Richard A Phillips

Jérôme Fort

Maria P Dias

Conservation status and threats to seabirds: an overview

Keywords: Climate change, Discarding, Disease, Disturbance, Habitat loss, Invasive alien species, Overfishing, Pollution, Seabird bycatch, Wind farms

whether they are published or not. The documents may come

Seabird status and trends

Seabirds are one of the most threatened groups of vertebrates, with almost half of species listed as globally threatened with extinction or as Near threatened by the International Union for Conservation of Nature (IUCN) [START_REF] Dias | Threats to seabirds: A global assessment[END_REF]. Overall, 19 (5%), 36 (10%), 58 (16%) and 42 (12%) of a total of 362 species of seabirds are classified as Critically Endangered, Endangered, Vulnerable and Near Threatened, respectively (Figure 2.1). Over half (56%) of the 314 seabird species for which the population trend is known are in decline, including 80 species currently classified as Least Concern. The latter comprise 39% of the 205 seabird species in the Least Concern category. Declining trends are common across taxonomic groups, applying to the global populations of at least half the species in most seabird orders (Figure 2.2). Based on IUCN Red List categorizations, more species of albatrosses, petrels, and storm-petrels (Procellariiformes), and penguins (Sphenisciformes) are at higher risk of extinction than in other orders (Figure 2.2).

[Insert Figure 2.1 here] [Insert Figure 2.

here]

The main causes leading to poor conservation status of seabirds are generally well known and affect both their marine and terrestrial habitats [START_REF] Dias | Threats to seabirds: A global assessment[END_REF][START_REF] Phillips | The conservation status and priorities for albatrosses and large petrels[END_REF][START_REF] Rodríguez | Future directions in conservation research on petrels and shearwaters[END_REF][START_REF] Trathan | Pollution, habitat loss, fishing, and climate change as critical threats to penguins[END_REF]. The top three causes of population declines in seabirds are invasive alien species, incidental mortality (bycatch) in fisheries, and climate change, followed by overfishing, direct harvesting (hunting and trapping), and disturbance. Many other human activities also have negative impacts on their populations through sub-lethal or lethal effects (Dias et al. 2019: Figure XX.3). Although research and conservation often focus on threats with obvious impacts on survival or reproduction, many climate or human-mediated stressors affecting physiology, energetics, or behaviour have synergistic effects, likely enhancing sensitivity of seabirds to other environmental pressures and increasing vulnerability in a world that is increasingly perturbed by human activities.

[Insert Figure 2.3] Bycatch, climate change, and, to a less extent, overfishing and invasive alien species have a high impact1 across many groups (Figure 2.4). Direct harvesting and disturbance also affect a wide range of species from different orders, but typically with lower impacts. Albatrosses and petrels face a more limited number of threats than other taxa -each species is affected by 2.4 threats on average -although usually with high impacts. In contrast, penguin species are on average affected by >5 different threats (Figure 2.4). A high percentage (>60%) of species in most groups other than albatrosses and petrels is affected by both marine and terrestrial threats (Figure 2.4).

[Insert Figure 2.4 here]

Threats to seabirds

Bycatch

Bycatch in fisheries is amongst the most pervasive and severe threats to seabirds, and a major cause of population declines in many species, particularly albatrosses, large petrels, and penguins in southern oceans, and sea ducks in northern latitudes [START_REF] Dias | Threats to seabirds: A global assessment[END_REF][START_REF] Phillips | The conservation status and priorities for albatrosses and large petrels[END_REF][START_REF] Trathan | Pollution, habitat loss, fishing, and climate change as critical threats to penguins[END_REF]Chapter 3). Scavenging seabirds are attracted to vessels by the prospect of an easy meal in the form of unwanted catch, bait, and other offal discarded at any point during fishing operations, or bait taken from longlines during setting. In longline fisheries, most seabird bycatch occurs during setting, when birds swallow the hook or are entangled, then dragged under and drowned [START_REF] Da Rocha | Reduction in seabird mortality in Namibian fisheries following the introduction of bycatch regulation[END_REF][START_REF] Jiménez | Bycatch of great albatrosses in pelagic longline fisheries in the southwest Atlantic: Contributing factors and implications for management[END_REF]). However, live captures, mainly during hauling, are also common, amounting to 40% and 11% of all birds bycaught in demersal and pelagic longline fisheries, respectively; this is also a serious problem given that over half of the birds may die subsequently from their injuries [START_REF] Phillips | Variation in live-capture rates of albatrosses and petrels in fisheries, post-release survival and implications for management[END_REF]. In trawl fisheries, the main issue is injury or collision with trawl warp and monitoring cables during hauling, although birds can also be trapped in the net [START_REF] Sullivan | Seabird mortality on factory trawlers in the Falkland Islands and beyond[END_REF][START_REF] Watkins | Interactions between seabirds and deep-water hake trawl gear: an assessment of impacts in South African waters[END_REF].

Although there are many limitations in observer data, the minimum level of mortality in longline and gillnet fisheries worldwide is estimated at >160,000 and >400,000 seabirds per year, respectively [START_REF] Anderson | Global seabird bycatch in longline fisheries[END_REF][START_REF] Žydelis | The incidental catch of seabirds in gillnet fisheries: a global review[END_REF]). Seabird mortality is likely of a similar order in trawl and artisanal fisheries, given the high levels of bycatch recorded for some fleets and the huge fishing effort in many regions [START_REF] Alfaro-Shigueto | Where small can have a large impact: structure and characterization of small-scale fisheries in Peru[END_REF][START_REF] Da Rocha | Reduction in seabird mortality in Namibian fisheries following the introduction of bycatch regulation[END_REF][START_REF] Watkins | Interactions between seabirds and deep-water hake trawl gear: an assessment of impacts in South African waters[END_REF].

Demographic studies often find relationships between annual changes in fishing effort in seabird foraging areas -likely correlated with bycatch rates if mitigation measures are inadequate -and adult or juvenile survival [START_REF] Cleeland | Disentangling the Influence of Three Major Threats on the Demography of an Albatross Community[END_REF][START_REF] Pardo | Additive effects of climate and fisheries drive ongoing declines in multiple albatross species[END_REF][START_REF] Rolland | Combined effects of fisheries and climate on a migratory long-lived marine predator[END_REF]. Impacts of this mortality at the population level are exacerbated if the bycatch is strongly biased towards one sex [START_REF] Gianuca | Global patterns of sex-and age-specific variation in seabird bycatch[END_REF][START_REF] Mills | Modelling impacts of long-line fishing: what are the effects of pair-bond disruption and sex-biased mortality on albatross fecundity[END_REF].

The burgeoning of tracking studies in the early 2000s resulted in a huge increase in knowledge of where and when adult seabirds overlap with particular fisheries in the breeding and nonbreeding seasons [START_REF] Clay | A comprehensive large-scale assessment of fisheries bycatch risk to threatened seabird populations[END_REF][START_REF] Delord | Areas of importance for seabirds tracked from French southern territories, and recommendations for conservation[END_REF][START_REF] Grémillet | Persisting worldwide seabird-fishery competition despite seabird community decline[END_REF][START_REF] Phillips | Year-round distribution of white-chinned petrels from South Georgia: relationships with oceanography and fisheries[END_REF]. In contrast, details on the fisheries representing the highest risk for juveniles and immatures is generally lacking [START_REF] Carneiro | A framework for mapping the distribution of seabirds by integrating tracking, demography and phenology[END_REF].

Once problematic fisheries or fleets have been identified, pressure can be exerted on management authorities to mandate the use of bycatch mitigation measures, of which the most effective are: heavier line weighting, streamer (tori) lines, and nightsetting in demersal and pelagic longline fisheries [START_REF] Jiménez | Towards mitigation of seabird bycatch: large-scale effectiveness of night setting and Tori lines across multiple pelagic longline fleets[END_REF]; streamer lines, net binding, and banning of offal discharge during hauling in trawl fisheries [START_REF] Da Rocha | Reduction in seabird mortality in Namibian fisheries following the introduction of bycatch regulation[END_REF][START_REF] Maree | Significant reductions in mortality of threatened seabirds in a South African trawl fishery[END_REF]; and restriction of setting times and areas in gillnet fisheries, for which technical solutions have proven more challenging (O'Keefe et al. in press). If compliance is high, seabird bycatch can be reduced to low or negligible levels [START_REF] Collins | Mitigating the impact of longline fisheries on seabirds: Lessons learned from the South Georgia Patagonian toothfish fishery (CCAMLR Subarea 48.3)[END_REF][START_REF] Da Rocha | Reduction in seabird mortality in Namibian fisheries following the introduction of bycatch regulation[END_REF] [START_REF] Maree | Significant reductions in mortality of threatened seabirds in a South African trawl fishery[END_REF][START_REF] Melvin | Lessons from seabird conservation in Alaskan longline fisheries[END_REF]). However, progress has been very slow in some fisheries, particularly in the High Seas under the jurisdiction of the Regional Fisheries Management Organisations (RFMOs), where mitigation regulations are not bestpractice, and the levels of observer coverage in terms of monitoring compliance and bycatch rates remain woefully inadequate [START_REF] Phillips | Requisite improvements to the estimation of seabird by-catch in pelagic longline fisheries[END_REF].

Overfishing

The depletion of stocks by excessive fishing is a major threat to seabirds because of the direct competition for food resources [START_REF] Grémillet | Persisting worldwide seabird-fishery competition despite seabird community decline[END_REF]Chapter 3).

Overfishing can have a high impact on seabird populations, particularly during the breeding season when foraging ranges are restricted by central-place constraints [START_REF] Barbraud | Density dependence, prey accessibility and prey depletion by fisheries drive Peruvian seabird population dynamics[END_REF][START_REF] Carroll | Kittiwake breeding success in the southern North Sea correlates with prior sandeel fishing mortality[END_REF][START_REF] Grémillet | Starving seabirds: unprofitable foraging and its fitness consequences in Cape gannets competing with fisheries in the Benguela upwelling ecosystem[END_REF][START_REF] Pichegru | Overlap between vulnerable top predators and fisheries in the Benguela upwelling system: implications for marine protected areas[END_REF].

Overfishing is the main cause of population declines of at least 24 species (Dias et al. 2019); penguins, cormorants, frigatebirds, gannets, boobies, and sea ducks are particularly vulnerable (Figure 2.4). The effect of overfishing is often aggravated by other causes of prey depletion, such as those resulting from changes in oceanographic conditions (Grémillet and Boulinier 2009, and see below). Given difficulties in quantifying the effects of overfishing on seabird populations, the magnitude can be underestimated. Nevertheless, it is clearly a growing problemthe number of globally threatened species affected by overfishing increased from 10 to 22 in less than a decade [START_REF] Croxall | Seabird conservation status, threats and priority actions: a global assessment[END_REF][START_REF] Dias | Threats to seabirds: A global assessment[END_REF], and fishing pressure on stocks of shoaling, coastal fish species (forage fish), a food resource shared between humans and seabirds, is likely to intensify in the future [START_REF] Grémillet | Persisting worldwide seabird-fishery competition despite seabird community decline[END_REF]. Of major concern is also the predicted expansion of fisheries targeting mesopelagic species [START_REF] Sutherland | A horizon scan of emerging issues for global conservation in 2019[END_REF], which are an important component of the diet of many pelagic seabirds [START_REF] Waap | The diet of a nocturnal pelagic predator, the Bulwer's petrel, across the lunar cycle[END_REF][START_REF] Watanuki | Factors affecting the importance of myctophids in the diet of the world's seabirds[END_REF].

Ensuring sustainable harvesting of fish stocks and minimization of impacts on nontarget species such as seabirds are at the heart of ecosystem-based fisheries management, but will require much improved governance, and a more effective national and international policy framework [START_REF] Garcia | Food security and marine capture fisheries: characteristics, trends, drivers and future perspectives[END_REF].

Invasive alien species

Invasive alien species at breeding colonies represent a huge threat to seabirds (Chapter 4), worldwide, affecting almost ⅔ of all species [START_REF] Dias | Threats to seabirds: A global assessment[END_REF]). The impacts are particularly severe on islands, which often host endemic seabird species that evolved in the absence of mammalian ground-predators and have limited natural defenses [START_REF] Courchamp | Mammal invaders on islands: impact, control and control impact[END_REF]. Invasive alien species affect all seabird groups, with a particularly high impact on small petrels (including storm-petrels), shearwaters, and albatrosses [START_REF] Dias | Threats to seabirds: A global assessment[END_REF]. The invasive species with the greatest impacts are predators, particularly rats Rattus spp., house mice Mus musculus, and cats Felis cattus, which affect seabird species ranging in size from small, burrow-nesting petrels, to large, surface-nesting albatrosses [START_REF] Angel | Review of impacts of the introduced house mouse on islands in the Southern Ocean: are mice equivalent to rats?[END_REF]Barbraud et al. 2021b;[START_REF] Cleeland | Disentangling the Influence of Three Major Threats on the Demography of an Albatross Community[END_REF]. Other problematic invasive alien species include ants, which can attack nesting seabirds [START_REF] Plentovich | Detrimental effects of two widespread invasive ant species on weight and survival of colonial nesting seabirds in the Hawaiian Islands[END_REF], and mammalian herbivores and plants, which damage habitats and reduce breeding propensity or success [START_REF] Cleeland | Introduced species and extreme weather as key drivers of reproductive output in three sympatric albatrosses[END_REF][START_REF] Mcchesney | History and status of introduced mammals and impacts to breeding seabirds on the California Channel and northwestern Baja California Islands[END_REF].

Although the scale of the threat to seabirds from invasive alien species remains high, there have been many notable successes in eradication or control in recent decades [START_REF] Russell | Invasive alien species on islands: impacts, distribution, interactions and management[END_REF]. Despite the many challenges, areas ranging in size from small islands to entire island groups have been cleared of one or more of rats, mice, cats, and rabbits through spreading of poison bait (often by helicopter), trapping, or shooting campaigns [START_REF] Martin | Rodent eradication scaled up: clearing rats and mice from South Georgia[END_REF][START_REF] Nogales | A review of feral cat eradications on islands[END_REF][START_REF] Springer | Methodology and challenges of a complex multi-species eradication in the sub-Antarctic and immediate effects of invasive species removal[END_REF]. Although this sometimes results in high levels of non-target mortality of scavenging seabirds, the impacts tend to be short-lived [START_REF] Alderman | Macquarie Island's northern giant petrels and the impacts of pest eradication on population abundance[END_REF][START_REF] Travers | Bottom-up effect of eradications: The unintended consequences for top-order predators when eradicating invasive prey[END_REF]. Often the overall benefits become very clear in terms of population recovery or recolonization in the following years (Barbraud et al. 2021a;[START_REF] Benkwitt | Rat eradication restores nutrient subsidies from seabirds across terrestrial and marine ecosystems[END_REF][START_REF] Brooke | Seabird population changes following mammal eradications on islands[END_REF]. However, the restoration process can be slow, particularly for burrow-nesting petrels, if other invasive meso-predators are still present [START_REF] Dilley | Modest increases in densities of burrow-nesting petrels following the removal of cats (Felis catus) from Marion Island[END_REF], underlining the advantages of eradicating all invasive predators simultaneously [START_REF] Baker | Simultaneous invasive alien predator eradication delivers the best outcomes for protected island species[END_REF]. Single or multispecies eradications are now technically feasible over very large areas, but the barriers include funding, garnering support from locals on inhabited islands [START_REF] White | Alien invasive vertebrates in ecosystems: pattern, process and the social dimension[END_REF], and the need on some islands to take vulnerable species into captivity that would otherwise suffer major non-target mortality [START_REF] Oppel | Temporary captive population and rapid population recovery of an endemic flightless rail after a rodent eradication operation using aerially distributed poison bait[END_REF].

Changing climate

Changing climatic conditions are likely to have multiple, increasing, direct and indirect effects on seabird populations (Chapter 8). Hence, around 100 seabird species (27%), of which penguins and albatrosses are considered to be the most vulnerable, are expected to be impacted by changes to oceanography or other aspects of climatic variation [START_REF] Dias | Threats to seabirds: A global assessment[END_REF]. Warming temperatures will likely have direct physiological impacts, particularly on thermoregulatory processes in seabirds given they are endothermic [START_REF] Oswald | Direct impacts of climatic warming on heat stress in endothermic species: seabirds as bioindicators of changing thermoregulatory constraints[END_REF]. Heat-stress events will increase, impacting behavior, reproduction, and survival [START_REF] Cook | Parenting in a warming world: thermoregulatory responses to heat stress in an endangered seabird[END_REF][START_REF] Oswald | Heat stress in a high-latitude seabird: effects of temperature and food supply on bathing and nest attendance of great skuas Catharacta skua[END_REF]. Polar species, which are often ill-adapted to cope with heat stress and inhabit regions where temperatures are warming twice as fast as much of the rest of the planet, are expected to be more vulnerable to changes in their thermal landscape [START_REF] Blix | Adaptations to polar life in mammals and birds[END_REF][START_REF] Choy | Limited heat tolerance in a cold-adapted seabird: implications of a warming Arctic[END_REF][START_REF] Oswald | Direct impacts of climatic warming on heat stress in endothermic species: seabirds as bioindicators of changing thermoregulatory constraints[END_REF]. Forecasted changes in precipitation patterns may also affect seabird reproduction and chick survival through increased thermoregulatory costs or flooding of nests [START_REF] Chambers | Observed and predicted effects of climate on Australian seabirds[END_REF][START_REF] Demongin | Effects of severe rains on the mortality of southern rockhopper penguin (Eudyptes chrysocome) chicks and its impact on breeding success[END_REF]. In polar regions, nesting seabirds can be affected by heavy snowfall, particularly in the early breeding period, reducing nest-site accessibility and delaying egg-laying, or entombing incubating adults, all of which reduce reproductive output [START_REF] Michielsen | Nest characteristics determine nest microclimate and affect breeding output in an Antarctic seabird, the Wilson's storm-petrel[END_REF][START_REF] Moe | Climate change and phenological responses of two seabird species breeding in the high-Arctic[END_REF][START_REF] Schmidt | An ecosystem-wide reproductive failure with more snow in the Arctic[END_REF]. Finally, the prevailing winds affect energy balance of seabirds, year round [START_REF] Amélineau | Windscape and tortuosity shape the flight costs of northern gannets[END_REF][START_REF] Fort | Thermodynamic modelling predicts energetic bottleneck for seabirds wintering in the northwest Atlantic[END_REF]). As such, predicted increases in storm frequency in some regions will have major consequences for flight costs and survival [START_REF] Clairbaux | Meeting Paris agreement objectives will temper seabird winter distribution shifts in the North Atlantic Ocean[END_REF][START_REF] Guéry | Winter extratropical cyclone influence on seabird survival: variation between and within common eider Somateria mollissima populations[END_REF].

There are also multiple indirect effects of climate change, including warming temperatures which can result in habitat loss or fragmentation. Sea-level rise and storm surges are increasing threats on tropical atolls and other low-lying islands [START_REF] Hatfield | Population dynamics of Hawaiian seabird colonies vulnerable to sea-level rise[END_REF][START_REF] Reynolds | Will the effects of sea-level rise create ecological traps for Pacific island seabirds?[END_REF][START_REF] Vanderwerf | Establishing Laysan and black-footed albatross breeding colonies using translocation and social attraction[END_REF][START_REF] Von Holle | Effects of future sea level rise on coastal habitat[END_REF]). Melting sea ice can affect access to sympagic prey resources [START_REF] Amélineau | Arctic climate change and pollution impact little auk foraging and fitness across a decade[END_REF], reduce availability of floating ice for resting [START_REF] Lovvorn | Modeling marine protected areas for threatened eiders in a climatically changing Bering Sea[END_REF], modify exposure to predators [START_REF] Chaulk | Landscape features and sea ice influence nesting common eider abundance and dispersion[END_REF][START_REF] Dey | Forecasting the outcome of multiple effects of climate change on northern common eiders[END_REF], and, ultimately, determine population trajectories [START_REF] Descamps | Species and spatial variation in the effects of sea ice on Arctic seabird populations[END_REF][START_REF] Dey | Increasing nest predation will be insufficient to maintain polar bear body condition in the face of sea ice loss[END_REF].

Warming temperatures will modify the energetic landscape of seabirds, thus altering their movements and distribution at large spatial scales [START_REF] Clairbaux | Meeting Paris agreement objectives will temper seabird winter distribution shifts in the North Atlantic Ocean[END_REF][START_REF] Clairbaux | Climate change could overturn bird migration: Transarctic flights and highlatitude residency in a sea ice free Arctic[END_REF], generating new threats to seabird populations such as increased inter-specific competition (Bonnet-Lebrun et al. in press), or the introduction of new pathogens (see below). Finally, changing climate greatly impacts physical and biological oceanography, thereby altering abundance and distribution of the prey of seabirds [START_REF] Cheung | Projecting global marine biodiversity impacts under climate change scenarios[END_REF][START_REF] Poloczanska | Global imprint of climate change on marine life[END_REF]. This can result in spatial and temporal mismatches between seabird demand and their favoured resources, with major impacts on species or populations unable to adjust to new environmental conditions [START_REF] Keogan | Global phenological insensitivity to shifting ocean temperatures among seabirds[END_REF]. Such effects include decreased breeding success [START_REF] Cury | Global seabird response to forage fish depletion-one-third for the birds[END_REF][START_REF] Frederiksen | From plankton to top predators: bottom-up control of a marine food web across four trophic levels[END_REF] or increased adult mortality [START_REF] Jones | mortality of a planktivorous seabird in response to a marine heatwave[END_REF], and can leading to population decline [START_REF] Descamps | Circumpolar dynamics of a marine top-predator track ocean warming rates[END_REF][START_REF] Erasmus | Predatory species left stranded following the collapse of the sardine Sardinops sagax (Pappe, 1854) stock off the northern Benguela upwelling system: a review[END_REF].

Disease

Although potential pathogen or vectors (viruses, bacteria, fungi, protozoa, gastrointestinal parasites, and ectoparasites) are widespread among seabirds [START_REF] Barbosa | Health of Antarctic birds: a review of their parasites, pathogens and diseases[END_REF][START_REF] Uhart | Review of diseases (pathogen isolation, direct recovery and antibodies) in albatrosses and large petrels worldwide[END_REF], disease is considered to be a current threat for only 15 species, predominantly albatrosses and penguins, with a high impact in most cases (Dias et al. 2019: Figure XX.4). However, there are many knowledge gaps in terms of prevalence and effects of pathogens (Chapter 5). Of the pathogens, avian cholera is the most widespread spatially and taxonomically among seabirds, and is responsible for most disease outbreaks that have involved high mortality [START_REF] Barbosa | Health of Antarctic birds: a review of their parasites, pathogens and diseases[END_REF]. In recent decades, avian cholera has been the main driver of steep population declines of albatrosses and penguins breeding at Amsterdam Island [START_REF] Jaeger | Avian cholera outbreaks threaten seabird species on Amsterdam Island[END_REF]. Some viruses also cause diseases in seabirds, typically showing a pattern of high variability among years in prevalence and in the level of impact on chicks, and generally with sub-lethal effects on adults [START_REF] Vanderwerf | Juvenile survival, recruitment, population size, and effects of avian pox virus in Laysan Albatross (Phoebastria immutabilis) on Oahu, Hawaii, USA[END_REF][START_REF] Work | Necrotizing enteritis as a cause of mortality in Laysan albatross, Diomedea immutabilis, chicks on Midway Atoll, Hawaii[END_REF][START_REF] Young | Prevalence of avian pox virus and effect on the fledging success of Laysan Albatross[END_REF].

However, avian pox reduced first-year survival and recruitment probability in the Laysan Albatross Phoebastria immutabilis (VanderWerf and Young 2016).

Pathogens can be spread through various routes, including ectoparasites, invasive alien species such as rats, physical contact, ingestion of infected carcasses (by skuas in particular), and by aerosol transmission [START_REF] Gamble | Predator and scavenger movements among and within endangered seabird colonies: Opportunities for pathogen spread[END_REF][START_REF] Uhart | Review of diseases (pathogen isolation, direct recovery and antibodies) in albatrosses and large petrels worldwide[END_REF][START_REF] Vanderwerf | Juvenile survival, recruitment, population size, and effects of avian pox virus in Laysan Albatross (Phoebastria immutabilis) on Oahu, Hawaii, USA[END_REF][START_REF] Young | Prevalence of avian pox virus and effect on the fledging success of Laysan Albatross[END_REF]. Diseases are potentially a worsening problem for seabirds as pathogens or vectors are likely to become more widespread, or pathogens more virulent as a consequence of climatic and other environmental changes, such as increased pollution and other stressors, and higher connectivity associated with expanding human activities [START_REF] Grimaldi | Infectious diseases of Antarctic penguins: current status and future threats[END_REF]. Although there are some partial solutions such as vaccination programmes for some species at specific sites [START_REF] Bourret | Vaccination protects endangered albatross chicks against avian cholera[END_REF], or application of avian insecticide to kill ectoparasite vectors [START_REF] Alderman | Developing a climate adaptation strategy for vulnerable seabirds based on prioritisation of intervention options[END_REF][START_REF] Vanderwerf | Establishing Laysan and black-footed albatross breeding colonies using translocation and social attraction[END_REF]), the first line of defense should be to minimize transfer risks associated with human activities by stringent biosecurity measures.

Pollution

Environmental pollution can have a major impact on seabirds, which are exposed to thousands of anthropogenic contaminants (Chapter 6). Chemical or light pollution are considered to be a threat to >50 seabird species, particularly penguins, cormorants, and small petrels [START_REF] Dias | Threats to seabirds: A global assessment[END_REF]. Effects are likely to worsen, given the global increase in chemical production [START_REF] Wilson | Toward a new US chemicals policy: rebuilding the foundation to advance new science, green chemistry, and environmental health[END_REF], loss of plastic into the environment [START_REF] Borrelle | Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution[END_REF][START_REF] Ostle | The rise in ocean plastics evidenced from a 60-year time series[END_REF], and the development of coastal or offshore structures and associated artificial lights [START_REF] Davies | Frame Conflicts' in Natural Resource Use: Exploring Framings Around Arctic Offshore Petroleum Using Q-Methodology[END_REF].

Seabirds are long-lived species at the apex of marine food webs and thus show very high levels of some chemical contaminants, which magnify along food chains and accumulate in their tissues [START_REF] Cherel | Accumulate or eliminate? Seasonal mercury dynamics in albatrosses, the most contaminated family of birds[END_REF]). As a consequence, these contaminants can have substantial sub-lethal effects on seabirds, affecting their physiology and behavior, and ultimately their reproduction, survival, and population dynamics [START_REF] Mills | Mercury exposure in an endangered albatross: long-term changes and consequences for breeding success[END_REF]. Contaminant exposure can affect bird metabolism through endocrine disruption [START_REF] Blévin | Contaminants and energy expenditure in an Arctic seabird: Organochlorine pesticides and perfluoroalkyl substances are associated with metabolic rate in a contrasted manner[END_REF][START_REF] Melnes | Dissimilar effects of organohalogenated compounds on thyroid hormones in glaucous gulls[END_REF], or reduce hatching success due to an inability to sustain energetically-intensive incubation behavior [START_REF] Blévin | Contaminants and energy expenditure in an Arctic seabird: Organochlorine pesticides and perfluoroalkyl substances are associated with metabolic rate in a contrasted manner[END_REF][START_REF] Tartu | Endocrine and Fitness Correlates of Long-Chain Perfluorinated Carboxylates Exposure in Arctic Breeding Black-Legged Kittiwakes[END_REF]. Laboratory evidence from Herring Gulls Larus argentatus suggest that neurotoxic contaminants could impair learning and cognition in developing birds [START_REF] Burger | Effects of lead on learning in herring gulls: an avian wildlife model for neurobehavioral deficits[END_REF], with potential effects on the capacity for behavioral plasticity and thus survivorship. Beyond these sublethal impacts, thousands of seabirds suffer direct mortality annually as a consequence of acute pollution of the environment following oil spills that resulted from illegal or accidental discharges from shipping, oil and gas exploitation [START_REF] Chilvers | Sources and reporting of oil spills and impacts on wildlife 1970-2018[END_REF]Wiese and Robertson 2004;[START_REF] Wilhelm | Comparing an estimate of seabirds at risk to a mortality estimate from the November 2004 Terra Nova FPSO oil spill[END_REF], with potentially substantial impacts on demography [START_REF] Votier | Oil pollution and climate have wide-scale impacts on seabird demographics[END_REF].

Plastic pollution is an increasing threat to seabirds, with plastic ingestion recorded all over the world and in diverse species [START_REF] Baak | Plastic ingestion by four seabird species in the Canadian Arctic: comparisons across species and time[END_REF][START_REF] Kühn | Quantitative overview of marine debris ingested by marine megafauna[END_REF][START_REF] Phillips | Albatrosses and petrels at South Georgia as sentinels of marine debris input from vessels in the southwest Atlantic Ocean[END_REF]. Although there are still few examples to-date of well-documented population-level impacts [START_REF] Lavers | Plastic ingestion by flesh-footed shearwaters (Puffinus carneipes): Implications for fledgling body condition and the accumulation of plastic-derived chemicals[END_REF][START_REF] Roman | A quantitative analysis linking seabird mortality and marine debris ingestion[END_REF], plastic ingestion has various effects from physical damage such as gastrointestinal obstruction [START_REF] Ryan | The lifespan of ingested plastic particles in seabirds and their effect on digestive efficiency[END_REF], to toxicological, as the original additives or the organic pollutants adsorbed on floating plastics from the surrounding seawater transfer to seabird tissues after ingestion [START_REF] Tanaka | Piece-by-piece analysis of additives and manufacturing byproducts in plastics ingested by seabirds: Implication for risk of exposure to seabirds[END_REF][START_REF] Yamashita | Plastic additives and legacy persistent organic pollutants in the preen gland oil of seabirds sampled across the globe[END_REF].

Artificial lights can be a major source of pollution, and there are frequent records of major mortality events involving collisions, particularly of shearwaters, petrels, and storm-petrels attracted to ships, oil platforms, and other structures at sea, and buildings and streetlights in coastal areas (Reed et al. 1985;[START_REF] Rodríguez | Future directions in conservation research on petrels and shearwaters[END_REF][START_REF] Rodríguez | Seabird mortality induced by land-based artificial lights[END_REF]Chapter 6). Environmental conditions, such as the phase of the moon and weather conditions, can have a strong influence on the collision probability, with most incidents on darker nights, in fog or rain [START_REF] Montevecchi | Influences of artificial light on marine birds[END_REF][START_REF] Rodríguez | Factors affecting mortality of shearwaters stranded by light pollution[END_REF]. Artificial lights may also alter the navigational ability of birds migrating along coastal areas [START_REF] Van Doren | High-intensity urban light installation dramatically alters nocturnal bird migration[END_REF]. By disrupting sleep, light pollution could also have cascading behavioral and physiological impacts, as shown experimentally and in free-living birds [START_REF] Dominoni | Artificial light at night advances avian reproductive physiology[END_REF][START_REF] Sanders | A metaanalysis of biological impacts of artificial light at night[END_REF][START_REF] Svechkina | The impact of artificial light at night on human and ecosystem health: a systematic literature review[END_REF]. Whether the faculties of seabirds are similarly impaired by artificial lights has not been tested. Human communities on islands, and responsible shipping operators and crew, are increasingly aware of the problem of light pollution, and change the types of light or its orientation, use shields, or turn off lights during sensitive periods (chick fledging, or on vessels at night) to reduce impacts [START_REF] Rodríguez | Seabird mortality induced by land-based artificial lights[END_REF][START_REF] Rodríguez | Future directions in conservation research on petrels and shearwaters[END_REF][START_REF] Rodríguez | Seabird mortality induced by land-based artificial lights[END_REF]. However, the impacts of light pollution on birds at sea through shipping and offshore infrastructure remain greatly underestimated [START_REF] Montevecchi | Influences of artificial light on marine birds[END_REF][START_REF] Rodríguez | Future directions in conservation research on petrels and shearwaters[END_REF].

Habitat loss and development

Seabird habitat loss has been accelerated by the combined effect of climate change and the expansion of human populations and industries. At-sea foraging habitats used during both the breeding and non-breeding periods are threatened by the growing number of offshore wind farms that exclude seabirds from foraging areas (Peschko et al. 2020a; Chapter 7), although the impacts of this habitat loss on populations are currently unknown (see below). In the polar regions, melting sea ice leads to the loss of essential foraging and resting habitats, with clear impacts on seabird populations (see above). The main effects of habitat loss are at breeding sites on land. Agriculture or the introduction of grazing animals substantially change vegetation cover, affecting the breeding success and survival [START_REF] Trathan | Pollution, habitat loss, fishing, and climate change as critical threats to penguins[END_REF].

Likewise, seabirds nesting in forests, such as the marbled murrelet Brachyramphus marmoratus, can be impacted by the timber industry [START_REF] Betts | Squeezed by a habitat split: Warm ocean conditions and old-forest loss interact to reduce long-term occupancy of a threatened seabird[END_REF]. The continued urbanization of coastlines by humans also destroys breeding habitat, forcing nesting seabirds to relocate, and impacting productivity of birds that rely on spatial knowledge to find food [START_REF] Kavelaars | Breeding habitat loss reveals limited foraging flexibility and increases foraging effort in a colonial breeding seabird[END_REF][START_REF] Salas | Forced nest site relocations negatively affect reproductive investment in a colonial seabird species[END_REF]. Breeding habitats on tropical atolls and other low-lying coastal areas will be further impacted in coming decades by climate-induced rises in global sea level and storm surges [START_REF] Hatfield | Population dynamics of Hawaiian seabird colonies vulnerable to sea-level rise[END_REF][START_REF] Reynolds | Will the effects of sea-level rise create ecological traps for Pacific island seabirds?[END_REF][START_REF] Vanderwerf | Establishing Laysan and black-footed albatross breeding colonies using translocation and social attraction[END_REF][START_REF] Von Holle | Effects of future sea level rise on coastal habitat[END_REF].

Seabird and egg harvesting

The legal consumption of seabirds and their eggs has been much reduced worldwide over the last century, following the implementation of protective regulations (Chapter 7). This reduction likely led to rapid population recoveries [START_REF] Merkel | Evidence of recent population recovery in common eiders breeding in western Greenland[END_REF], although the full impacts of legal harvest on populations dynamics still remain poorly evaluated, particularly as there are often other pressures involved [START_REF] Chen | Human harvest, climate change and their synergistic effects drove the Chinese Crested Tern to the brink of extinction[END_REF][START_REF] Frederiksen | Quantifying the relative impact of hunting and oiling on Brünnich's guillemots in the North-west Atlantic[END_REF]Wiese et al. 2004). Nonetheless, hunting of seabirds at colonies is still considered to be the second largest threat on land in terms of number of species affected, and the top threat to coastal species that are globally threatened [START_REF] Dias | Threats to seabirds: A global assessment[END_REF]. For instance, traditional hunting of seabirds and egg harvesting is still a common practice in the Arctic where this resource can be fundamental [START_REF] Naves | Geographic and seasonal patterns of seabird subsistence harvest in Alaska[END_REF][START_REF] Renner | Connecting subsistence harvest and marine ecology: a cluster analysis of communities by fishing and hunting patterns[END_REF]. Additionally, because many seabirds are longdistance migrants, multiple breeding populations over a wide area could be impacted by local hunting during the non-breeding season, as demonstrated recently for Thickbilled Murre Uria lomvia, with local hunting pressure in West Greenland and Eastern Canada during winter affecting growth rates, and potentially causing declines in populations from Svalbard to Canada [START_REF] Frederiksen | Quantifying the relative impact of hunting and oiling on Brünnich's guillemots in the North-west Atlantic[END_REF]. Moreover, populations may also suffer from unquantified illegal hunting, the impact of which is even harder to estimate. Highly threatened or small populations of seabirds are particularly at risk from illegal consumption [START_REF] Awkerman | Incidental and intentional catch threatens Galápagos waved albatross[END_REF][START_REF] Mondreti | Illegal egg harvesting and population decline in a key pelagic seabird colony of the Eastern Indian Ocean[END_REF][START_REF] Smart | A survey of Grenadians on seabird harvest in the Grenada Grenadines[END_REF]. Management of seabird and egg harvesting is a complex issue, and ensuring sustainability requires conservation measures to be developed in close consultation between local communities and authorities.

Disturbance

Disturbance at colonies can have a negative impact on the breeding success of seabird or even result in the abandonment of colonies [START_REF] Carney | A review of human disturbance effects on nesting colonial waterbirds[END_REF][START_REF] Giese | Effects of human activity on Adelie penguin Pygoscelis adeliae breeding success[END_REF]Chapter 7). It is a widespread problem that affects more than 20% of seabird species (Figure 2.4), particularly terns, gulls, and cormorants, and including half of the 26 coastal species that are globally threatened and many penguin species [START_REF] Dias | Threats to seabirds: A global assessment[END_REF][START_REF] Trathan | Pollution, habitat loss, fishing, and climate change as critical threats to penguins[END_REF]. There are growing concerns that widespread increases in ecotourism activities have exacerbated this threat [START_REF] Ellenberg | Physiological and reproductive consequences of human disturbance in Humboldt penguins: the need for species-specific visitor management[END_REF].

Loss of subsurface predators in tropics

Loss of subsurface predators is an indirect threat to many seabirds, particularly boobies, shearwaters, frigatebirds, and terns in the tropics. There, most species are unable to dive more than a few metres and forage over large areas of relatively unproductive ocean; hence they take advantage of forage fish driven to the surface by tuna, billfishes, and dolphins [START_REF] Au | Seabird interactions with dolphins and tuna in the eastern tropical Pacific[END_REF][START_REF] Danckwerts | Biomass consumption by breeding seabirds in the western Indian Ocean: indirect interactions with fisheries and implications for management[END_REF][START_REF] Jaquemet | Seabird community structure in a coastal tropical environment: importance of natural factors and fish aggregating devices (FADs)[END_REF]). Long-term declines in tuna abundance following overexploitation in purse-seine, longline and, until they were banned in the 1990s, longdrift-net fisheries in High Seas (which also bycaught numerous dolphins and whales) [START_REF] Lotze | Historical baselines for large marine animals[END_REF] have greatly reduced accessibility of prey for tropical seabirds. Commercial whaling is likely to have had a similar effect on surfacefeeding seabirds over wide areas. More emphasis on ecosystem-based management of tuna fisheries, which also considers indirect impacts, would improve the situation, but progress at RFMOs has been slow [START_REF] Juan-Jordá | Report card on ecosystem-based fisheries management in tuna regional fisheries management organizations[END_REF]). On the plus side, the recovery of great whales [START_REF] Lotze | Historical baselines for large marine animals[END_REF], and associated facilitation of surface feeding by seabirds may to some extent offset other threats in temperate and polar regions.

Guano harvesting

Guano reserves -the accumulated excrement of seabirds and bats rich in nutrients such as nitrogen or phosphate -has a high value as organic fertilizer (Chapter 7).

The guano harvesting industry boomed during the 19 th and 20 th century in seabird colonies of Peru and Chile, declining when replaced by industrial manufacture of fertilizers [START_REF] Schnug | Guano: The White Gold of the Seabirds[END_REF]. Peru is still the largest producer of guano in the world, originating mostly from colonies of Guanay Cormorants Phalacrocorax bougainvillii, Peruvian Pelicans Pelecanus thagus and Peruvian Boobies Sula variegata [START_REF] Weimerskirch | Foraging in Guanay cormorant and Peruvian booby, the major guano-producing seabirds in the Humboldt Current System[END_REF]. Guano harvesting can disturb breeding birds and lead to nest abandonment, chick mortality, or loss of entire colonies [START_REF] Du Toit | Conservation assessment and management plan for southern African coastal seabirds[END_REF][START_REF] Duffy | The guano islands of Peru: the once and future management of a renewable resource[END_REF], and is considered to be the main driver of the historical population decline of the Peruvian Diving-petrel Pelecanoides garnotii [START_REF] Cristofari | Unexpected population fragmentation in an endangered seabird: the case of the Peruvian diving-petrel[END_REF].

Emerging and under-studied threats Hybridization

Hybridization may be a threat to seabirds of conservation concern because introgression degrades the gene pool [START_REF] Seehausen | Conservation: losing biodiversity by reverse speciation[END_REF]). However, mixed-species pairing and production of hybrid offspring is uncommon in seabirds other than gulls and terns, probably because of behavioral barriers and generally high natal philopatry [START_REF] Brown | Hybridization and back-crossing in giant petrels (Macronectes giganteus and M. halli) at Bird Island, South Georgia, and a summary of hybridization in seabirds[END_REF][START_REF] Phillips | Breeding-site vagrancy and hybridization in albatrosses[END_REF]). As such, hybridisation probably represents a potential threat only to a few, very rare species, such as the Chinese Crested Tern Thalasseus bernsteini [START_REF] Yang | Genetic evidence of hybridization of the world's most endangered tern, the Chinese Crested Tern Thalasseus bernsteini[END_REF].

Offshore wind farms

The number and extent of offshore wind farms is expected to grow substantially over the next decades at a global scale (Chapter 7). The turbines can affect seabirds via collision or, more often, displacement from good-quality habitats [START_REF] Cook | Quantifying avian avoidance of offshore wind turbines: current evidence and key knowledge gaps[END_REF][START_REF] Furness | Assessing vulnerability of marine bird populations to offshore wind farms[END_REF]). However, the consequences of these and other offshore marine infrastructures for seabird populations are still poorly understood [START_REF] Green | Lack of sound science in assessing wind farm impacts on seabirds[END_REF]. Coastal species, including sea ducks, terns, and shags are particularly affected [START_REF] Dias | Threats to seabirds: A global assessment[END_REF], but highly migratory species can also be at risk due to cumulative impacts across their ranges [START_REF] Busch | Approaching population thresholds in presence of uncertainty: Assessing displacement of seabirds from offshore wind farms[END_REF]. Advances in tracking technology have greatly increased our understanding of potential risks of new offshore windfarms on seabird populations [START_REF] Lane | Vulnerability of northern gannets to offshore wind farms; seasonal and sex-specific collision risk and demographic consequences[END_REF]Peschko et al. 2020b); however, there is a clear need to expand knowledge to less studied regions, including Asia and South America, where the offshore wind market is expected to grow exponentially.

Discarding

Commercial fisheries provide enormous quantities (>10 million tonnes) of supplementary food for scavenging seabirds in the form of discards (unwanted catch, spent bait, and offal), including from large, demersal species that would otherwise be unavailable [START_REF] Mcinnes | DNA metabarcoding as a marine conservation and management tool: A circumpolar examination of fishery discards in the diet of threatened albatrosses[END_REF][START_REF] Real | Discard-ban policies can help improve our understanding of the ecological role of food availability to seabirds[END_REF][START_REF] Tasker | The impacts of fishing on marine birds[END_REF] Chapter 3). Discarding can be highly beneficial, depending on the balance between seabird bycatch that may result from increased vessel interactions, and higher breeding success associated with diets that include a high proportion of discards [START_REF] Mcinnes | DNA metabarcoding as a marine conservation and management tool: A circumpolar examination of fishery discards in the diet of threatened albatrosses[END_REF][START_REF] Oro | Effects of a trawling moratorium on the breeding success of the yellow-legged gull Larus cachinnans[END_REF][START_REF] Phillips | Diet of the northern fulmar Fulmarus glacialis: reliance on commercial fisheries?[END_REF]). As such, there are concerns that reductions in discarding associated with better fisheries management could have negative repercussions for some seabird populations [START_REF] Bicknell | Potential consequences of discard reform for seabird communities[END_REF][START_REF] Real | Discard-ban policies can help improve our understanding of the ecological role of food availability to seabirds[END_REF]. However, availability of discards affects food web and community structure by increasing dietary overlap [START_REF] Bugoni | The importance of pelagic longline fishery discards for a seabird community determined through stable isotope analysis[END_REF][START_REF] Jiménez | Inter-specific niche partitioning and overlap in albatrosses and petrels: dietary divergence and the role of fishing discards[END_REF], and potentially influences distributions by concentrating seabirds in productive areas favoured by fisheries, exacerbating bycatch risk [START_REF] Ryan | Effect of trawling on bird and seal distributions in the southern Benguela region[END_REF].

Discards may also be nutritionally poorer than natural prey, resulting in lower growth rates of chicks [START_REF] Grémillet | A junk-food hypothesis for gannets feeding on fishery waste[END_REF]. There are also indirect effects from associated increases in abundance of predator-scavengers (particularly gulls and skuas) on smaller seabirds that are their alternative prey in years when availability of discards or natural prey resources such as forage fish are reduced [START_REF] Stenhouse | Indirect effects of the availability of capelin and fishery discards: gull predation on breeding storm petrels[END_REF][START_REF] Votier | Changes in fisheries discard rates and seabird communities[END_REF]).

Conclusions

Although this overview has considered each threat largely as independent, it is important to note that many seabird populations are subject to multiple threats that may be additive or synergistic. As such, addressing population declines will often require a suite of management measures, and potentially compensatory mitigation for threats such as climate change that will only be alleviated by decades of human effort at a global scale. Moreover, much of the focus of conservation research has been on obvious impacts of human activities, and there are many sub-lethal or emerging threats that are likely undocumented or little understood. There have also been strong biases towards temperate and polar, rather than tropical species, and to surface-nesting and larger burrowing seabirds, rather than storm petrels. Hence more balance is required in terms of investment into both research and conservation action. the percentage of species affected by each threat in 3 classes: <15%, 15%-30%, >30% (y axis in the legend). Values at the bottom (text in grey) represent the average number of threats affecting each species in that order and the percentage of species in each order that are affected by both marine and terrestrial threats, regardless of impact. Based on [START_REF] Dias | Threats to seabirds: A global assessment[END_REF] and sources mentioned therein.

Figure 2 . 2 .

 22 Figure 2.2. Percentage of seabird species by taxonomic order in each IUCN Red List category. A. All species. "Others" include Gaviiformes, Pelecaniformes, Phaethontiformes and Podicipediformes. B. Families within the order Procellariiformes. C. Families within the order Charadriiformes. Values at the top represent the percentage of species with declining trends, with the percentage of declining species among those with known trends in parentheses. LC = Least Concern; NT = Near Threatened; VU = Vulnerable; EN = Endangered; CR = Critically Endangered; CR(PE) = Critically Endangered (Possibly Extinct); DD = Data Deficient. Data source: BirdLife International (2021).

Figure 2 .

 2 Figure 2.3. Infographic illustrating the most important threats and emerging threats to seabirds at sea and on land.

Figure 2 .

 2 Figure 2.4. Threats affecting seabird species by taxonomic group. "Others" include Gaviiformes, Pelecaniformes, Phaethontiformes, and Podicipediformes. Color gradients represent, from lighter to darker colours, 1) the average impact of each threat (see Dias et al. 2019) in 3 classes: <4, 4-6, >6 (x axis in the legend), and 2)

Impact is measured as the product of the scope of the threat (percentage of the total population affected) and its severity (rate of population decline caused by the threat over three generations; see Garnett et al.

[START_REF] Naves | Geographic and seasonal patterns of seabird subsistence harvest in Alaska[END_REF] , Dias et al. 2019).


Z Hadjou Belaid

L Mechernene
email: l_mechernene@yahoo.fr

F Z Abdoune

A Berrayah
email: a_berrayah@yahoo.frulrich.maschke@univ-lille1.fr

U Maschke

Photo-Differential Scanning Calorimetry Study on Photopolymerization of Polyacrylate/E7 Liquid Crystal Blends

Keywords: Ttripropylene glycol diacrylate, liquid crystal, photopolymerization, phase separation kinetics, photo-differential-scanning-calorimetry (Photo-DSC)

Photo-differential scanning calorimetry (photo-DSC) was used to investigate the cure kinetics of a photo-initiated polymerization of tripropylene glycol diacrylate (TPGDA) monomers mixed with a eutectic liquid crystal mixture, E7, chosen due to its significant optical properties. The thermal photo-polymerization reactions were performed in isothermal mode in order to identify and evaluate the different thermal effects occurring during the photo-polymerization reaction under UV light. The results obtained help to better understand the curing mechanisms as well as the kinetics of the curing reactions. The isothermal photo-DSC provided complex thermograms showing the presence of two thermal effects: an exothermic heat (positive activation energy) generated during the conversion of monomers and an endothermic heat produced by the UV lamp. The latter phenomenon was observed in our previous work and was associated to the heating of the polymer dispersed liquid crystals (PDLC) material by the UV irradiation lamp. The dependence of photopolymerization on the time and intensities of the curing UV light were investigated. The results showed that the high intensity UV light heats the material and slows the process of phase separation which, in turn, affects the electro-optical properties of the material under study. The photo DSC results also revealed that the heating of the system was insignificant and can, therefore, be neglected, for materials exposed to UV radiation with an irradiation time of 5 seconds and a low intensity of 13.80 mW/cm 2 .

Introduction

Polymer/liquid crystal composite materials, also called polymer dispersed liquid crystals (PDLC), are generally in the form of thin films consisting of nano/micro-sized liquid crystal droplets dispersed in a more or less flexible amorphous polymer matrix [1,[START_REF] Deshmukh | Novel techniques of PDLC film preparation furnishing manifold properties in a single device[END_REF]. These multiphase materials, with an electro-optical functionality, have many interesting optical applications [START_REF] Deshmukh | Effect of temperature on the optical and electro-optical properties of poly(methyl methacrylate)/E7 polymer-dispersed liquid crystal composites[END_REF][START_REF] Lampert | Chromogenic smart materials[END_REF], such as the glazing with controllable transparency: they can toggle from an opaque state (Off state) to a transparent state (On state) by the simple application of a voltage.

The methods used in preparing PDLC films are divided into two classes. The first one uses an emulsion method [START_REF] He | The effect of molecular weight of polymer matrix on properties of polymer-dispersed liquid crystal[END_REF][START_REF] Doane | Polymer dispersed liquid crystal[END_REF][START_REF] Zhang | Effect of electric field on phase separation of polymer dispersed liquid crystal[END_REF][START_REF] Formentín | Polymerdispersed liquid crystals based on E7: Morphology and characterization[END_REF][START_REF] Ahmad | Surfactant-doped reverse-mode polymer-dispersed liquid crystal display with enhanced properties[END_REF][START_REF] Ding | A study of electro-optical properties of PDLC films prepared by dual UV and heat curing[END_REF] which consists in obtaining a polymer including LC domains, after evaporation of the solvent, from a homogeneous mixture of the polymer and the liquid crystal (LC) material. The second one includes all the processes that involve the phase separation of the LC and the polymer matrix from a homogeneous solution. The most common method, which belongs to the second class, was selected for the preparation of our materials. It involves photopolymerization under UV irradiation, which induces phase separation, also called polymerization-induced phase separation (PIPS) [START_REF] Yaroshchuk | Control of phase separation and morphology of thiol-ene based PDLCs by curing light intensity[END_REF][START_REF] Malik | Effects of polymer viscosity on the polymerization switching and electro-optical properties of unaligned liquid crystal/UV curable polymer composites[END_REF][START_REF] Hadjou Belaid | Relaxation Behaviour of Monomer/Liquid Crystal Blends after Exposure to Ultraviolet Light[END_REF], between the resulting polymer and the liquid crystals. This method allows producing films with optimal thermo-physical properties. The samples, obtained by phase separation, still show some fine liquid crystal droplets [START_REF] Chun | Polymer-Dispersed Liquid Crystal Applied in Active-Matrix Transparent Display[END_REF][START_REF] Kemiklioglu | Effects of photoinitiator on electro-optical properties of polymerization-induced phase separation blue-phase liquid crystals[END_REF] of various sizes, dispersed more or less homogeneously inside the polymer matrix. The size and shape of the liquid crystal domains are essential parameters to be considered in order to obtain films with optimal electro-optical properties. These two factors are strongly influenced by the phase separation kinetics occurring within the material [START_REF] Malik | Droplet orientation and optical properties of polymer dispersed liquid crystal composite films[END_REF][START_REF] Serbutoviez | Polymerization-Induced Phase Separation. 2. Morphology of Polymer-Dispersed Liquid Crystal Thin Films[END_REF]. Indeed, rapid phase separation kinetics generally gives small-sized droplets, while large-sized droplets are obtained with slower kinetics. In previous works [START_REF] Hadjou Belaid | Relaxation Behaviour of Monomer/Liquid Crystal Blends after Exposure to Ultraviolet Light[END_REF][START_REF] Chun | Polymer-Dispersed Liquid Crystal Applied in Active-Matrix Transparent Display[END_REF][START_REF] Kemiklioglu | Effects of photoinitiator on electro-optical properties of polymerization-induced phase separation blue-phase liquid crystals[END_REF][START_REF] Malik | Droplet orientation and optical properties of polymer dispersed liquid crystal composite films[END_REF][START_REF] Hadjou Belaid | Unusual Phase Separation Kinetics of Polyacrylate/E7 Blends[END_REF] the kinetics of the photopolymerization reaction of the monomer and liquid crystal mixtures under UV radiation were investigated optically by measuring the variations of light transmitted by the sample as a function of curing time.

The results showed a rapid and expected decrease of the transmitted light intensity was observed during the first milliseconds after the beginning of the UV irradiation. This decrease was explained by the formation of the liquid crystal domains which were formed progressively; with the advance of the polymerization the system becomes more and more inhomogeneous. This effect was then followed by an unexpected enhancement of the transmitted light [START_REF] Hadjou Belaid | Unusual Phase Separation Kinetics of Polyacrylate/E7 Blends[END_REF]. This phenomenon, whose duration was longer, from a few seconds to a few tens of seconds, depended on the time and intensity of irradiation of the UV source. The increase in transmitted light, which means that the system gradually returned from the anisotropic state to the isotropic state, thus becoming more homogeneous, was explained as a thermal effect: The sample heats up under the UV lamp light during the photopolymerization. Temperature measurements, performed by means of a thermal probe attached to the sample, showed an unexpected endothermic effect due to the high intensity of the UV lamp used [START_REF] Hadjou Belaid | Relaxation Behaviour of Monomer/Liquid Crystal Blends after Exposure to Ultraviolet Light[END_REF]. The aim of the present work was to try to better know and quantify the energies that are involved in the polymerization of our systems and which strongly control the morphology of our materials. This was made possible by the use of the photodifferential scanning calorimetry (photo-DSC) technique [START_REF] Jakubiak | Isothermal Photo Differential Scanning Calorimetry. Crosslinking polymerization of multifunctional monomers in presence of visible light photoinitiators[END_REF][START_REF] Ciprian | Influence of temperature and UV intensity on photo-polymerization reaction studied by photo-DSC[END_REF] which allowed obtaining the evolution of the thermograms during photopolymerization.

The photo-DSC thermograms clearly showed two types of heat involved in the polymerization under UV irradiation, namely an exothermic heat flow from the polymerization reaction and an endothermic heat flow from the UV lamp heat the sample as would be expected. Calorimetric measurements were carried out by photo-DSC as a function of the intensity and time irradiation of the UV lamp for the various samples [START_REF] Ciprian | Influence of temperature and UV intensity on photo-polymerization reaction studied by photo-DSC[END_REF][START_REF] He | Photopolymerization and structure formation of methacrylic acid based hydrogels: The effect of light intensity[END_REF]. These measurements showed that the exothermic and endothermic energy flows more or less overlapped in time, depending on the time and intensity of the UV irradiation. The goal of the present study was to identify and understand the two types of heat flows involved in the preparation of our materials in order to improve their electro-optical properties.

Experimental

Tripropylene glycol diacrylate (TPGDA, M n = 300 g/mol) from TOTAL Cray Valley (France), was used as a difunctional monomer. The commercial liquid crystal (LC) E7, provided by Merck KGaA (Darmstadt, Germany), consists of a mixture of four cyanobiphenylene derivatives (Scheme 1). This liquid crystal mixture exhibits a nematic-isotropic phase transition temperature, T NI = 60°C, and has a large nematic phase temperature range, between -62°C and +60°C. The Initial mixtures, containing 30 wt.-% of monomer, 70 wt.-% of LC E7, and 1wt.-% of the mass of monomer, of photoinitiator (Lucirin TPO, from BASF AG, Germany) were kept under mechanical stirring at room temperature (20°C) for several hours until a homogeneous mixture was obtained.

Scheme 1 that should be inserted in the Experimental section The polymerization and crosslinking processes were induced by a LC-08 irradiation source (from Hamamatsu Photonic K.K., Japan), equipped with a Xenon lamp, covering the spectrum between 250 and 800 nm, and an optical fiber. The solution samples prepared as above were exposed to the UV irradiation. In this study, the photo-polymerization kinetics were monitored by means of a differential scanning calorimeter (DSC 8500 -Perkin Elmer Inc, USA) adapted for photochemical measurements with a Perkin Elmer photocalorimeter accessory sample-holder cover.

The heat flow released during the photopolymerization reaction, using isothermal photo differential scanning calorimetry, was measured for various irradiation times and intensities of the UV light. The intensities used in this study are listed in Table 1:

The masses of the samples were 1mg (uncertainty ±0.1mg). The polymerization was carried out at a temperature of 20±1 °C. The isothermal photo-differential scanning calorimetry allowed us to measure, at the temperature of 20°C, the energy difference between the crucible containing the sample (reactive mixture) and the reference crucible, as a function of time. The UV irradiation was equally transmitted via a two-output optical fiber; one went to the sample and the other one to the reference cell. The differential scanning calorimeter used was an instrument controlled by a computer and equipped with a liquid nitrogen cooling system. The Perkin Elmer's specialized thermal analysis software allowed us to measure the flows of both types of energies, exothermic and endothermic, from a thermogram. These were the energies absorbed and released by the sample under UV irradiation.

A complementary study was carried out to measure the glass transition temperature, T g , of the resulting composite materials prepared with various irradiation times and intensities of the UV light. The DSC technique was used to investigate the glass transition temperatures, T g , and the isotropic-nematic temperatures, T NI , of the composite materials previously prepared under the UV irradiation. First, the sample was cooled down to a temperature of -80°C, for 2 min, until thermal equilibrium was reached. Then the temperature was increased from -80 °C to min, after which it was cooled at a rate of 10 ° C / min to -80 ° C and the cooling scan recorded. The purpose of this last operation was to watch the behavior of the glass transition temperature T g and the isotropic-nematic temperature, T NI , under the effect of the heat provided by the UV lamp, and also to check the influence of that lamp on the morphological properties of the systems which significantly controlled the electro-optical properties of the PDLC.

Results and discussion

The kinetics of the photopolymerization reaction has been found to depend on several parameters, including the photo-initiator selected, the nature of the monomer used and its concentration in the monomer/liquid crystal mixture [START_REF] Roussel | Photopolymerization kinetics and phase behaviour of acrylate based polymer dispersed liquid crystals[END_REF][START_REF] Hyun | Investigation of the photopolymerizationinduced phase separation process in polymer dispersed liquid crystal[END_REF]. This polymerization is also highly influenced by the parameters of the preparation process, such as the time and intensity of the UV irradiation and the temperature in the polymerization-induced phase separation (PIPS) method used here [START_REF] Smith | A Calorimetric Study of Phase Separation in Liquid Crystal/Matrix Systems: Determination of the Excess Specific Heat of Mixing[END_REF][START_REF] White | Contribution of monomer functionality and additives to polymerization kinetics and liquid crystal phase separation in acrylate-based polymer-dispersed liquid crystals (PDLCs)[END_REF].

In our previous study [START_REF] Hadjou Belaid | Unusual Phase Separation Kinetics of Polyacrylate/E7 Blends[END_REF] on the optical transmission of these systems under UV irradiation, it showed an unexpected thermal effect which was associated with the heating of the PDLC material by the UV lamp at relatively high intensities. It was observed that the optical transmission capacity of these materials increased with the UV dose, which could be explained by the high temperature of the material as a result of its irradiation by the UV lamp. If the irradiation time is relatively long then the sample reaches a temperature exceeding the nematicisotropic phase transition temperature (T NI ); the sample becomes isotropic although the polymerization/crosslinking process continues during this isotropic phase. In order to study the endothermic heat flow released by the UV lamp, which affects the electro-optical properties of the prepared composites, an isothermal analysis was carried out by the photo-DSC technique. This operation was performed at various UV radiation intensities but with a fixed time of 20 s; it was then repeated for different UV radiation time and a fixed intensity of 13.80 mW/cm 2 , which is the lowest intensity delivered by the UV lamp used. The exposure time of 20s (which was the average of the times used) and UV intensity of 13.80 mW/cm 2 (which was the lowest intensity released by the UV lamp) were chosen in order to have a minimum heating of the sample by the UV lamp; it was assumed that this would help to obtain two separate heat flow thermograms. Figure 2-A illustrates the thermal behavior of the TPGDA/TPO/E7 mixtures, for various UV radiation intensities, and an irradiation time of 20 s. This figure shows that the photopolymerization kinetics of the reactive mixtures, in the presence of the liquid crystal (LC), strongly depended on the intensity of the UV radiation emitted by the LC8 UV lamp. The endothermic flow increased significantly with the intensity of the UV irradiation. The exothermic part of the thermograms, corresponding to the heat released in the photopolymerization reaction, decreased with the increasing intensity of the UV lamp which heated the samples. This causes the polymerization to slow down.

The effect of varying the irradiation time, at an intensity fixed at 13.80 mW/cm2, is shown by the thermograms in Fig. 2-B, in which it is easy to note that the time of UV radiation did not significantly affect the rate of the exothermic energy flow but not its total value, measured from the area of the exothermic flux, which varied with the irradiation time. This is due to the very high rate of the photopolymerization reaction, which lasted only a few milliseconds, well below the different irradiation times used. The large difference (about 5 mw . s) observed in the middle for the area of the exothermic flow, as a function of the irradiation time, for these systems, could be explained by a possible change in the intensity of the UV lamp with changing irradiation time, although it was well fixed at the beginning. As expected, the endothermic flow was not very sensitive to the variation of the irradiation time before stopping the irradiation. Indeed, the sample can reach thermal equilibrium in the same way for the low intensity of 13.80mW / cm 2 . The endothermic effect, observed in the photo-DSC thermograms, stems from the fact that the amount of heat transferred to the DSC by the polymer/LC mixture (of high calorific capacity) per unit time was less than that transferred directly by the reference cell made of aluminum (of a more or less low calorific capacity). This is due to the fact that aluminum is a good conductor as compared to polymers which are poor thermal conductors.

A similar comparative study was conducted on the same reactive mixtures but without the LC, in order to understand its effect on the thermograms. The results obtained, as a function of the intensity and the irradiation time, are shown in Figs. 3-A and3-B, respectively. A large exothermic flux was observed for these systems as compared to those containing the liquid crystals. This result may probably be explained by the monomer mass of 1 mg which was much larger than that used in the previous mixtures containing a LC; that was only 0.3 mg. The amount of monomers to be polymerized was therefore larger, thus generating a higher exothermic energy [START_REF] Jakubiak | Isothermal Photo Differential Scanning Calorimetry. Crosslinking polymerization of multifunctional monomers in presence of visible light photoinitiators[END_REF], which varied little with the intensity of the UV irradiation. However, the endothermic contribution was much less important as compared with that observed in the case of the mixtures with a liquid crystal. The endothermic flow varied a little with the intensity of the UV lamp, but it did not change when the irradiation time was changed, as expected. For the mixtures without liquid crystals the exothermic effect was more important than the endothermic effect, while the opposite was observed for the mixtures containing the liquid crystals.

The results showed that the observed behavior was somewhat unexpected (for the systems with LC), where the endothermic flow grew with the intensity of the UV irradiation. It is also remarked that there was a virtual disappearance of the exothermic flow, and probably no PDLC material was formed, for a high intensity of UV light, in which case the endothermic flow predominated. This phenomenon, which is due to the infrared part of the UV light spectrum, was responsible for the heating of the low-weighting samples.

One can say that, for a liquid crystal mixture, the polymeric network formation was slowed down due to the presence of the LC domains, and that the exothermic flow was largely dominated by the endothermic flow which varied greatly with the intensity of the UV lamp. Moreover, the higher the intensity of the UV lamp, the warmer the system become. On the other hand, for a fixed intensity of the lamp, the endothermic flow was the same for any irradiation time. The mixtures containing LC absorbed more than 2.5 times more energy, released by the UV lamp, than those without LC. The PDLC composites absorbed much more energy from the irradiation source than did the polymeric network TPGDA without LC. Figures 4-A and4-B present the values of the exothermic and endothermic flows for the TPGDA/TPO/E7 systems as a function of the time and intensity of the UV radiation, respectively. These values can be calculated from the areas of the thermograms plotted in Figs. 2-A and2-B, for the mixtures containing a liquid crystal. The energy absorbed by the system increased significantly with increasing irradiation time, and this increase became more and more important with increasing intensity of the UV lamp (Figure 2-A).

On the other hand, the energy released (exothermic) decreased when the intensity of the UV radiation increased (Figure 4-B). When this intensity reached high values, the temperature of the material increased considerably and this caused a decline in the conversion rate of the monomers. The excessive heating of the system can be explained by the high intensity of the UV radiation. This heating begins before the complete formation of the PDLC composite film. However, the decline of the exothermic energy as a function of the irradiation time, for a fixed intensity (13.80 mW/cm 2 ), as shown in Fig. 4-A, is inexplicable, since the photopolymerization, which ends in a few milliseconds, must release the same energy, given that all the doses used have a minimum duration of 5 seconds which is well above the polymerization time. The energy absorbed (endothermic) by the sample containing the liquid crystals (LC) varied significantly with the intensity of the UV source; the lowest intensity emitted by the LC8 lamp used in this study was relatively high. The intensity of the UV source thus played an important role in the preparation of the PDLC composite materials by the polymerization-induced phase separation (PIPS) method using this powerful UV source.

For the reactive mixtures without LC, the values H Exo of the exothermic energy released by the photopolymerization reaction, calculated from the exothermic peak area of Fig. 3-A, varied slightly as a function of the intensity of the UV lamp (Figure 5-B). Quantitatively, the energy absorbed (endothermic) for all of the intensities presented the same variation as a function of the irradiation time until the irradiation was stopped. The energy generated by the UV lamp was much smaller than that obtained for mixtures with LC. Similarly to the mixtures with LC, the time of UV light exposure was found to only slightly influence the energy flow released by the photopolymerization.

A complementary study was conducted on the behavior of the glass transition temperature, T g , and the nematic-isotropic temperature, T NI , in order to understand the effects of the intensity and time of the UV irradiation on the morphology of the prepared materials on which their electro-optical properties crucially depend. The classical DSC study of the photopolymerization reaction can give insights on the mechanical and electro-optical properties [START_REF] Abdoune | Investigation of the electro-optical behaviour of UV-cured polymer/liquid crystal systems[END_REF][START_REF] Bouchakour | Optical properties of electron beam-and UV-cured polypropyleneglycoldiacrylate/liquid crystal E7 systems[END_REF]. This investigation consisted first of carrying out the classical DSC experiments, on several PDLC materials prepared at different UV intensities and irradiation time, then of measuring the possible changes in T g and T NI , and characteristics of the morphology. This last study made it possible to find the transition temperatures, Tg and T NI , of our materials as a function of the time and the intensity of UV radiation. Figure 6-A shows the influence of the irradiation time emitted by the UV lamp on the glass transition temperature, T g , and the isotropic-nematic temperature, T NI , of the TPGDA/E7 materials (30/70 Wt.-%), while the intensity of the UV radiation was maintained fixed at 13.80 mW/cm 2 .

Figure 6-A shows that the isotropic-nematic temperature T NI , for TPGDA/E7 materials was practically insensitive to the variations in the irradiation times, particularly beyond the time of 5s. The temperature, T NI , increased slightly with the intensity. This increase, which was more pronounced for low intensities, reached a steady value of ca. 60.5 °C for intensities exceeding 30% (Figure 6B). This temperature increased by about 3.5 °C when for intensities exceeding 30% of Fig. 6-B. This temperature increased by about 4 °C when the intensity rose from 10 to 30%.

On the other hand, the glass transition temperature, T g , showed no change with respect to the irradiation time. This was expected, as the formation of the network, which ended within a few milliseconds, depended very little on the irradiation time, especially since the UV intensity of 13.80 mW/cm 2 used with these times was still significant. There are strong reasons to believe that the irradiation time (dose), or the time during which the sample is subjected to UV lamp irradiation, had an effect on the morphology of the liquid crystal domains. The formation of the liquid crystal droplets in the original homogeneous solution may have been slowed down relative to that of the polymer matrix when the irradiation time or dose was increased. The effect of the intensity of the UV irradiation is represented in Fig. 6-B, where it is clearly seen that the glass transition temperature, T g , remained constant. This may be explained by the fact that, since all the intensities used were relatively high, this led to mechanically identical networks, previously obtained through polymerization processes completed with the same kinetics. The polymer matrices of the PDLC materials must have similar mechanical properties, particularly the property of rigidity, independently of the relatively strong intensities of the UV irradiation used in the preparation of our materials since they had the same T g s. The results presented in both Figs. 6-A and 6-B supports those found by the Photo-DSC, which showed that the exothermic flux of the thermograms depended weakly on the UV irradiation time and slightly on the intensity. This is in good agreement with the results found by the classical DSC.

The results reported in this article indicate that, in order to distinguish between the two thermal effects involved in the polymerization of the PDLC, namely the one resulting from the photopolymerization reaction and the one which is due to the high intensity of the UV lamp, it was necessary to use the irradiation time of 5s and an intensity of 13.80 mW/cm 2 of the UV radiation. In this case, the endothermic heat flow obtained was due to the infrared part of the UV spectrum of the lamp LC8. This probably affects the morphology and electro-optical properties (response time) of these materials materials but needs further examination.

Conclusions

The kinetics of the photopolymerization reaction of PDLC composite materials, prepared by a polymerization induced phase separation (PIPS) process, was investigated to quantify and understand the two types of thermal contributions observed on the thermograms obtained during the preparation of these materials by the Photo-DSC technique. The polymerization rate under high UV intensity was rapid; this automatically led to a rapid separation of the two phases, namely the liquid crystal molecules domains and the polymeric network formed. The results obtained by the Photo-DSC technique indicated that photopolymerization was a rapid, exothermic reaction which was nearly independent of the dose and intensity of the UV radiation, except for the smallest irradiation time and the lowest intensity for which a slight change was observed. Note that even the smallest intensity of the UV lamp used remained high. The presence of an endothermic heat flow on the thermograms indicated that significant heating of the samples occurred under the high intensity of the UV lamp. This endothermic energy flow was due to the infrared part of the spectrum of the powerful UV source. The exothermic energy released during the photopolymerization reaction was small relative to the endothermic energy absorbed by the sample for the TPGDA / E7 composite materials as compared to the liquidcrystal free systems. It is worth mentioning that for these PDLC materials, prepared under UV radiation, the thermograms exhibited a predominant endothermic effect which increased with the intensity of the UV lamp. In one of these TPGDA / E7 composites (30/70%) it was found that the temperatures T g and T NI increased slightly for low UV intensities and low irradiation time, respectively. This confirmed the results found by the Photo-DSC (Figure 2 B).

Figure 2 .

 2 Figure 2. Isothermal Photo-DSC (IDSC) of photopolymerization for the TPGDA/TPO/E7 mixture, -A-for various UV irradiation intensities during 20s, -B-for various UV irradiation times for an intensity fixed at 13.80 mW/cm 2 .

Figure 3 .

 3 Figure 3. Isothermal Photo-DSC (IDSC) of photopolymerization for the TPGDA/TPO mixture, -Afor various UV irradiation intensities during 20s, -B-for various UV irradiation times for an intensity fixed at 13.80 mW/cm 2 .

Figure 4 .

 4 Figure 4. Photopolymerization enthalpies of TPGDA/E7/TPO mixture, H Exo and H Endo , -A-as a function of the UV irradiation time for a UV light intensity of 13.80 mW/cm 2 , -B-as a function of the intensity during 20s.

Figure 5 .

 5 Figure 5. Photopolymerization enthalpies of TPGDA/TPO mixture, H Exo and H Endo , -A-as a function of the UV irradiation time for a UV light intensity of 13.80 mW/cm 2 , -B-as a function of the UV intensity during 20s.

Figure 6 .

 6 Figure 6. Effect of UV irradiation on temperatures T g and T NI of the TPGDA/E7 systems, -A-for various irradiation times for a UV intensity of 13.80mW/Cm 2 , -B-for various UV intensities during 20s.