal of its eggs[END_REF] on Bird Island, one of the largest and more seasonal sooty tern colonies of the archipelago [START_REF] Feare | The biology of the Sooty tern Sterna fuscata in the Seychelles and the effects of experimental removal of its eggs[END_REF][START_REF] Feare | Breeding schedules and feeding strategies of Seychelles seabirds[END_REF][START_REF] Rocamora | Seychelles. In Africa and associated islands[END_REF].

Surface oceanographic conditions

We used sea-surface temperatures (SST) and chlorophyll concentration in a 500 km radius circle centred on each breeding site, assuming that this is the maximum foraging range of breeding sooty terns [START_REF] Flint | Time and energy limits to the foraging radius of Sooty Terns Sterna fuscata[END_REF]. The SST data were obtained from the IGOSS database (http://ingrid.ldgo.columbia.edu) from January 1997 to December 2003. Although the chlorophyll concentration changes seasonally in response to the cycle of environmental forcing, the distribution of phytoplankton is very inhomogeneous. This is manifested by pronounced phytoplankton growth along the coasts (Figure 1b) prompted by local upwelling of nutrients or riverine input. Assuming that terns can adopt an optimal foraging strategy, we determine the appropriate measure of productivity of the environs by the 90th percentile of the chlorophyll a within 500 km of the island. This is determined from a composite of measurements for each fifth of a month, and the median of these measurements taken as an indicator of the peak productivity within the region. Thus, for example, a high productivity is determined for Juan de Nova (Figure 2), as a sizeable part of the Madagascar coastline lies within reach of the tern colony on that island. The productivity determined for Europa, on the other hand, is much lower but has significant short-term (weekly) variability because of the pronounced effects of ephemeral events causing the advection of chlorophyll into mid channel, either from the west coast of Madagascar (Figure 1b) or, more commonly, from the Mozambican coast [START_REF] Quartly | Eddies in the southern Mozambique Channel[END_REF]. [START_REF] Zubkov | Ultraplankton distribution in the surface waters of the Mozambique Channel -flow cytometry and satellite imagery[END_REF] showed that in this region the ultraplankton (those ranging in size from 0.2µm to 5µm) were composed principally of the eukaryotic algae Synechococcus and Prochlorococcus in equal proportions. However, whilst the abundance of Synechococcus related closely to the satellite measurements the abundance of Prochlorococcus was not easily determined from satellite data. Thus changes in phytoplankton composition may not always be reflected in satellitederived values of chlorophyll.

Detection of changes in trends

We firstly analyse the monthly values of SST and chlorophyll concentration over the period 1997-2003 to identify the temporal variability of these parameters and describe the oceanic conditions around the colonies during breeding seasons. We use the time lag in the breeding peak between two successive seasons to evaluate the seasonality of the breeding on the different islands. Then we correlate the local trend changes (monthly) in the time series of SST and chlorophyll concentration with the laying dates by using the cum-sum method [START_REF] Ibanez | Application de la méthode des sommes cumulées à l'analyse de séries chronologiques océanographiques[END_REF]. This method, which calculates the sum of the deviations of all individual data from the mean of the series, allows us to detect dates of the changes of trend in the time series.

S p = x i -p k ! i=1 p
where S p is the cum-sum value, x i the i th observation and k the mean of the series

Results

Oceanographic conditions

The monthly values of SST and chlorophyll calculated over the period 1997-2003 show clear seasonal signals (Figure 2). The SST data show two well marked seasons and short transition periods, with the coldest period during austral winter, the hottest during austral summer and temperatures increasing with latitude from the south of the Mozambique Channel to the north of the Seychelles Archipelago. Over the study period, the amplitude of the seasonal cycle, calculated from the differences of SST for successive months (i.e., monthly variations), does not differ significantly between locations (all test t, p > 0.05). However, it could be noted that the waters surrounding Europa Island have slightly greater short-term variation (intra-monthly) and seasonal change in temperature than the 3 other locations. In the same way, Lys and Bird Islands have smaller short-term variation and seasonal amplitude, respectively.

Although seasonal cycles in chlorophyll occurred, they are independent of the latitude (Figure 2). Around both Europa and Bird Islands, which are the furthest apart, a bloom of phytoplankton starts in early winter and reaches a maximum in mid to late winter. This annual peak of chlorophyll is more marked around Bird Island than Europa Island. In addition to having their greatest seasonal variability, waters surrounding Bird Island have a greater interannual variability. Around Juan de Nova, over the year, the chlorophyll concentration is higher than around the other locations and shows two distinct periods. Between January and May (i.e. summer), when the SST are the warmest, the chlorophyll concentration is higher than during the second part of the year. Around Lys Island the concentration of chlorophyll is highest in late winter, and the seasonal peak of chlorophyll lasts longer than around the other islands.

Sooty tern breeding phenology in relation to the marine environment

Our data on laying dates for the sooty terns are highly correlated between years for Europa (all Spearman correlation rank rs, p < 0.001) and Juan de Nova (rs = 0.98, p < 0.001) but at the limit of the threshold of significance for Lys (rs = 0.68, p = 0.048). Table 1 shows the mean laying peak by island and year, and the time lag between successive mean peaks of laying. The small difference in dates between two successive laying peaks at Europa and Juan de Nova Islands indicates seasonal reproduction, which also occurs at Bird Island [START_REF] Feare | The biology of the Sooty tern Sterna fuscata in the Seychelles and the effects of experimental removal of its eggs[END_REF][START_REF] Feare | Breeding schedules and feeding strategies of Seychelles seabirds[END_REF]. On the other hand, the time lag between successive years in the mean laying dates at Lys Island is higher and close to one month (32 days). This result suggests a nonseasonal breeding regime there, as reinforced by previous observations (Table 1).

As expected, the analysis of the trend in the time series of SST and chlorophyll concentration indicates seasonal signals around all islands. Results of this analysis, however, give new information on the variability of the marine environment around the breeding locations. Hence, during periods with laying data for Europa, Juan de Nova and Bird Islands, the results reveal a clear and evident relation between laying dates and both SST and chlorophyll (Figure 3). The laying begins on average one month after the change of trend in the SST, and the chlorophyll trend reaches a maximum a short time after the laying period. In all cases, this maximum corresponds to the highest peak of chlorophyll during the year. An interesting difference between these locations is that the trends of SST and chlorophyll concentration are in opposition at Europa and Bird Islands, whereas they are in phase at Juan de Nova, with a slight lag in the response of the chlorophyll compared to the SST in all the cases. This result confirms the difference of regime in the productivity of these oceanic areas, already noted previously, but also the relation between the SST and the concentration of chlorophyll. In 2003, around Lys Island, SST and chlorophyll also showed seasonal trends and relation between the two parameters. However, there is no clear relation between the laying period and the variation of the marine environment (Figure 3).

Discussion

The oceanographic conditions around the sites we studied are different but show marked seasonal signals characterized in particular by annual blooms of phytoplankton. These blooms are suitable for the aggregation of zooplankton, small nekton, and larger carnivorous nektonic species (Piontkovki and Williams 1995) that are preyed on by top predators like seabirds. Our study demonstrates that the timing of reproduction of the sooty terns in the southwest Indian Ocean matches with these seasonal peaks of chlorophyll, except at Lys Island. In all the cases, the peaks of chlorophyll follow significant variations of SST, themselves induced by seasonal changes of the marine environment. Thus we suggest that the seasonal breeding regime at Europa, Juan de Nova and Bird Islands is driven by climate signals that strongly modify the marine environment and lead to favourable distribution of food around the breeding sites. Conversely, the non-seasonal breeding regime suspected at Lys Island (Le Corre & Jaquemet 2005) and exhibited in this study occurs when seasonal changes of the environment do not significantly affect the aggregation of organisms preyed on by sooty terns around the island.

Local environmental variations

We clearly identify a correlation between the breeding season of sooty terns from Europa, Juan de Nova and Bird Islands and the seasonal variability of the local marine environment as was already demonstrated for other species at different latitudes [START_REF] Forchhammer | Breeding phenology and climate[END_REF][START_REF] Ramos | Influence of local and large-scale weather events and timing of breeding on tropical roseate tern reproductive parameters[END_REF][START_REF] Frederiksen | Scale-dependent climate signals drive breeding phenology of three seabirds species[END_REF]. The relatively rapid variation in SST (change of trend) before the beginning of the reproduction indicates strong modification of the environment, which precedes the increase in marine productivity. This situation favours prey concentration and acts as the starter of the breeding period for sooty terns. The maximum of productivity is reached after the laying peak, during the rearing period, when the requirements of food for the colonies are the highest. In other tropical areas like the Galapagos Archipelago, a correlation between the change of SST and the breeding activity has already been reported [START_REF] Boersma | Breeding patterns of Galapagos Penguins as indicator of oceanographic conditions[END_REF]. These strong modifications of the environment revealed by the SST are often favourable to the enhancement of productivity of the ocean. Although the SST and the chlorophyll concentration around Lys Island show seasonal signals similar to those observed at the other locations, they do not seem to induce a seasonal breeding regime there. This suggests that they do not induce sufficiently favourable seasonal conditions to optimise the breeding success of the terns, or that there is no reliable proximal cue for conditions when young will be present. Such a situation is similar to that observed at Ascension Island in the equatorial Atlantic Ocean, where sooty terns breed independently of the seasonal variation in the environment and breed approximately every 10 months [START_REF] Ashmole | The biology of the wideawake or sooty tern Sterna fuscata on Ascension Island[END_REF].

Sooty terns are commensal of surface schools of tuna [START_REF] Schreiber | Sooty Tern (Sterna fuscata)[END_REF], and an increase in fish catches could be expected during the breeding seasons. However, surprisingly, the main catches of surface tropical tuna around the colonies do not closely match the breeding activity of the sooty terns (i.e., [START_REF] Marsac | Etude des relations entre l'hydroclimat et la pêche thonière hauturiere tropicale dans l'océan Indien occidental[END_REF][START_REF] Fonteneau | Atlas of tropical tuna fisheries[END_REF]. In winter, the main fishing activity of the studied area is centred on the Somalia upwelling, north of Bird Island. In the northern part of the Mozambique Channel, the fishing of surface tuna occurs between January and May, and the activity moves southward during this period. It is centred on the Juan de Nova sector after the breeding period of the terns (April-May). In both cases, however, the density of schools of tuna may be high enough for terns. It is quite likely that there is only a weak connection between fishing statistics and the real abundance of tuna. In the south of the Mozambique Channel, the commercial fishing activity is almost non-existent, but a survey has demonstrated a strong association between surface schools of predators, including tuna and sooty tern flocks during their breeding period [START_REF] Jaquemet | Foraging habitat of the seabird community of Europa Island (Mozambique Channel)[END_REF], suggesting high tuna abundance.

Seasonal breeding regime and large-scale variability of the marine environment

The seasonal variability of the marine environment at coarse scale (500 km) around the breeding sites is as well marked as that observed in the tropical western Indian Ocean, which is under the control of the monsoon phenomenon and the southern climate [START_REF] Donguy | Seasonal variations of sea-surface salinity and temperature in the tropical Indian Ocean[END_REF]. the contrasted breeding regimes between Europa and Bird Islands on the one hand and Juan de Nova on the other seem to be consequences of the seasonal variation of the oceanic conditions in the area and the development of oceanic structures that favour the aggregations of marine organisms, including prey and commensal surface tuna.

In austral winter in the southern Mozambique Channel and in the north of the Seychelles Archipelago, the intensification and the northward drift of the Subtropical Convergence [START_REF] Villacastin-Herrero | Sea surface temperature of oceans surrounding subequatorial Africa: seasonnal patterns, spatial coherence and long-term trends[END_REF] and the activation of the southwest monsoon [START_REF] Cushing | Survey of resources in the Indian Ocean and Indonesian sea[END_REF]) respectively, favour mixing of the euphotic layer leading to a decrease of SST and an increase of sea-surface salinity [START_REF] Donguy | Seasonal variations of sea-surface salinity and temperature in the tropical Indian Ocean[END_REF]. These conditions allow the development of phytoplankton blooms highlighted by the peaks of chlorophyll. In the sea around the Seychelles, consequently, primary, secondary, and tertiary production are higher between May and October than during the November-March period [START_REF] Cushing | Survey of resources in the Indian Ocean and Indonesian sea[END_REF][START_REF] Feare | Breeding schedules and feeding strategies of Seychelles seabirds[END_REF], and such processes probably also occur around Europa Island. These blooms, therefore, tend to induce the aggregation of marine organisms like flying fish (Piontkovki and Williams 1995), which are largely preyed on by sooty terns of these islands (Feare 1976, Jaquemet unpubl. data). The result is a large amount of food available around the colonies for breeding seabirds, which are limited in their foraging range [START_REF] Flint | Time and energy limits to the foraging radius of Sooty Terns Sterna fuscata[END_REF]. The plentiful catches of pelagic fishes observed south of the Mozambique Channel and Madagascar (Le Corre 2001) reinforce the idea of a higher productivity of the marine environment at this season.

Although the summer seasonal reproduction of sooty terns on Juan de Nova was already known (Le Corre 2001), the size of the colony was only recently estimated (Le Corre & Jaquemet 2005). The breeding of this huge colony (2,000,000 breeding pairs, the largest of the Indian Ocean) in austral summer, in the warm pool of the southwest Indian Ocean [START_REF] Donguy | Seasonal variations of sea-surface salinity and temperature in the tropical Indian Ocean[END_REF], suggests a large amount of food available at this season in the surface layer. Although SST and chlorophyll are usually negatively correlated, the peak in chlorophyll and the plentiful long-line catches of tuna for the area [START_REF] Fonteneau | Atlas of tropical tuna fisheries[END_REF]) confirm the probably large availability of food in the vicinity of the island. These observations suggest particular ocean forcing that induces favourable conditions for the development of large phytoplankton biomass. It has been recently demonstrated that around five anticyclonic eddies per year pass through or form at the narrows of the Channel (Ridderinkhof and de Ruitjer 2003). This southward train of eddies has maybe led previous researchers to infer a small cyclonic eddy centred on the Davie Ridge in summer [START_REF] Saetre | The circulation of the Mozambique Channel[END_REF] and could explain the divergence of currents found by [START_REF] Piton | Les conditions favorables à la présence de thons de surface dans les parages de Madagascar[END_REF]. The interactions of these features advect water into the central part of the Channel and also lead to upwelling and enhancement of the biological production [START_REF] Mcgillicuddy | Influence of mesoscale eddies on new production in the Sargasso Sea[END_REF][START_REF] Lima | Biological response to frontal dynamics and mesoscale variability in oligotrophic environments: Biological production and community structure[END_REF]. Anticyclonic structures favour the aggregation and the development of zooplanktonic organisms by generating retention of phytoplankton biomass [START_REF] Lima | Biological response to frontal dynamics and mesoscale variability in oligotrophic environments: Biological production and community structure[END_REF] and passive drifting organisms like larvae, and such an effect is corroborated by the numerical dominance of fish larvae and post-juveniles in the diet of the sooty terns of Juan de Nova (Jaquemet unpubl. data). Finally, since Juan de Nova is on the edge of the continental shelf, the outflows of the Madagascan rivers during the rainy season (November to March, [START_REF] Jury | A study of climate and weather variability over the tropical southwest Indian Ocean[END_REF] could increase the input of nutrients on to the shelf and also favour primary production.

Non-seasonality of Lys Island breeding regime

Conversely to Europa, Juan de Nova and Bird Islands, which are located in seasonally productive areas induced by large-scale oceanic forcing, Lys Island is less affected by major oceanic variations. The island, indeed, is south of the south-equatorial divergence, north of the Mozambique Channel circulation and far from seasonally existing oceanic features such as fronts or upwellings [START_REF] Piton | Les conditions favorables à la présence de thons de surface dans les parages de Madagascar[END_REF]. Moreover, the island is on the path of the northern branch of the SEC all year round. This leads to a more extended primary production period with a less marked temporal variation than at southern and northern locations. Such a situation is responsible for a greater unpredictability of the optimal oceanic conditions to breed and for irregular fluctuations of the feeding opportunities all year round. This could explain the non-seasonal breeding regime there. Indeed, if seasonal changes occurred but have rather little effect on breeding success at different times of the year, breeding at less than annual intervals might be advantageous for sooty terns [START_REF] Ashmole | The biology of the wideawake or sooty tern Sterna fuscata on Ascension Island[END_REF]. In this case the marine environment has favoured individuals that attempt to breed as often as they can, and it could explain the non-seasonality of the breeding regime of the sooty terns already noted at Lys Island [START_REF] Diamond | The biology of terns nesting at Aldabra Atoll, Indian Ocean, with particular reference to breeding seasonality[END_REF][START_REF] Le Corre | Breeding seasons of seabirds of Europa Island (southern Mozambique Channel) in relation to seasonal changes in the marine environment[END_REF], Le Corre & Jaquemet 2005). A similar situation in the breeding regime also occurs in the Seychelles Archipelago with a closely related species, the bridle tern (Sterna anaethetus), which breeds every 8 months at Cousin Island [START_REF] Diamond | Subannual breeding and moult cycles in the Bridled tern (Sterna anaethetus) in the Seychelles[END_REF]). However the reasons why bridled terns breed every 8 months at a place where most other seabird species breed seasonally during austral winter is not known [START_REF] Diamond | Subannual breeding and moult cycles in the Bridled tern (Sterna anaethetus) in the Seychelles[END_REF]). Interestingly, the reproduction at Lys Island is synchronised, and the species is known to have little success with second nesting attempts if the first one fails [START_REF] Feare | The biology of the Sooty tern Sterna fuscata in the Seychelles and the effects of experimental removal of its eggs[END_REF], Schreiber et al 2002). Similar observations have been reported for sooty terns on Ascension Island (Atlantic), which have a non-seasonal reproduction but co-ordinate their breeding [START_REF] Ashmole | The biology of the wideawake or sooty tern Sterna fuscata on Ascension Island[END_REF]. This suggests that colonialism is a more important selective pressure than the food availability for the species. It has been proposed that the colonies may act as information centres, which help individuals to locate the sources of food [START_REF] Ward | The importance of certain assemblages of birds as 'information centres' for food-finding[END_REF]. In the case of Lys Island, where the availability of prey may be hard to predict, colonialism may increase the breeding success of individuals. However, recent results do not support the information centre hypothesis [START_REF] Davoren | Search strategies of a pursuitdiving marine bird and the persistence of prey patches[END_REF]. It has even been demonstrated that the size of the colony is negatively correlated to the foraging activity of individuals [START_REF] Lewis | Evidence of intra-specific competition for food in a pelagic seabird[END_REF][START_REF] Ainley | Prey resources, competition, and geographic structure Kittiwakes colonies in Prince William Sound[END_REF]. On the other hand, sooty terns adopt a network foraging strategy at sea, which means cooperation [START_REF] Jaquemet | Foraging habitat of the seabird community of Europa Island (Mozambique Channel)[END_REF]. Hence, the high density of individuals may confer enough advantage in the search for food sources to outweigh the impact of intraspecific competition. Indeed, food resources are very patchy and mostly ephemeral in tropical oceanic waters, even when their production is at its highest.

These differences of reproductive strategies involve different life history traits between individuals of the populations. It shows the importance of intraspecific comparisons of the reproduction at different locations to understand the influence of the environment on the breeding, as already studied in the Southern Ocean on an albatross species [START_REF] Pinaud | Ultimate and proximate factors affecting the breeding performance of a marine top-predator[END_REF].

In conclusion, we suggest that there is oceanic control of the breeding phenology of the sooty terns of the southwest Indian Ocean. When the marine environment surrounding the colonies is seasonal and leads to the development of predictable oceanic features aggregating prey such as fronts, seasonal reproduction occurs. Conversely, when the seasonality of the marine environment does not lead to development of such features, non-seasonality of reproduction occurs. This hypothesis is reinforced by the seasonal reproduction, during winter, at Cosmoledo Atoll (9°43'S, 47°35E, Figure 1) of a huge sooty tern colony [START_REF] Rocamora | The breeding avifauna of Cosmoledo Atoll (Seychelles) with special reference to seabirds: conservation status and international importance[END_REF] in the southern part of the Seychelles Archipelago. Indeed, during the breeding period, the colony is on the edge of the divergence between the SEC and the Equatorial Counter Current [START_REF] Piton | Les conditions favorables à la présence de thons de surface dans les parages de Madagascar[END_REF].

In the light of our results and in the context of climate change, we could ask how sooty terns are able to respond to large climatic fluctuations of the ocean like El Niño Southern Oscillation. Indeed, several studies have already demonstrated the influence of large-scale weather events on the reproductive performances and dynamics of animal populations in different locations [START_REF] Schreiber | Central seabirds and the El Nino Southern Oscillation: 1982-1983 perspectives[END_REF][START_REF] Forchhammer | Breeding phenology and climate[END_REF][START_REF] Post | Synchronization of animal population dynamics by large-scale climate[END_REF][START_REF] Ramos | Influence of local and large-scale weather events and timing of breeding on tropical roseate tern reproductive parameters[END_REF][START_REF] Frederiksen | Scale-dependent climate signals drive breeding phenology of three seabirds species[END_REF]. Further studies will help discern the advantages and disadvantages of seasonal and non-seasonal reproduction regime in response to unpredictable climatic fluctuations. Conversely, the breeding biology of seabirds could be useful to monitor changes of long-term environmental conditions.

Figure 1

 1 Figure 1

  Figure 1 Click here to download Figure: Figure 1.doc

  Figure 3 Click here to download Figure: Figure 3.doc

Acknowledgements

The field work was supported financially by the DIREN-Réunion and logistically by the FAZSOI, to whom we are grateful. SJ benefited from a PhD grant from the French Ministère de l'Enseignement Supérieur de la Recherche. Help on field work from M. Salamolard, M. Charrier, F. Saint-Ange, T. Gesthemme S. Caceres, E. Robert and J. Emeras is appreciated. We are indebted to F. Marsac and A. Fonteneau from IRD-UR Thétis and the IOTC for their constructive comments on the manuscript. SeaWiFS data were processed and provided by NASA/GSFC under the 'Mission to Planet Earth', with data management at NOCS by L. Marsh. Comments by R. Wynn on an early draft are greatly appreciated, as are constructive comments from 3 anonymous reviewers.


A Edalati

H Boutayeb

T A Denidni

BAND STRUCTURE ANALYSIS OF RECONFIGURABLE METALLIC CRYSTALS: EFFECT OF ACTIVE ELEMENTS

The band structure for the normal propagation of crystals formed by periodically loaded metallic wires is analyzed for different wire diameters and for various loads, which are assimilated as diodes. The diodes are simulated by using an equivalent R-C circuit. The influences of the values of the R-C elements in On-state and Off-state are studied, and the results are compared with the band structures obtained for continuous and discontinuous-wire materials. To validate the proposed analysis, experimental results for a reconfigurable crystal are presented.

INTRODUCTION

Electromagnetic Band Gap (EBG) materials, also called Photonic Crystals, are periodic structures that offer pass-bands and stop-bands to electromagnetic waves [START_REF] Sakoda | Optical Properties of Photonic Crystal[END_REF][START_REF] Boutayeb | Multi-layer crystals of metallic wires: analysis of the transmission coefficient for outside and outside excitation[END_REF][START_REF] Dmitriev | Magnetic photonic crystals with square latticegroup theoretical standpoint[END_REF]. They have potential applications in microwave and antenna domains, such as designing high-gain antennas [START_REF] Boutayeb | Analysis and Design of a High-Gain Antenna based on Metallic Crystals[END_REF][START_REF] Pirhadi | An Analytical Investigation of the Radiation Characteristics of Infinitesimal Dipole Antenna Embedded in Partially Surfaces to Obtain High Directivity[END_REF][START_REF] Li | Study on High Gain Circular Waveguide Array Antenna with Metamaterial Structure[END_REF][START_REF] Weily | High-Gain 1D EBG resonator antenna[END_REF], suppressing surface waves [START_REF] Yang | Microstrip Antennas Integrated With Electromagnteic Band-Gap (EBG) Structures: A Low Mutual Coupling Design for Array Applications[END_REF], increasing antenna bandwidth [START_REF] Pirhadi | Using Electromagnetic Bandgap Superstrate to Enhance the Bandwidth of Probe-FED Microstrip Antenna[END_REF], designing Artificial Magnetic Conductors (AMC) for low-profile antennas [START_REF] Sohn | Comparative Study on Various Artficial Magnetic Conductors for Low-Profile Antenna[END_REF], and creating controllable microwave components [START_REF] De Lustrac | Experimental demonstrattion of electrically controllable photonic crystals at centimeter wavelenghts[END_REF][START_REF] Lourtioz | Toward controllable photonic crystals for centimeter and millimeter wave devices[END_REF][START_REF] Djermoun | Negative refraction device with electrically controllable permittivity and negative permeability[END_REF][START_REF] Boutayeb | Focusing Characteristics of a Metallic Cylindrical Electromagnetic Band Gap Structure with Defects[END_REF][START_REF] Karim | A novel reconfigurable filter using periodic structures[END_REF]. The objective of this paper is to describe the effect of the diodes in a reconfigurable EBG structure composed of metallic wires loaded periodically with diodes. In particular, we analyze the effect of the wire diameter and the effect of the equivalent capacitance of the diode. The band theory is an efficient method to analyze active or non-active EBG structures [START_REF] Sakoda | Optical Properties of Photonic Crystal[END_REF]. The dispersion characteristics of periodic structures formed by infinitely long metallic wires have been widely analyzed [START_REF] Simovski | The average field approach for obtaining the band structure of crystals with conducting wire inclusions[END_REF][START_REF] Silveirinha | A Hybrid method for the efficient calculation of the band structure of 3D-metallic crystal[END_REF][START_REF] Hudlicka | A Triple Wire Medium as an Isotropic Negative Permittivity Metamaterial[END_REF]. By neglecting the impedance of the active elements, the band structure of the discontinuous wire medium has been studied in [START_REF] Boutayeb | Band structure analysis of crystals with discontinuous metallic wires[END_REF], whereas in [START_REF] Belov | Twodimensional electromagnetic crystals formed by reactively loaded wires[END_REF][START_REF] Tretyakov | An analytical model of metamaterials based on loaded wire dipole[END_REF], studies of the dispersion characteristics of crystals with loaded wires have been reported. However, to our knowledge, the influence of the impedance of active elements in reconfigurable EBG structures has not been studied enough. Indeed, as it is shown in this paper, the influence of the diodes is not the same for large or small wire diameter. In addition, the value of the equivalent capacitance of the diode should be taken into account in the design of a reconfigurable EBG structure, and the frequency shift due to this capacitance is evaluated in this work. In this paper, new numerical results are presented to analyze the effect of active elements on the dispersion characteristics of reconfigurable crystals. To compute the propagation constant, a transmission line model is used, where a 2-D periodic structure (grid) of loaded wires is modeled by a T-circuit. The T-circuit parameters are expressed in terms of the S-parameters of the grid, which are computed with the Finite Difference Time domain (FDTD) method. To validate our proposed approach, experimental results are also presented and discussed.

COMPUTATION OF THE PROPAGATION CONSTANT

Figure 1 shows the 3-D periodic structure composed of periodically loaded metallic wires. The periods in x, y and z directions are called P x , P y and P z , respectively. w is the width of the discontinuities and a is the wire diameter. The loads are active elements, modeled by RC circuits. The propagation of the transverse electric field in x-direction is considered. To compute the propagation constant β x , a transmission line model is used, where a 2-D periodic structure in y-direction is modeled by a T-circuit [START_REF] Boutayeb | Band structure analysis of crystals with discontinuous metallic wires[END_REF].β x is obtained from the following dispersion equation cos (

β x P x ) = (1 + ZY ) cos (kP x ) + j Z + Y 2 1 + Z 2 sin (kP x ) (1)
where k is the free space wave number and the coefficients Z and Y are derived from the reflection and transmission coefficients of the grid, r and t:

Y = (r -t -1)(r + t -1) 2t (2)
Z = -r + t -1 r -t -1 (3)
The coefficients r and t were computed with a home-made Finite Difference Time Domain (FDTD) code, where image boundary conditions and a thin mesh (△ = P eriod/80) were used. The scheme introduced in [START_REF] Piket-May | FDTD modeling of digital signal propagation in 3-D circuits with passive and active loads[END_REF] was used to model the R and C elements. In the followings sections, we consider P y = P x = P z = P . Because only the fundamental mode is considered, the limitation P z ≤ λ is used

NUMERICAL ANALYSIS

The two first band limits for the structure with active elements in Onand Off-states are shown in Figs. 2 and3, respectively, versus the fill factor. In On-state, we consider R = 10 Ω, while in Off-state we use a parallel RC circuit with R = 30 kΩ and different values of C are tested. For comparison, the bands of structures with continuous and discontinuous wires are also presented. According to [START_REF] Lourtioz | Toward controllable photonic crystals for centimeter and millimeter wave devices[END_REF], for small fill factor (wire diameter), these two structures present a dual behavior in their two first bands. From Fig. 2, it can be noted that the active element has a negligible effect in On-state when the fill factor is small. This can be explained by the fact that there is a small difference between diameters of the active element and the wire. However, for large fill factor, this difference becomes important, and the band structures of the active crystal and the continuous wire medium becomes different. From Fig. 3, in Off-state, the active element modifies the band structure only for small wire diameter. This is due to the fact that, for large diameter, the diode capacitance is negligible compared to the capacitance of the wire. Because of the capacitance, the bands are shifted to lower frequencies by comparison with the bands obtained with the ideal discontinuous wire structure (without active elements). This reduces the band presenting a dual behavior. As a conclusion, for large fill factor, active elements have an effect only in On-state, whereas for small fill factor, they have an influence only in Off-state. Now, let us consider that the fill factor is chosen small, such that only the influence of the capacitance is relevant. The shift of the lower stop-band versus the capacitance is plotted in Fig. 4. This curve is important for the design of active crystals with dual behavior of their bands, in On-or Off-states.

EXPERIMENTAL RESULTS

To validate the proposed analysis, three structures composed of five parallel sheets of metallic strips of 2 mm width printed on RO3003 Rogers flexible substrates with P = 20 mm were fabricated. The first structure is composed of continuous strips, the second one is made of discontinuous strips, and the last one contains silicon PIN diodes BAP65-03 from Philips. Strips are used instead of rods to facilitate the soldering of surface mounted diodes. The substrate is chosen flexible for future fabrication of a reconfigurable cylindrical crystal not presented here. The substrate has a permittivity of 3, loss tangent of 0.0013 and a thickness of 0.254mm. In the discontinuous strip structure, the discontinuity width is w = 2 mm. Figure 5 shows the photograph of the fabricated crystal. The transmission coefficients of these structures were measured in an anechoic chamber, using two horn antennas and a network analyzer. The distances between the structures and the transmitting and receiving antennas are 2 m and 2 cm, respectively. The structure with diodes has the same parameters than the structure with discontinuous wires expect for the added diodes. To impose the same current in each diode, resistors of 6.2 KΩ are also added in both side of each strip. Note that in [START_REF] De Lustrac | Experimental demonstrattion of electrically controllable photonic crystals at centimeter wavelenghts[END_REF][START_REF] Lourtioz | Toward controllable photonic crystals for centimeter and millimeter wave devices[END_REF][START_REF] Djermoun | Negative refraction device with electrically controllable permittivity and negative permeability[END_REF] inductors have been used to play this role. In On-state, a voltage supply of +13.7 V olt is used, and the current is about 0.4 mA. For this amount of current, the diode resistance is about 1.6 Ω. In Off-state, the power supply is switched off, and for simulations, we considered C = 600 f F and R = 30 KΩ, corresponding to the typical values given by the sheet from the manufacturer. Figures 7 (a is observed only in the band 0 GHz -1.8 GHz. To increase this band, one has to choose active elements that have low capacitance. According to the numerical results presented in Section 3, a value of the capacitance less than C = 100 f F gives nearly the same dual behavior than the continuous and discontinuous strip structures.

CONCLUSION

The band structure of active metallic crystals has been studied, for different fill factors and different values of the loads. It has been shown that resistive loading has an influence only for large fill factor, and capacitive loading has an effect only for small fill factor. To validate the proposed analysis, experimental results have been presented and they show a good agreement with the theoretical ones.

Figure 1 .

 1 Figure 1. Infinite 3-D periodic structure of loaded metallic wires in air and equivalent RC circuits for numerical simulations.

Figure 2 .

 2 Figure 2. Two first band limits for structures with continuous wires and for wires periodically loaded with R = 10 Ω versus fill factor a/P .

Figure 3 .

 3 Figure 3. Two first band limits for structures with continuous, discontinuous and loaded wires with R = 30 KΩ , for different values of C, vs. fill factor a/P .

Figure 4 .

 4 Figure 4. Frequency shift of lower stop-band versus capacitance C, for a/P = 10%.

Figure 5 .

 5 Figure 5. Photograph of the fabricated reconfigurable EBG structure.

Figure 6 .

 6 Figures 6 (a) and (b) show the simulated and measured transmission coefficients of the structures with continuous and discontinuous strips, respectively. A good agreement is observed between theoretical and experimental data. From Fig.6, the continuous strip structure has a stop-band from 0 GHz to 4.5 GHz and a pass-band from 4.5 GHz to 7 GHz. In these two bands, the discontinuous strip structure has a dual behavior. The structure with diodes has the same parameters than the structure with discontinuous wires expect for the added diodes. To impose the same current in each diode, resistors of 6.2 KΩ are also added in both side of each strip. Note that in[START_REF] De Lustrac | Experimental demonstrattion of electrically controllable photonic crystals at centimeter wavelenghts[END_REF][START_REF] Lourtioz | Toward controllable photonic crystals for centimeter and millimeter wave devices[END_REF][START_REF] Djermoun | Negative refraction device with electrically controllable permittivity and negative permeability[END_REF] inductors have been used to play this role. In On-state, a voltage supply of +13.7 V olt is used, and the current is about 0.4 mA. For this amount of current, the diode resistance is about 1.6 Ω. In Off-state, the power supply is switched off, and for simulations, we considered C = 600 f F and R = 30 KΩ, corresponding to the typical values given by the sheet from the manufacturer. Figures7 (a) and (b) show the transmission coefficient of the proposed structure, in On-and Off-states, respectively, confirming that the band structure is shifted due to the diode capacitance. The dual behavior

Figure 7 .

 7 Figure 7. Transmission coefficient for an active crystal with PIN diodes: (a) On-state (b) Off-state.

Acknowledgement

This work was supported by National Science Engineering Research Council of Canada (NSERC) and Ultra-Electronics.


Alix Goguey

Géry Casiez

Daniel Vogel
email: dvogel@uwaterloo.ca

Fanny Chevalier

Thomas Pietrzak
email: thomas.pietrzak@lifl.fr

Nicolas Roussel
email: nicolas.roussel@inria.fr

A Three-Step Interaction Pattern for Improving Discoverability in Finger Identification Techniques

Keywords: Multi-touch, finger identification, shortcuts, command selection, parameter control, direct manipulation ACM Classification H.5.2 Information interfaces and presentation: User interfaces

Figure 1: Our three-application demo: (a) SMS application: the thumb can be used to feed-forward the current mapping between indivdual fingers and commands; here, the index triggers the copy command. (b) PDF annotation application: (b1) thumb showing the different subsets of commands available for selection; after sliding the thumb in the corresponding direction, (b2) updated feed-forward corresponding to the top-right set. (c) Vectorial drawing application demo: non-dominant thumb and index select a command mapping enabling the dominant ring finger to draw an ellipse.

INTRODUCTION

Finger identification is a way to increase the multi-touch input space. Significant previous work has tackled the technical sensing problem with various approaches including inferring finger identity based on geometric relationships between touch contact points [START_REF] Wagner | Multi-finger chords for hand-held tablets: Recognizable and memorable[END_REF], using an overhead camera to track Submitted to UIST 2014 -Please do not cite or redistribute.

bare hands [START_REF] Malik | Interacting with large displays from a distance with vision-tracked multi-finger gestural input[END_REF] or fingers with coloured rings [START_REF] Wang | FingerSense: augmenting expressiveness to physical pushing button by fingertip identification[END_REF], wearing gloves with fiducial markers [START_REF] Marquardt | Designing user-, hand-, and handpart-aware tabletop interactions with the TouchID toolkit[END_REF], recognizing fingerprints [START_REF] Holz | Fiberio: a touchscreen that senses fingerprints[END_REF], and forearm electromyography [START_REF] Benko | Enhancing input on and above the interactive surface with muscle sensing[END_REF]. It seems inevitable that one day finger identification will be a standard feature of consumer multi-touch devices.

Robust finger identification sensing is on the horizon [START_REF] Benko | Enhancing input on and above the interactive surface with muscle sensing[END_REF][START_REF] Holz | Fiberio: a touchscreen that senses fingerprints[END_REF], but its implications for interaction have only been considered in isolated point designs where chords were mapped to commands. Marquardt et al. built an exploratory toolkit for tabletops [START_REF] Marquardt | Designing user-, hand-, and handpart-aware tabletop interactions with the TouchID toolkit[END_REF] and explored a number of interaction technique ideas to emphasize the toolkit's expressiveness. However, these explorations did not address the problem of discoverability brought by finger identification: discriminating fingers on a touch screen theoretically enables the access to up to 1023 finger combinations, a too large of a set to choose from for one who is not aware of the fingers-to-command mapping.

To adress the discoverability problem associated with this large input space, we propose a three-step interaction pattern inspired by the combined use of keyboard shortcuts and a mouse in traditional graphical applications where 1) the nondominant hand activates a mode with a key, 2) the dominant hand manipulates the mouse, and 3) the non-dominant hand optionally constrains the manipulation with a modifier key.

In our case, on step 1, a set of (selector) fingers determines a command mapping for another set of (trigger) fingers. On step 2, trigger fingers select a command and manipulate its parameters in a single stroke (Figure 1-c). On step 3 (optional), selector fingers can constrain the manipulation. This three-step pattern only requires users to remember which fingers are in which one of the selectors set or the triggers set. More importantly, the separation between selectors and trig-gers allows the introduction of on-demand feed-forward: the command mapping can be visualized on-screen as a crib sheet if the user pauses after the first step. This on-demand feedforward makes it easy for novice users to browse the available commands, without hindering expert performance.

Depending on the context (e.g. smartphone, tablet, tabletop), the crib shit can take different forms and be invoked in different ways by one or more selector fingers. It may be triggered by a chord (Figures 1-a, 1-c) or a selection in a marking menu [START_REF] Kurtenbach | User learning and performance with marking menus[END_REF], for example (Figures 1-b1, 1-b2). We prototyped several applications to illustrate this three-step pattern in different contexts, that we describe in the following.

PROOF OF CONCEPT

We developed three applications: an SMS editor on a smartphone, a PDF annotation application on a tablet, and a vectorial drawing application on a tabletop system. For the smartphone and the tablet, considering the devices' form factors and the chosen applications, we assigned the selectors and triggers to a single hand and used only the thumb as a selector. For the tabletop application, we used the non-dominant hand's fingers as selectors and the dominant ones as triggers.

Hardware and low-level software configuration

Our focus is not on the technical problems of finger identification, but to experiment with our approach, we need to reliably identify which fingers are in contact with the display.

For this demonstration, we chose to attach different coloured rings to the user's fingers and to track their 2D position using a 640×480px RGB camera running at 20Hz and computer vision code using OpenCV. This technique can reliably track up to 6 fingers (Figure 2). The tracked positions are transformed into the display reference frame using an homography computed from a short calibration procedure. Our software makes it possible to generate custom touch events with hand and finger IDs in Qt applications running on Android or iOS devices.

Webcam Color Ring

Applications

The SMS writing application (Figure 1-a) uses our three-steps pattern to support commands such as copy/cut/paste, spelling corrections and undo/redo, which are traditionally accessed on smartphones through contextual menus, dwell or gestures (e.g. shaking the device to undo). Considering the relatively low number of commands, we used one selector finger only for command selection -enabling two mappings of trigger fingers: with or without the selector.

For the PDF annotation application, we identified 19 common commands in our informal observations. Considering this number and inherent relationships between the commands, we decided to break down this set into three subsets that can be selected by moving the thumb in different directions: when the thumb crosses a threshold distance represented by the dotted circle in Figure 1-b1, the corresponding subset is selected (e.g., all different ink tools). Once a subset is selected, the feed-forward is updated to show the four tools associated to the remaining trigger fingers (Figure 1-b2). Some commands can also be constrained using the thumb (e.g., freeform vs. by-line highlighting).

For the vectorial drawing application, we identified 26 common commands. We grouped these commands into seven categories comprising at most five commands each. Chord combinations of three fingers of the non-dominant hand are used to select command mappings (i.e. categories), while the five fingers of the dominant hand are used to trigger the commands (Figure 1-c). Shape creation tools can be constrained in several ways (e.g. aspect ratio, snapping) and those constraints can be combined.

NOTE

The demonstration will be interactive. We will give people a take away set so they can try at the conference and at home.

Figure 2 :

 2 Figure2: Demonstration setup: the user wear color rings that are tracked by the computer using the webcam; the tablet sends the touch inputs to the computer via the router; the computer then merges both webcam and tablet information and identifies the touchs which are sent back to the tablet via the router.