Junaid S Khan
email: junaid.khan@canada.ca

Jennifer F Provencher
email: jennifer.provencher@canada.ca

Mark R Forbes
email: mark.forbes@carleton.ca

Mark L Mallory
email: mark.mallory@acadiau.ca

Camille Lebarbenchon
email: camille.lebarbenchon@univ-reunion.fr

Karen D Mccoy
email: karen.mccoy@ird.fr

Parasites of seabirds: a survey of effects and ecological implications

Keywords: Bacteria, bird, ectoparasite, endoparasite, helminth, marine, pathogen, parasitism, seabird, virus

Parasites are ubiquitous in the environment, and can cause negative effects in their host species. Importantly, seabirds can be long-lived and cross multiple continents within a single annual cycle, thus their exposure to parasites may be greater than other taxa. With changing climatic conditions expected to influence parasite distribution and abundance, understanding current levels of infection, transmission pathways and population-level impacts are integral aspects for predicting ecosystem changes, and how climate change will affect seabird species. In particular, a range of micro-and macro-parasites can affect seabird species, including ticks, mites, helminths, viruses and bacteria in gulls, terns, skimmers, skuas, auks and selected phalaropes (Charadriiformes), tropicbirds (Phaethontiformes), penguins (Sphenisciformes), tubenoses (Procellariiformes), cormorants, frigatebirds, boobies, gannets (Suliformes), and pelicans (Pelecaniformes) and marine seaducks and loons (Anseriformes and Gaviiformes). We found that the seabird orders of Charadriiformes and Procellariiformes were most represented in the parasite-seabird literature. While negative effects were reported in seabirds associated with all the parasite groups, most effects have been studied in adults with less information known about how parasites may affect chicks and fledglings. We found studies most often reportred on negative effects in seabird hosts during the breeding season, although this is also the time when most seabird research occurs. Many studies report that external factors such as condition of the host, pollution, and environmental conditions can influence the effects of parasites, thus cumulative effects likely play a large role in how parasites influence seabirds at both the individual and population level. With an increased understanding of parasite-host dynamics it is clear that major environmental changes, often those associated with human activities, can directly or indirectly affect the distribution, abundance, or virulence of parasites and pathogens.

Introduction

Parasites, defined as organisms that live at the expense of their hosts [START_REF] Combes | Parasitism: the ecology and evolution of intimate interactions[END_REF][START_REF] Poulin | Meta-analysis and research on host-parasite interactions: past and future[END_REF], form diverse and ubiquitous components of all ecosystems, and include both macroparasites (e.g., helminths, arthropods) and microparasites (e.g., bacteria, viruses, fungi, protists). While most ecological studies focus on megafauna, these organisms can exert a significant influence on ecosystem composition and function via their diversity, abundance and impact on other members of the community [START_REF] Thomas | Parasitism and Ecosystems[END_REF]. While much is known regarding parasites in some taxa (i.e. captive mammals), considerably less is known in others. Given that changing climatic conditions are expected to influence parasite distribution and abundance, understanding current levels of infection, transmission pathways and population-level impacts are integral aspects for predicting ecosystem changes (eg. Leger et al. 2013). Seabirds are a particularly interesting group to study in this respect as they often traverse large distances along their migratory routes, and thus may be exposed to, and disperse, many more parasites when compared to other groups [START_REF] Boulinier | Migration, prospecting, dispersal? What host movement matters for infectious agent circulation?[END_REF].

Additionally, seabirds are long-lived and typically breed in large, dense colonies, traits that can favour parasite maintenance within populations [START_REF] Mccoy | The role of seabirds of the Iles Eparses as reservoirs and disseminators of parasites and pathogens[END_REF]. Finally, some species of seabirds frequently come into contact with humans (e.g., gulls or harvested species), and thus may be a source of zoonotic disease.

Although the impact of parasites on captive (e.g., poultry) and terrestrial (e.g., Passeriformes) bird species has been relatively well-studied, less has been done on seabird parasites. Recent research has provided an overview of some of these relationships (e.g. Granoth-Wilding et al. 2014;[START_REF] Provencher | Antiparasite treatment, but not mercury burdens, influence nesting propensity dependent on arrival time or body condition in a marine bird[END_REF], whereas others, such as Grimaldi et al. (2015), focused on the impact of parasites in specific bioregions and orders/genera. Except under extreme conditions where the presence of a parasite has a devastating impact causing widespread mortality (e.g. avian cholera; [START_REF] Butler | Handling stress affects survival and reproduction of breeding female Common Eiders during an avian cholera outbreak[END_REF][START_REF] Descamps | Avian cholera, a threat to the viability of an Arctic seabird colony?[END_REF][START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF], little is known about how interactions with these organisms alter seabird health, reproductive success, and ultimately, seabird population viability and evolution. The impact of a parasite will depend on the nature of the parasite in question, and may be influenced by interactions with other local environmental factors, such as the quantity and quality of food resources, levels of local pollution, habitat quality, and presence of predators, competitors and/or other parasites (e.g., [START_REF] Provencher | Direct and indirect causes of sex differences in mercury concentrations and parasitic infections in a marine bird[END_REF][START_REF] Butler | Handling stress affects survival and reproduction of breeding female Common Eiders during an avian cholera outbreak[END_REF].

Understanding seabird-parasite interactions is particularly important in regions that are expected to experience great degrees of change in the near future. For example, Polar Regions are expected to warm at a more rapid rate than temperate or tropical regions [START_REF] Francis | Evidence for a wavier jet stream in response to rapid Arctic warming[END_REF]. Generally, parasitism is thought to be relatively depauperate in polar regions because of the lower diversities of suitable intermediate hosts, coupled with conditions that may be unsuitable for certain parasite groups [START_REF] Brooks | Finding them before they find us: informatics, parasites, and environments in accelerating climate change[END_REF][START_REF] Davidson | Arctic parasitology: why should we care?[END_REF][START_REF] Kutz | A walk on the tundra: host-parasite interactions in an extreme environment[END_REF]). However, with changing climatic conditions, and corresponding distributional changes in different taxonomic groups, parasite diversity in the polar zones may increase substantially.

There is already evidence to suggest increased parasitism in northern wildlife due to changing climatic conditions (Brooks and Hoberg, 2007;[START_REF] Kutz | The Arctic as a model for anticipating, preventing, and mitigating climate change impacts on host-parasite interactions[END_REF]Kutz et al. , 2012)). Additionally, novel parasites may extend their ranges in polar regions, or potentially expand to new hosts introducing new interactions within ecosystems [START_REF] Davidson | Arctic parasitology: why should we care?[END_REF][START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF].

The purpose of the present work is to give an overview of known parasite taxa in the seven aforementioned seabird orders, to survey their current distributions, and examine the known effects on seabird populations worldwide. By doing so, we hope to identify the gaps in our current knowledge and shed light on the relationships between parasites and their seabird hosts, and how changing conditions and other stressors may alter the seabird host-parasite relationships.

Methods

This purpose of this paper is to provide a survey of available literature documenting parasites of seabirds, and their effects. We used a literature scanning approach using Web of Science Core Collection and Google Scholar as our main databases. Queries entered into both databases were a combination of general and taxon-specific search terms. All search results were scanned for relevance related to the diversity, abundance and relationships of parasites with seabird hosts. Publications that did not achieve these goals, or had more specific research focuses were not included. All relevant journal articles were coded and entered in a spreadsheet for further examination. For each article, the following information was recorded: the parasitic organism studied, a short summary of the article, any effects of the parasite on the birds, the seabird order and species infected by the parasite, the location where the study was conducted, the methods used in the study, the year the study was conducted, and citation information. All search terms used for this review can be found in Table 1.

For the purpose of this study, we consider seabirds as defined by Gaston (2004), with updated classifications proposed by the International Ornithological Committee World Bird List [START_REF] Gill | Higher classification of modern birds[END_REF]. This includes gulls, terns, skimmers, skuas, auks and selected phalaropes (Charadriiformes), tropicbirds (Phaethontiformes), penguins (Sphenisciformes), tubenoses (Procellariiformes), cormorants, frigatebirds, boobies, gannets (Suliformes), and pelicans (Pelecaniformes) as the main species. We also include, to a limited extent, seaducks and loons (Anseriformes and Gaviiformes) in this review, as they fulfill a very similar ecological niche.

Results

A total of 258 journal articles on the taxonomy, distribution, prevalence and abundance of parasites in seabirds were reviewed. Of these, 98 articles housed information on the impacts of parasites on seabirds specifically. Research results gathered on each of the parasite groups are summarized below.

Macroparasites

Ectoparasites

Ticks

Ticks are blood-feeding arachnids belonging to the order Parasitiformes, sub-order Ixodida and include approximately 900 described species [START_REF] Guglielmone | The Argasidae, Ixodidae and Nuttalliellidae (Acari: Ixodida) of the world: a list of valid species names[END_REF]. All species require at least one bloodmeal to complete the life cycle, feeding on a wide range of mammals, birds, reptiles and amphibians. Among arthropods, ticks transmit the greatest variety of infectious agents, including viruses, bacteria, protozoa and even helminths, but most research to date on seabirds has focused on Ixodes spp. vectors of Lyme disease bacteria (Borrelia burgdorferi s.l.; [START_REF] Dantas-Torres | Ticks and tick-borne diseases: a One Health perspective[END_REF][START_REF] Jongejan | The global importance of ticks[END_REF]; Table 1). Ticks can be divided into two distinct categories; hard ticks (Ixodidae) and soft ticks (Argasidae). Hard ticks have a single long bloodmeal in each of their three life stage (larva, nymph, adult), whereas soft ticks take several short bloodmeals in nymphal and adult stages and have a variable number of nymphal instars [START_REF] Dietrich | Worldwide distribution and diversity of seabird ticks: implications for the ecology and epidemiology of tick-borne pathogens[END_REF]. In both groups, the duration of attachment on the host determines the likelihood of disease transmission [START_REF] Dantas-Torres | Ticks and tick-borne diseases: a One Health perspective[END_REF].

Of the 17 identified species of hard ticks found on seabirds, all but one (Amblyomma loculosum) belong to the genus Ixodes. Of these, Ixodes uriae has the largest distribution, and is the most studied species (Figure 1a; Coulson et al. 2009;[START_REF] Mccoy | Host-dependent genetic structure of parasite populations: differential dispersal of seabird tick host races[END_REF]Munoz-Leal and Gonzalez-Acuna, 2015;Olsen et al. 1993). Hard ticks have been recorded feeding on seabirds belonging to the orders Charadriiformes, Phaethontiformes, Sphenisciformes, Procellariiformes, and Suliformes [START_REF] Dietrich | Worldwide distribution and diversity of seabird ticks: implications for the ecology and epidemiology of tick-borne pathogens[END_REF][START_REF] Gauthier-Clerc | Comparison of behaviour, body mass, haematocrit level, site fidelity and survival between infested and non-infested king penguin Aptenodytes patagonicus by ticks Ixodes uriae[END_REF][START_REF] Ramos | Productivity of white-tailed tropicbird on Aride Island, Seychelles[END_REF]. However, no observations of hard ticks on wild individuals from Pelicaniformes, Gaviiformes, or

Anseriformes have been reported to date. The taxonomic status of most seabird hard ticks is well-established, though work has shown that these ticks may form isolated host specific populations, even when different seabird species breed in sympatry (e.g., [START_REF] Mccoy | Host-dependent genetic structure of parasite populations: differential dispersal of seabird tick host races[END_REF];

the potential for host-associated speciation may be high in this group and overall diversity as measured using traditional morphological criteria may underestimate true diversity [START_REF] Moon | Phylogeographic structure in penguin ticks across an ocean basin indicates allopatric divergence and rare trans-oceanic dispersal[END_REF].

The 12 known species of soft ticks exploiting seabirds all belong to the genera Argas and Ornithodoros (Carios), and tend to be restricted to tropical and sub-tropical zones (Figure 2).

Soft ticks have been found parasitizing Charadriiformes, Phaethontiformes, Sphenisciformes, Procellariiformes, Suliformes, and Pelicaniformes [START_REF] Dietrich | Worldwide distribution and diversity of seabird ticks: implications for the ecology and epidemiology of tick-borne pathogens[END_REF][START_REF] Duffy | The ecology of tick parasitism on densely nesting Peruvian seabirds[END_REF][START_REF] Feare | Desertion and abnormal development in a colony of Sooty Terns Sterna fuscata infested by virus-infected ticks[END_REF] Figure 1); we found no study that reported the presence of soft ticks on wild individuals of Gaviiformes or Anseriformes. However, due to the short time that these ticks require for bloodfeeding (i.e., minutes) and their nocturnal activity, they are difficult to observe under natural conditions and their diversity, prevalence and local abundance in seabird colonies is surely underestimated. In addition, general questions related to their systematics remain open [START_REF] Guglielmone | The Argasidae, Ixodidae and Nuttalliellidae (Acari: Ixodida) of the world: a list of valid species names[END_REF][START_REF] Dupraz | Linking morphometric and genetic divergence with host use in the tick complex, Ornithodoros capensis sensu lato[END_REF], and much basic observational work is still required to clarify their diversity, specificity and impact on seabird population dynamics.

Many studies on tick-seabird systems have focused on the prevalence of human pathogens such as Soldado virus and B. burgdorferi within ticks, with few discussing the circulation of these agents among ticks and seabirds (e.g., [START_REF] Gylfe | Isolation of Lyme disease Borrelia from puffins (Fratercula arctica) and seabird ticks (Ixodes uriae) on the Faeroe Islands[END_REF][START_REF] Main | Avalon and Clo Mor: two new Sakhalin group viruses from the North Atlantic[END_REF]but see Nunn et al. 2006a andGomez-Diaz et al. 2010). As most research has targeted known pathogenic agents, the presence and diversity of other microparasites transmitted via ticks (or other potential vectors) remain to be investigated. The effect of known tick-borne pathogens on seabird populations is also poorly understood [START_REF] Dietrich | Worldwide distribution and diversity of seabird ticks: implications for the ecology and epidemiology of tick-borne pathogens[END_REF]. We discuss these pathogens in more detail in later sections.

Some work has evaluated the direct impact of ticks on seabird health, reproductive success, and population dynamics in relation to hyperinfestations (e.g., Boulinier et al. 1996;[START_REF] Duffy | The ecology of tick parasitism on densely nesting Peruvian seabirds[END_REF][START_REF] Monticelli | Juvenile survival in a tropical population of roseate terns: interannual variation and effect of tick parasitism[END_REF][START_REF] Ramos | Activity patterns and effect of ticks on growth and survival of tropical Roseate Tern nestlings[END_REF][START_REF] Ramos | Productivity of white-tailed tropicbird on Aride Island, Seychelles[END_REF]. Results vary greatly among species and orders, and are often related to other factors, such as food access and supply. For example, [START_REF] Ramos | Activity patterns and effect of ticks on growth and survival of tropical Roseate Tern nestlings[END_REF] and [START_REF] Monticelli | Juvenile survival in a tropical population of roseate terns: interannual variation and effect of tick parasitism[END_REF] reported that in years of poor food supply, populations of Roseate Terns (Sterna dougallii) experienced massive chick mortality and breeding failure in areas of high tick (Amblyomma loculosum) densities. However, similar studies conducted on White-tailed Tropicbirds (Phaethon lepturus) in the same region found no significant effect of tick parasitism/interference competition on productivity [START_REF] Ramos | Productivity of white-tailed tropicbird on Aride Island, Seychelles[END_REF].

There is evidence to suggest that species size may influence the effect that ticks have on individuals and populations. In a study conducted in the Huaura Islands, Peru nest desertion due to ticks was more common for Guanay Cormorant (Phalacrocorax bougainvilli) and Peruvian Booby (Sula variegata) populations than for the larger Peruvian Brown Pelican (Pelecanus occidentalis ambius; [START_REF] Duffy | The ecology of tick parasitism on densely nesting Peruvian seabirds[END_REF]. However, post-mortem investigations of three dead King Penguins (Aptenodytes patagonicus) found on the Crozet archipelago in the Sub-Antarctic region led researchers to conclude that hyperinfestation by ticks during incubation (when birds do not feed) was the most likely cause of death (Gauthier-Clerc et al. 1999).

Although no difference in body mass or behaviour of breeding King Penguins was found with respect to more commonly observed levels of tick infestation [START_REF] Gauthier-Clerc | Comparison of behaviour, body mass, haematocrit level, site fidelity and survival between infested and non-infested king penguin Aptenodytes patagonicus by ticks Ixodes uriae[END_REF], some impact on reproductive success has been suggested [START_REF] Mangin | Ticks Ixodes uriae and the breeding performance of a colonial seabird, king penguin Aptenodytes patagonicus[END_REF]. No such observations have been made for smaller penguin species.

A relatively well-studied aspect of tick-seabird systems is the cyclical dynamics of seabird populations in relation to tick abundance. As the number of ticks in a given colony increases, the likelihood of nest desertion and chick mortality increases (Boulinier and Danchin, 1996;[START_REF] Duffy | The ecology of tick parasitism on densely nesting Peruvian seabirds[END_REF][START_REF] Monticelli | Juvenile survival in a tropical population of roseate terns: interannual variation and effect of tick parasitism[END_REF][START_REF] Ramos | Activity patterns and effect of ticks on growth and survival of tropical Roseate Tern nestlings[END_REF][START_REF] Ramos | Tick infestation of chicks in a seabird colony varies with local breeding synchrony, local nest density and habitat structure[END_REF].

Conversely, breeding failures at the colony level also may also have a detrimental impact on local tick populations [START_REF] Danchin | The incidence of the tick parasite Ixodes uriae in Kittiwake Rissa tridactyla colonies in relation to the age of the colony, and a mechanism of infecting new colonies[END_REF][START_REF] Dietrich | Worldwide distribution and diversity of seabird ticks: implications for the ecology and epidemiology of tick-borne pathogens[END_REF][START_REF] Monticelli | Juvenile survival in a tropical population of roseate terns: interannual variation and effect of tick parasitism[END_REF]Olsen et al. 1993;[START_REF] Ramos | Activity patterns and effect of ticks on growth and survival of tropical Roseate Tern nestlings[END_REF]. This type of dynamic interaction between seabirds and ticks will also have consequences for the local transmission and large-scale circulation of infectious agents [START_REF] Boulinier | Dispersal and parasitism[END_REF].

Mites

Althoughthese are also small arachinds, unlike ticks, only a small fraction of mite species parasitize animals. Species exploiting seabirds as hosts belong to the orders Mesostigmata, Trombidiformes, and Sarcoptiformes. Among the Mesostigmata, it is species of the superfamily Dermanyssoidea (e.g., Dermanyssus gallinae, Ornithonyssus bursa) that are among the most studied. Species in this group have been reported to agitate adult birds, causing interruption in egg incubation, reducing nestling growth rates, and reducing overall reproductive success (Clayton and Thompkins, 1995;[START_REF] Moller | Effects of parasitism by a haematophagous mite on reproduction in the barn swallow[END_REF]. It is the cohort Astigmatina (order Sacroptiformes) however that houses the greatest diversity of bird-associated mites, including feather mites (Psoroptidia).

While there is no doubt on the parasitic status of blood-feeding mites, the status of feather mites as parasites is still debated. These mites are thought to mainly consume feather pith or skin, and in some cases, feather oils [START_REF] Stefan | Niche partitioning of feather mites within a seabird host, Calonectris borealis[END_REF]. In domestic birds, mites that feed on feather pith can weaken feathers and cause premature breakage and skin lesions [START_REF] Proctor | Feather mites (Acari: Astigmata): ecology, behavior, and evolution[END_REF]. Hyperinfestation can also result in severe itching, causing birds to pull feathers [START_REF] Proctor | Feather mites (Acari: Astigmata): ecology, behavior, and evolution[END_REF]. However, studies by [START_REF] Blanco | Feather mites on group-living red-billed choughs: a nonparasitic interaction?[END_REF]2001a;2001b) suggest that feather mites may help boost body condition by removing excess body oils and fungi.

Mite abundance on a bird host can be affected by season, temperature, light, humidity, and host body condition [START_REF] Proctor | Feather mites (Acari: Astigmata): ecology, behavior, and evolution[END_REF]. Mites are typically transmitted between hosts by close body contact, which can be frequent in colonial nesting seabirds [START_REF] Proctor | Feather mites (Acari: Astigmata): ecology, behavior, and evolution[END_REF]. Feather mites have been found on species belonging to the order Procellariformes, Gaviiformes, Pelecaniformes, Anseriformes, and Charadriiformes [START_REF] Proctor | Feather mites (Acari: Astigmata): ecology, behavior, and evolution[END_REF][START_REF] Stefan | Niche partitioning of feather mites within a seabird host, Calonectris borealis[END_REF]. A study by [START_REF] Stefan | Niche partitioning of feather mites within a seabird host, Calonectris borealis[END_REF] determined that a single bird can play host to multiple species of mites (Microspalax brevipes and Zachvatkinia ovate) due to resource partitioning and spatial segregation amongst the species. Indeed, a more recent study found up to 8 different mite species exploiting Cape Verde Shearwaters (Calonectris edwardsii) and Bulwer's Petrels (Bulweria bulwerii; [START_REF] Stefan | More Than Meets the Eye": Cryptic Diversity and Contrasting Patterns of Host-Specificity in Feather Mites Inhabiting Seabirds[END_REF]. However, the effects of mites on wild seabirds are largely unknown. Studies on mite-seabird interactions have focused on the phylogeny and coevolution of mites and their hosts, rather than the impact of mite infestation itself [START_REF] Choe | Ectoparasites of the pelagic cormorant, Phalacrocorax pelagicus, from the Pribilof Islands, Alaska[END_REF], 1988, 1989;Coulson et al. 2009;[START_REF] Stefan | More Than Meets the Eye": Cryptic Diversity and Contrasting Patterns of Host-Specificity in Feather Mites Inhabiting Seabirds[END_REF].

Fleas

Fleas are small (adults ~3mm), flightless, insects of the order Siphonaptera that parasitize mammal and bird populations worldwide. Of the 2,500 identified species, only those belonging to the genera Ceratophyllus, Parapsyllus, Mioctenopsylla, and Xenopsylla have been reported on seabirds (Coulson et al. 2009;[START_REF] Kane | Avian pox in Magellanic penguins (Spheniscus magellanicus)[END_REF]. Flea parasitism probably occurs in most seabird species and has been reported in wild seabirds belonging to the orders Sphenisciformes, Charadriiformes, Anseriformes, Procellariformes, and Suliformes [START_REF] Alonso | Arthropods in shag (Phalacrocorax aristotelis) nests from the national park of the Atlantic Islands (NW Spain): occurrence and abundance at different breeding phases[END_REF][START_REF] Choe | Ectoparasites of the pelagic cormorant, Phalacrocorax pelagicus, from the Pribilof Islands, Alaska[END_REF]Kristjansson et al. 2016). The egg, larval, and pupal stages of most fleas live off the host's body, but adults are obligate blood-feeders. Because a large part of the life cycle occurs in the habitat used by the host, the presence and abundance of fleas is closely related to the conditions of their breeding environment (Gómez-Díaz and González-Solís, 2010). Fleas can act as disease vectors for several bacteria, viruses and helminths [START_REF] Bitam | Fleas and flea-borne diseases[END_REF][START_REF] Kane | Avian pox in Magellanic penguins (Spheniscus magellanicus)[END_REF]. Although poorly studied in relation to seabirds, they are known mechanical vectors for avipoxviruses [START_REF] Kane | Avian pox in Magellanic penguins (Spheniscus magellanicus)[END_REF].

Many studies on fleas in seabird colonies focus on examining the diversity and trophic relationships within arthropod communities, rather than the effects of flea parasitism itself [START_REF] Alonso | Arthropods in shag (Phalacrocorax aristotelis) nests from the national park of the Atlantic Islands (NW Spain): occurrence and abundance at different breeding phases[END_REF][START_REF] Choe | Ectoparasites of the pelagic cormorant, Phalacrocorax pelagicus, from the Pribilof Islands, Alaska[END_REF]Coulson et al. 2009 terms of blood loss. These results fall in line with observations in passerine birds (eg. [START_REF] Fitze | Life history and fitness consequences of ectoparasites[END_REF].

Lice

Lice are obligate parasites belonging to the insect order Phthiraptera. Lice are further subdivided into two main groups: sucking lice (suborder Anoplura) and chewing lice (suborders Ischnocera, Amblycera and Rhynchophthirina). Anoplura are obligate blood-sucking parasites found almost exclusively on mammalian hosts. In contrast, species of chewing lice are commonly found on bird species, feeding on hair, feathers and epidermal skin/scales (Clay and Morby, 1967). Unlike ticks and fleas, lice spend their entire life cycle on a single host. Females lay their eggs, commonly known as nits, by gluing them to the hairs or feathers of their hosts through a secretion created in their accessory glands. Lice are typically host-specific, rather than geographically-specific, which makes estimating their global distribution challenging (Clay and Morby, 1967). Factors influencing the presence and abundance of lice on seabirds are not extensively studied. Rivera-Parra et al. ( 2014) sampled five seabird species across seven islands in the Galapagos, and found a connection between host density and louse infestation levels.

However, the study also pointed out the need for further research on the impact of local weather conditions and host density on lice loads; factors which have been found to affect louse loads in other bird groups [START_REF] Clayton | Coevolution of life on hosts: integrating ecology and history[END_REF].

Louse-seabird interactions are generally studied by collecting lice present on individual birds or in bird nests [START_REF] Alonso | Arthropods in shag (Phalacrocorax aristotelis) nests from the national park of the Atlantic Islands (NW Spain): occurrence and abundance at different breeding phases[END_REF]Coulson et al. 2009;Gomez-Diaz and Gonzalez-Solis, 2010;[START_REF] Rivera-Parra | Comparative ectoparasite loads of five seabird species in the Galapagos Islands[END_REF]. The most common methods of louse collection include dust ruffling with an insecticide and agitating the feathers until parasites fall off [START_REF] Mallory | Ectoparasites of northern fulmars Fulmarus glacialis (Procellariformes: Procellariidae) from the Canadian Arctic[END_REF][START_REF] Rivera-Parra | Comparative ectoparasite loads of five seabird species in the Galapagos Islands[END_REF], or collecting invertebrates from active nests [START_REF] Alonso | Arthropods in shag (Phalacrocorax aristotelis) nests from the national park of the Atlantic Islands (NW Spain): occurrence and abundance at different breeding phases[END_REF]Coulson et al. 2009). Although lice are prevalent in six of the eight main seabird orders that we consider here (Procellariformes, Charadriiformes, Pelicaniformes, Anseriformes, Suliformes, Sphenisciformes), most research on louse-seabird interactions has been focused on the co-evolution or phylogeny of lice species in relation to their hosts [START_REF] Paterson | Parasites, petrels and penguins: does louse presence reflect seabird phylogeny?[END_REF][START_REF] Paterson | Seabird and louse coevolution: Complex histories revealed by 12S rRNA sequences and reconciliation analyses[END_REF][START_REF] Page | Phylogeny of "Philoceanus complex" seabird lice (Phthiraptera: Ischnocera) inferred from mitochondrial DNA sequences[END_REF]).

Although rarely studied in seabirds, evidence suggests that lice can reduce host fitness in numerous ways (thermoregulatory stress due to feather condition, grooming costs related to higher metabolic rates, predator vigilance, etc.; [START_REF] Clayton | Coevolution of life on hosts: integrating ecology and history[END_REF]. We found one study that measured the impacts of louse loads on wild seabirds; [START_REF] Daunt | Parental age and offspring ectoparasite load in European Shags Stictocarbo aristotelis[END_REF] measured loads of Eidemanniella pellucida on breeding European Shags (Phalacrocorax aristotelis) and their chicks, and found no measurable effect of louse load on offspring growth.

Flies

Hippoboscid flies (order Diptera), commonly known as louse flies, are the most common family of parasitic flies found on birds and mammals. The Hippoboscidae are divided into three main subfamilies, only two of which (Ornithomyinae and Hippoboscinae) include obligate parasites of birds [START_REF] Duvallet | Hippoboscidae, Nycteribiidae et Streblidae (Diptera)[END_REF]. As hippoboscid flies are highly mobile (some species are winged) and can spend part of their lifecycle off host, host to host transmission can be very high [START_REF] Bequaert | The Hippoboscidae or louse-flies (Diptera) of mammals and birds. Part I. Structure, physiology and natural history[END_REF]. Factors affecting the parasitism by Hippoboscidae in seabirds are not well understood, and many species may have co-evolved with their specific hosts (Bakey, 1967;[START_REF] Levin | Comparative host-parasite population genetic structures: obligate fly ectoparasites on Galapagos seabirds[END_REF]. However, as mentioned above, humidity and oily plumage can be related to the prevalence of louse flies on seabirds [START_REF] Bequaert | The Hippoboscidae or louse-flies (Diptera) of mammals and birds. Part I. Structure, physiology and natural history[END_REF]. The annual colonial nesting behavior of seabirds may also be a key aspect supporting the maintenance and spread of hippoboscid flies [START_REF] Bastien | Haemoproteus iwa in great frigatebirds (Fregata minor) in the Islands of the western Indian Ocean[END_REF][START_REF] Levin | Comparative host-parasite population genetic structures: obligate fly ectoparasites on Galapagos seabirds[END_REF].

Hippoboscid flies have been recorded on seabirds belonging to the orders Suliformes, Procellariformes, Pelecaniformes, and Charadriiformes [START_REF] Bastien | Haemoproteus iwa in great frigatebirds (Fregata minor) in the Islands of the western Indian Ocean[END_REF][START_REF] Bequaert | The Hippoboscidae or louse-flies (Diptera) of mammals and birds. Part I. Structure, physiology and natural history[END_REF][START_REF] Whiteman | Cryptic host specificity of an avian skin mite (Epidermoptidae) vectored by louseflies (Hippoboscidae) associated with two endemic Galapagos bird species[END_REF]. Olfersia aenescens most commonly parasitizes Charadriiformes, Procellariformes, and Pelecaniformes. However, information on hippoboscid flies in other avian orders is less clear. For example, no reports were found of hippoboscids on any members of Anseriformes in this review; this is thought to be due to their oily plumage and damp nesting habitat preferences [START_REF] Bequaert | The Hippoboscidae or louse-flies (Diptera) of mammals and birds. Part I. Structure, physiology and natural history[END_REF]. Within avian orders, the diversity of hippoboscids can vary greatly from family to family. Among Pelicaniformes , all members of the family Fregatidae share the species Olfersia spinifera, whereas O. aenescens, O. fossulata and O. sordida parasitize Phalacrocoricidae, Sulicidae and Pelicanidae [START_REF] Bequaert | The Hippoboscidae or louse-flies (Diptera) of mammals and birds. Part I. Structure, physiology and natural history[END_REF].

The impact of Hippoboscidae on seabirds is also sparsely understood. [START_REF] Bequaert | The Hippoboscidae or louse-flies (Diptera) of mammals and birds. Part I. Structure, physiology and natural history[END_REF] suggested that since many louse fly species are host-specific, they are more dependent on host survival than other, more generalist, ectoparasites. However, their relatively high mobility may limit constraints on virulence. Recent studies on the impact of Hippoboscidae on seabirds have focused on the transmission of microparasites such as haemosporidia, rather than the direct impact of parasitism itself [START_REF] Bastien | Haemoproteus iwa in great frigatebirds (Fregata minor) in the Islands of the western Indian Ocean[END_REF][START_REF] Levin | Comparative host-parasite population genetic structures: obligate fly ectoparasites on Galapagos seabirds[END_REF]. The impact of these microparasites on seabirds is discussed in a later section.

Mosquitoes

There are approximately 3,500 named species of mosquitoes (Diptera: Culicidae), of which seven known genera (Aedes, Anopheles, Culex, Mansonia, Haemagogus, Psorophora, and Sabethes) transmit disease to animals, including humans [START_REF] Fang | Ecology: a world without mosquitoes[END_REF][START_REF] Gubler | Vector-borne diseases[END_REF]. Culicidae are the most prolific disease vectors within the order Diptera, and can transmit a range of parasites (e.g., Plasmodium relictum), viruses (alphaviruses, flaviviruses, etc) and helminthes within bird populations worldwide (more details outlined below). Of the four stages of their life cycles, mosquitoes spend the egg, larval and pupal stages in aquatic habitats, and only parasitize hosts as adults. Only the females bite and can transmit infectious agents (Gulber, 2009).

Mosquitoes are known to parasitize species belonging to Charadriiformes, Suliformes, Procellariiformes and Pelecaniformes. In studies relating to wild seabirds, only Aedes taeniorhynchus has been identified as a parasite [START_REF] Anderson | Differential responses of boobies and other seabirds in the Galapagos to the 1986-87 El Nino-Southern Oscillation event[END_REF][START_REF] Anderson | Waved albatross egg neglect and associated mosquito ectoparasitism[END_REF]. Most studies conducted on mosquito-bird systems have focused on birds acting as reservoirs for, or direct transmitters of, viruses and diseases that affect humans [START_REF] Parkinson | Potential impacts of climate change on infectious diseases in the Arctic[END_REF][START_REF] Reed | Birds, migration and emerging zoonoses: West Nile virus, Lyme disease, influenza A and enteropathogens[END_REF][START_REF] Tsiodras | Human infections associated with wild birds[END_REF]. Like other macroparasites, the direct impact of mosquitos on their hosts will depend on their relative abundance. Although factors affecting mosquito abundance are largely unknown, heat, humidity, and the availability of stagnant water for breeding are key determining factors [START_REF] Wegbreit | Relationships among weather, mosquito abundance, and encephalitis virus activity in California: Kern County 1990-98[END_REF]. Heat has been identified as a particularly important factor affecting mosquito parasitism of seabirds [START_REF] Gaston | Heat and mosquitoes cause breeding failures and adult mortality in an Arctic-nesting seabird[END_REF][START_REF] Gaston | Effects of climate-induced changes in parasitism, predation and predator-predator interactions on reproduction and survival of an Arctic marine bird[END_REF].

The colonial and often synchronized nature of seabird breeding supports the spread of mosquitoes among hosts within populations; birds born later in the breeding season can be prone to higher degrees of mosquito exposure due to a reduction in nestling hosts (Caillouet et al. 2013). In seabird populations, mosquito parasitism has been positively correlated with egg neglect in Waved Albatrosses (Diomedia irrorata) in the Galapagos Islands [START_REF] Anderson | Waved albatross egg neglect and associated mosquito ectoparasitism[END_REF]. In Hudson Bay, adult mortality and egg loss in a population of Thick-billed Murres (Uria lomvia) were also positively correlated with mosquito parasitism [START_REF] Gaston | Heat and mosquitoes cause breeding failures and adult mortality in an Arctic-nesting seabird[END_REF][START_REF] Gaston | Effects of climate-induced changes in parasitism, predation and predator-predator interactions on reproduction and survival of an Arctic marine bird[END_REF].

Endoparasites

Helminths

Helminths are a group of worms that are often parasitic in birds and include flatwormswhich further separate into cestodes (tapeworms) and trematodes (flukes) -nematodes (roundworms), and acanthocephala (thorny-headed worms; [START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF]Galaktionov and Bustnes, 1997). The life cycle and transmission pathways of helminths are often complex, with both direct transmission (i.e., from parent to chick during feeding), and indirect transmission via intermediate hosts or vectors. Helminths are typically studied in seabirds through carcass collection (e.g., [START_REF] Mallory | Breeding status, contaminant burden and endoparasites of northern fulmars (Fulmarus glacialis) from the Canadian high arctic[END_REF], although some studies have used regurgitation, endoscopic techniques, and anti-helminth treatments in seabirds, to assess parasitism in the gastro-intestinal tract and their effects [START_REF] Fonteneau | New data on gastrointestinal helminths in shags (Phalacrocorax verrucosus) at Kerguelen Archipelago[END_REF][START_REF] Granroth-Wilding | Betweenindividual variation in nematode burden among juveniles in a wild host[END_REF][START_REF] Provencher | Antiparasite treatment, but not mercury burdens, influence nesting propensity dependent on arrival time or body condition in a marine bird[END_REF]. Information on the impact of helminths on some species are more available because they are hunted or often beach cast, but overall the need for carcasses poses a challenge to explore helminth-seabird relationships in wild populations, as lethal sampling is most often used. In situations where carcasses are available, a range of helminths have been found in host species (e.g. [START_REF] Mallory | Breeding status, contaminant burden and endoparasites of northern fulmars (Fulmarus glacialis) from the Canadian high arctic[END_REF][START_REF] Tourangeau | Sources of variation in endohelminth parasitism of common eiders over-wintering in the Canadian Arctic[END_REF].

Cestodes and Trematodes

Cestodes and trematodes (Phylum Platyhelminthes) are characterized by their lack of a body cavity, hermaphroditism, and the use of suckers (borthridia) to attach themselves within their host [START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF]. Flatworms can be most easily distinguished by the segmentation in their bodies; trematodes have unsegmented, smooth bodies, whereas cestodes have segmented bodies [START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF]. The lifecycle of trematodes and cestodes often involves two, sometimes three, intermediate hosts (e.g., copepod infected by feeding on cestode eggs, which in turn are ingested by cephalopods or fish) before reaching maturity within a seabird [START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF]. However, only trematodes require the obligatory participation of molluscs to complete their lifecycle [START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF].

The prevalence and impacts of cestodes and trematodes vary by species, region, and sex. In Double-crested Cormorants (Phalacrocorax auritus) in southern Canada, [START_REF] Robinson | Helminth pathogen cathepsin proteases: it'sa family affair[END_REF] found that males had significantly higher levels of the trematode Drepanocaphalus spathans when compared with females. [START_REF] Mallory | Breeding status, contaminant burden and endoparasites of northern fulmars (Fulmarus glacialis) from the Canadian high arctic[END_REF] found a higher frequency of occurrence of cestodes in Northern Fulmars (Fulmarus glacialis) during the post-breeding season as compared with the pre-breeding season. The diversity of cestodes and trematodes has been shown to vary across a host species range. For example, birds inhabiting the southern extent of the Common Eider (Somateria mollissima) range in the North Sea have as many as 15 trematode species, whereas at the northern extent of the range in Franz Josef Land only, a single trematode species has been reported [START_REF] Galaktionov | Life cycles and distribution of seabird helminths in arctic and subarctic regions[END_REF][START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF]. In terms of the impact of these worms on seabird health and condition, as for most macroparasites, it is expected that negative impacts may only occur at very high parasite loads (see [START_REF] Galaktionov | Life cycles and distribution of seabird helminths in arctic and subarctic regions[END_REF], but detailed studies in seabirds are limited and difficult to attribute to cestodes and trematodes in particular.

Nematodes

Nematodes or roundworms (phylum Nematoda) are characterized by their round body shape and the presence of a body cavity. Nematodes also have distinct male and female organisms [START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF], similar to cestodes and trematodes, however, the prevalence of nematodes in seabirds is highly variable. Even individuals nesting at nearby colonies can exhibit strong differences in parasitism levels. [START_REF] Fonteneau | New data on gastrointestinal helminths in shags (Phalacrocorax verrucosus) at Kerguelen Archipelago[END_REF]

Acanthocephalans

Much like cestodes and trematodes, acanthocephalans are parasitic worms that lack a mouth, and instead attach to their hosts using a proboscis that is covered with spiny hooks [START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF][START_REF] Hoberg | Aspects of ecology and biogeography of Acanthocephala in Antarctic seabirds[END_REF]. Acanthocephalans also have distinct male and female sexes, and their life cycle requires intermediate hosts, most commonly thought to be benthic crustaceans [START_REF] Galaktionov | Patterns and processes influencing helminth parasites of Arctic coastal communities during climate change[END_REF]. Although acanthocephalans have been identified in many Antarctic and Arctic seabird fauna (Charadriiformes, Sphenisciformes, Anseriformes; Brandao et al. 2014;[START_REF] Hoberg | Aspects of ecology and biogeography of Acanthocephala in Antarctic seabirds[END_REF], they are best studied as one of the most prolific parasites of Common Eiders in the northern hemisphere [START_REF] Galaktionov | Life cycles and distribution of seabird helminths in arctic and subarctic regions[END_REF][START_REF] Provencher | Direct and indirect causes of sex differences in mercury concentrations and parasitic infections in a marine bird[END_REF][START_REF] Tourangeau | Sources of variation in endohelminth parasitism of common eiders over-wintering in the Canadian Arctic[END_REF]; Figure 3).

A sex bias in the prevalence of acanthocephalans in seabird species has been identified, and may be attributable to differing life history strategies between males and females, especially during the breeding season, caring of young and feeding in distinct areas during the moulting season [START_REF] Skirnisson | Association of helminth infections and food consumption in common eiders Somateria mollissima in Iceland[END_REF]. Similar to the other parasitic worms, the distribution and number of species in acanthocephalans vary by region, with the number of species of acanthocephalans being higher in the sub-Arctic regions than in Arctic environments [START_REF] Galaktionov | Life cycles and distribution of seabird helminths in arctic and subarctic regions[END_REF]. Again, there is very little work done on how this particular group may negatively affect seabirds given the co-occurrence with other species, and the difficulty of examining this group in isolation in relation to potential impacts on species.

Helminths interactions

While a single host can be affected by the number of helminths it carries, there is no clear relationship demonstrated in the literature on how helminths may interact with respect to each other within a host. For example, [START_REF] Robinson | Helminth pathogen cathepsin proteases: it'sa family affair[END_REF] found no correlation between the most abundant helminths (nematodes and cestodes) in Double-crested Cormorants. In contrast, [START_REF] Provencher | Direct and indirect causes of sex differences in mercury concentrations and parasitic infections in a marine bird[END_REF] found that the abundance of cestodes and acanthocephalans were negatively correlated with each other in Common Eiders, and suggested a possible negative interaction between helminth species within a host, at least in some species. Northern Fulmars from the high Canadian Arctic harboured several helminth species including cestodes, trematodes, nematodes, and acanthocephalans in varying levels of infection [START_REF] Mallory | Breeding status, contaminant burden and endoparasites of northern fulmars (Fulmarus glacialis) from the Canadian high arctic[END_REF]). [START_REF] Tourangeau | Sources of variation in endohelminth parasitism of common eiders over-wintering in the Canadian Arctic[END_REF] similarly found that Common Eiders in northern Canada to have several cestodes and acanthocephalans with varying levels of infection in relation to age and sex. Due to the difficulty in obtaining reliable estimates of parasite diversity and loads, the impact of these species on seabird health and population dynamics have received relatively little attention.

In some cases, only the presence or absence of helminths is reported without any additional information. For example, in [START_REF] Provencher | Trace element concentrations and gastrointestinal parasites of Arctic terns breeding in the Canadian High Arctic[END_REF], helminths are reported in Arctic Terns (Sterna paradisaea), but specific parasite identification and enumeration were not possible during the dissection of the birds. Globally, the lack of trained parasite taxonomists may also be hampering the identification and reporting of seabird parasites [START_REF] Hopkins | Declines in the numbers of amateur and professional taxonomists: implications for conservation[END_REF]. Furthermore, some of the most comprehensive parasite publications are only available in Russian where there is still a strong tradition of parasitologists interested in systematics and natural history (e.g., Kuklin, 2017).

Microparasites

Arboviruses

Although there are cases where viruses are transmitted by direct contact with an infected individual, most viruses in wild seabird populations are vector-borne (see [START_REF] Van Riper | Epizootiology and effect of avian pox on Hawaiian forest birds[END_REF]. Arboviruses, or arthropod-borne viruses, comprise close to 50 different viruses that have been identified in wild seabirds [START_REF] Beckham | Arbovirus infections[END_REF][START_REF] Nuttall | Tick-borne viruses in seabird colonies[END_REF][START_REF] Mackenzie | The zoonotic flaviviruses of Southern, South-Eastern and Eastern Asia, and australasia: the potential for emergent viruses[END_REF]. This section focuses on some of the most common types of arboviruses found in seabirds.

Avipoxviruses

Avipoxviruses, a subgroup of poxviruses, are most notable for causing avian pox in birds around the globe [START_REF] Bolte | Avian host spectrum of avipoxviruses[END_REF]). Ectoparasites such as mosquitoes are the most common mechanical vectors for this group of viruses in wild seabirds, although a flea species (Parapsyllus longicornis) has been suggested as a possible vector as well [START_REF] Kane | Avian pox in Magellanic penguins (Spheniscus magellanicus)[END_REF].

Avian pox is a slow-developing disease, the effects of which vary by species. The most common form of avian pox affects the skin, where the development of warts around featherless regions of birds (face, legs, and feet) is a common occurrence [START_REF] Tripathy | Avipox viruses[END_REF]. These lesions often disappear with time but can leave behind scarred and damaged skin. However, in cases where these warts become enlarged and clustered, loss of sight, breathing impairment, and difficulty feeding are common side effects. It is in the case of secondary infections due to a dampened immune system, and the development of scar tissue, where avian pox can contribute to bird mortality [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF][START_REF] Kane | Avian pox in Magellanic penguins (Spheniscus magellanicus)[END_REF].

Infections are also highest during wet seasons and virus transmission is mostly hostdensity dependent [START_REF] Van Riper | Epizootiology and effect of avian pox on Hawaiian forest birds[END_REF]. This is a key factor to note in the spread of avipoxviruses among seabird populations, many of which form dense breeding colonies. Avian pox has been reported in Sphenisciformes, Anseriformes, Charadriiformes, and

Procellariiformes. A study of Magellanic Penguins (Spheniscus magellanicus) from two colonies in Argentina revealed 108 chicks with avian pox lesions; however, the prevalence of lesions was 0.002 infected chicks to every handled chick. Of the afflicted chicks, 18 were found dead, but whether the cause of death was due to avian pox or secondary infections is unknown [START_REF] Kane | Avian pox in Magellanic penguins (Spheniscus magellanicus)[END_REF]). Avian pox is also the most common viral infection in albatrosses and large petrels (Uhart et al. 2017). It has been reported from Wandering Albatross (Eudyptes chrysocome) in the Prince Edward Islands in the Indian Ocean, to Laysan Albatross (Phoebastria immutabilis) on Oahu, Hawaii [START_REF] Young | Prevalence of avian pox virus and effect on the fledging success of Laysan Albatross[END_REF], suggesting that albatross populations may be exposed to these viruses worldwide.

Flaviviruses

Flaviviruses (family: Flaviviridae) have been mostly studied for their impacts on human health, in particular, dengue fever, yellow fever, Japanese encephalitis, Zika virus, and West Nile virus [START_REF] Gaunt | Phylogenetic relationships of flaviviruses correlate with their epidemiology, disease association and biogeography[END_REF][START_REF] Jaeger | Serological evidence for the circulation of flaviviruses in seabird populations of the western Indian Ocean[END_REF]. Ticks and mosquitoes are the most common vectors for flaviviruses [START_REF] Gaunt | Phylogenetic relationships of flaviviruses correlate with their epidemiology, disease association and biogeography[END_REF][START_REF] George | The isolation of arboviruses including a new flavivirus and a new Bunyavirus from Ixodes (Ceratixodes) uriae (Ixodoidea: Ixodidae) collected at Macquarie Island, Australia, 1975-1979[END_REF].

Although and is suggested to have caused mortality in adults and nestlings [START_REF] Rocke | The impact of disease in the American white pelican in North America[END_REF]. Antibodies known to neutralize West-Nile virus were also detected in blood sera from adult Great

Frigatebirds (Fregata minor) in the Western Indian Ocean, with no apparent sign of disease [START_REF] Jaeger | Serological evidence for the circulation of flaviviruses in seabird populations of the western Indian Ocean[END_REF].

In wild seabirds, flaviviruses such as Meaban virus, Usutu virus, Saumarez Reef virus, Tyuleniy virus, and Gadget's Gully virus have been identified in species belonging to Charadriiformes and Sphenisciformes [START_REF] Gaunt | Phylogenetic relationships of flaviviruses correlate with their epidemiology, disease association and biogeography[END_REF][START_REF] George | The isolation of arboviruses including a new flavivirus and a new Bunyavirus from Ixodes (Ceratixodes) uriae (Ixodoidea: Ixodidae) collected at Macquarie Island, Australia, 1975-1979[END_REF][START_REF] Mackenzie | The zoonotic flaviviruses of Southern, South-Eastern and Eastern Asia, and australasia: the potential for emergent viruses[END_REF]Williams, 2009, Arnal et al. 2014;[START_REF] Jaeger | Serological evidence for the circulation of flaviviruses in seabird populations of the western Indian Ocean[END_REF]. Only the pathogenicity of Saumarez Reef virus has been tested in seabirds through small-scale experimental infections of Little Blue Penguins (Eudyptula minor). Wild-captured individuals inoculated with the virus succumbed to disease and mortality within 9-13 days of the infection [START_REF] Morgan | Viral infections of little blue penguins (Eudyptula minor) along the southern coast of Australia[END_REF]. Although research on factors affecting the spread of flaviviruses amongst wild seabirds is lacking, one can presume that warm weather, damp/wet habitats, and host density favour the transmission of these vector-borne viruses [START_REF] Van Riper | Epizootiology and effect of avian pox on Hawaiian forest birds[END_REF].

Other Arboviruses

There are at least 40 other types of arboviruses identified from seabird ticks alone, with their pathogenicity and spread in wild seabird populations largely unknown [START_REF] Nuttall | Tick-borne viruses in seabird colonies[END_REF]. Lab tests of the Avalon nairovirus isolated from Black-legged Kittiwakes (Rissa tridactyla) on suckling mice exhibited little to no effect [START_REF] Quillien | Avalon virus, Sakhalin group (Nairovirus, Bunyaviridae) from the seabird tick Ixodes (Ceratixodes) uriae White 1852 in France[END_REF][START_REF] Spence | Replication and polypeptide synthesis of Mill Door/79, an orbivirus isolated from ticks from a seabird colony in Scotland[END_REF]. Sakhalin virus, named after the location it was first discovered (Sakhalin Island, Sea of Okhotsk), was pathogenic in lab-tested suckling mice after intracerebral inoculation, but the effects of the virus on the host Common Murres (Uria aalge) are unidentified (L'Vov et al. 1972). Other arboviruses isolated directly from wild seabirds include Nugget virus, Tagart virus, and an unnamed species identified in Black-legged Kittiwakes in Brittany [START_REF] Chastel | Infestation and hyperinfestation of Kittiwake, Rissa tridactyla L, by ticks [Ixodes-(c)-uriae, Ornithodoros-(A)-maritimus] infected by arboviruses-pathological effects[END_REF]Doherty et al. 1972). The effects of any of these viruses on their host populations are also unknown.

Avian Influenza

The first isolation of Influenza A virus (Orthomyxoviridae) in wild birds was reported in 1961, following an epizootic among Common Terns (Sterna hirundo) in South Africa [START_REF] Rowan | Mass mortality among European common terns in South Africa in April-May 1961[END_REF]Becker, 1966). Since that date, Influenza A viruses have been detected in more than 60 seabird species worldwide, mostly in gulls and terns (see [START_REF] Arnal | Circulation of a Meaban-like virus in yellow-legged gulls and seabird ticks in the western Mediterranean Basin[END_REF][START_REF] Lang | Assessing the role of seabirds in the ecology of influenza A viruses[END_REF] for review in gulls and other seabirds, respectively). Birds of the Anseriformes and Charadriiformes orders are the main hosts for influenza A viruses [START_REF] Olsen | Global patterns of influenza A virus in wild birds[END_REF]). Significant variation in virus prevalence have been found between closely related species, but also season and bird colony, suggesting that the transmission dynamics of avian influenza are strongly associated to the ecology of their hosts. Annual epidemics have been found in Black-headed Gulls (Chroicocephalus ridibundus), with peaks of transmission during the breeding season and, in particular, fledgling birds [START_REF] Verhagen | Epidemiology of influenza A virus among black-headed gulls, the Netherlands, 2006-2010[END_REF]. This supports the idea that bird age is an important factor involved in influenza virus infection. Investigations of virus transmission on tropical oceanic islands have also shown that terns could be involved in the introduction and maintenance of avian influenza viruses in these remote locations [START_REF] Lebarbenchon | Influenza A virus on oceanic islands: host and viral diversity in seabirds in the Western Indian Ocean[END_REF]).

Among the 16 described avian influenza virus hemagglutinin subtypes, H13 and H16 are associated to seabirds (Olsen et al. 2016). Although low pathogenic virus subtypes do not induce clinical signs of disease [START_REF] Brown | Susceptibility of avian species to North American H13 low pathogenic avian influenza viruses[END_REF][START_REF] Verhagen | Long-term effect of serial infections with H13 and H16 low pathogenic avian influenza viruses in black-headed gulls[END_REF], infection with highly pathogenic avian influenza viruses (H5 and H7 subtypes) can induce severe clinical signs. These subtypes can cause cloudy eyes, ruffled feathers, weakness, a lack of coordination, torticollis, and mortality [START_REF] Brown | Susceptibility of North American ducks and gulls to H5N1 highly pathogenic avian influenza viruses[END_REF]). To date, there is only limited evidence that these poultryadapted viruses can be maintained in wild birds for long periods of time [START_REF] Krauss | The enigma of the apparent disappearance of Eurasian highly pathogenic H5 clade 2.3. 4.4 influenza A viruses in North American waterfowl[END_REF][START_REF] Lebarbenchon | Persistence of highly pathogenic avian influenza viruses in natural ecosystems[END_REF]. The increasing number of spillover events nevertheless highlights the need to further investigate the role of seabirds in the spread of both low and highly pathogenic avian influenza viruses during epizootics.

Although it is likely that other directly transmitted viruses affect seabird populations, particularly in dense breeding colonies where contact rate is very high, current knowledge on these pathogens remains very limited. In addition to avian influenza, other viruses have been documented in seabirds in the past, such as the one causing puffinosis in Manx Shearwaters (Puffinus puffinus) in Europe [START_REF] Harris | Puffinosis among manx shearwaters on Skokholm[END_REF]Macdonald et al. 1967), or coronaviruses in gulls in the Bering Strait area [START_REF] Muradrasoli | Prevalence and phylogeny of coronaviruses in wild birds from the Bering Strait area (Beringia)[END_REF]. The epidemiology, ecology and evolution of these viruses would require further investigation in order to precisely assess the physiological and behavioural effects in their natural hosts.

Bacteria and Parasites

Avian Cholera

Avian cholera is a highly contagious disease resulting from infections of the Pasteurella multicoda bacterium. DNA fingerprinting has resulted in the identification of at least 16 different serotypes of P. multicoda [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. Avian cholera has become the most prolific infectious disease of waterbirds, even though it first appeared in North America as recently as 1944. Since then, the disease has expanded globally, and outbreaks have increased in frequency since the 1970s [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. One of the key reasons avian cholera has been so successful in its spread is due to the multitude of ways P. multicoda can make its way into a host's system. Bird-to-bird contact, ingestion of carcasses or contaminated food/water, insect bites, and even the aerosolization of the bacteria in heavily contaminated regions can all lead to avian cholera in wild birds [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. Infections of P.

multicoda can be acute or chronic, but in most species studied to date, infections result in death within 24-48 hours. Mortality in 6-12 hours has also been reported in wild birds, with peak mortality exceeding more than 1,000 birds a day in some cases [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF].

Avian cholera is identified in wild seabirds almost exclusively through necropsies, and has been reported in species belonging to Anseriformes, Procellariiformes, Phaethontiformes, Sphenisciformes, Suliformes, Pelicaniformes and Charadriiformes [START_REF] Crawford | High mortality of Cape Cormorants (Phalacrocorax capensis) off western South Africa in 1991 caused by Pasteurella multocida[END_REF][START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]Leotta and Giacobani, 2006). Environmental contamination from diseased birds is a primary source of infection. Due to the dense nesting colonies formed by wild seabirds, and the highly contagious nature of the disease, avian cholera can lead to rapid colony death and even extinction. The only known way to control its spread in these populations is by removing afflicted and dead birds from the region [START_REF] Descamps | Avian cholera, a threat to the viability of an Arctic seabird colony?[END_REF][START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF].

Avian cholera has been responsible for annual colony collapses in Common Eiders in Nunavut (Figure 4), Cape Cormorants (Phalacrocorax capensis) in South Africa, Common Murres in the Baltic Sea, Yellow-Nosed and Amsterdam Albatrosses (Diomedea chlororhynchos and D.

amsterdamensis) on Amsterdam Island, and Macaroni Penguins (Eudyptes chrysolophus) in the Antarctic [START_REF] Cooper | Disease outbreaks among penguins at sub-Antarctic Marion Island: a conservation concern[END_REF][START_REF] Crawford | High mortality of Cape Cormorants (Phalacrocorax capensis) off western South Africa in 1991 caused by Pasteurella multocida[END_REF][START_REF] Descamps | Avian cholera, a threat to the viability of an Arctic seabird colony?[END_REF]Osterblom et al. 2004;[START_REF] Waller | Management of avian cholera Pasteurella multocida outbreaks on Dyer Island, South Africa, 2002-2005[END_REF][START_REF] Weimerskirch | Diseases threaten southern ocean albatrosses[END_REF]). Long term the effects of these annual outbreaks is still being studied, but at least in some cases colonies persist but at reduced numbers.

Lyme Disease

Lyme disease, caused by the bacteria of the complex Borrelia burgdorferi sensu lato is the most widespread tick-borne disease in North America and Europe. The transmission of the infectious agent is best understood in terrestrial systems involving humans, wild birds and small mammals, and Ixodes ricinus and I. scapularis ticks [START_REF] Anderson | Involvement of birds in the epidemiology of the Lyme disease agent Borrelia burgdorferi[END_REF]Kurtenback et al. 2006). Lyme disease symptoms in humans and dogs range from complex mix of neurological disorders, cardiac-associated complications, joint pain, and skin rash in humans, to lameness, vomiting, diarrhea, lack of appetite, weight loss, and inflammation in dogs [START_REF] Klempner | Two controlled trials of antibiotic treatment in patients with persistent symptoms and a history of Lyme diseaseN[END_REF][START_REF] Littman | ACVIM small animal consensus statement on Lyme disease in dogs: diagnosis, treatment, and prevention[END_REF]. Despite having now been isolated from dozens of wild terrestrial birds, the effects of B. burgdorferi in these populations is not well understood. Most research on bird-tick systems studying the disease focus on the role of wild birds as reservoirs, with the potential to spread it across large spatial scales [START_REF] Clow | Northward range expansion of Ixodes scapularis evident over a short timescale in Ontario, Canada[END_REF][START_REF] Smith | Role of bird migration in the long-distance dispersal of Ixodes dammini, the vector of Lyme disease[END_REF].

In seabirds B. burgdorferi has been isolated from I. uriae ticks in the Northern and Southern hemispheres exploiting species belonging to Charadriiformes, Procellariformes, and Sphenisciformes [START_REF] Gylfe | Isolation of Lyme disease Borrelia from puffins (Fratercula arctica) and seabird ticks (Ixodes uriae) on the Faeroe Islands[END_REF][START_REF] Olsen | Transhemispheric exchange of Lyme disease spirochetes by seabirds[END_REF][START_REF] Duneau | Prevalence and diversity of Lyme borreliosis bacteria in marine birds[END_REF]. More generally, evidence of the widespread presence of this infectious agent in seabirds is accumulating [START_REF] Gasparini | Induced maternal response to the Lyme disease spirochaete Borrelia sensu lato in a colonial seabird, the kittiwake Rissa tridactyla[END_REF][START_REF] Dietrich | Worldwide distribution and diversity of seabird ticks: implications for the ecology and epidemiology of tick-borne pathogens[END_REF][START_REF] Staszewski | Variable exposure and immunological response to Lyme disease Borrelia among North Atlantic seabird species[END_REF]. However, for now, no evidence suggests a pathogenic effect of infection for the birds; in particular, a long-term capture-mark-recapture analysis of breeding Black-legged kittiwakes found no association between exposure to B. burgdorferi s.l. and survival [START_REF] Chambert | Exposure of black-legged kittiwakes to Lyme disease spirochetes: dynamics of the immune status of adult hosts and effects on their survival[END_REF].

Avian Botulism

Bacteria belonging to the genus Clostridium are responsible for an immense amount of wild bird deaths. Clostridium botulinum is the most prevalent species of bacteria from this genus found in wild birds, causing mortality through infecting hosts with botulinum toxins.

Although there are seven types of botulinum toxins created by this bacterium -labeled A to Git is the Type C botulinum toxin that has caused the highest mortality in wild birds [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. Before avian cholera outbreaks increased in frequency in the past quarter century, avian botulism was considered the most destructive disease in wild birds [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. Botulism outbreaks can regularly result in the death of thousands of waterbirds and shorebirds in a region and up to a million birds have been lost in a single outbreak of the disease [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. Blood samples from live birds are needed to identify the toxin in populations, making the study of avian botulism in wild birds a resource dependant endeavour [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF].

Factors affecting the spread of botulism in wild birds include colony density, water-level changes, water quality, and the prevalence of C. botulinum toxins in food or water [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. Temperature also plays a critical role in the spread of botulism, with most outbreaks taking place during summer and fall [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. In wild seabirds, botulism has been isolated from species belonging to Charadriiformes, Pelecaniformes, and Anseriformes [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF][START_REF] Neimanis | An outbreak of type C botulism in herring gulls (Larus argentatus) in southeastern Sweden[END_REF][START_REF] Rocke | The impact of disease in the American white pelican in North America[END_REF]. The symptoms of the disease in wild seabirds (e.g, Herring Gulls; Larus argentatus) include decreased physical coordination, paralysis of legs and wings, dehydration, and a decrease in food consumption. Avian botulism has caused mass mortality in Herring Gulls in Sweden, and American White Pelicans in North America [START_REF] Neimanis | An outbreak of type C botulism in herring gulls (Larus argentatus) in southeastern Sweden[END_REF][START_REF] Rocke | The impact of disease in the American white pelican in North America[END_REF].

Chlamydiosis

Chlamydiosis is an infection associated with bacteria belonging to the genus Chlamydia, which typically live within animal cells. Chlamydia psittaci is the most common species associated with the disease in wild birds. Chlamydiosis can range in pathogenicity in wild birds from inapparent infections to an acute disease with high mortality, depending on the strain of C. psittaci affecting the individual/population [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. Symptoms in wild birds range from decreased food consumption, pus discharges from the eyes and nares, and bloody diarrhea, to becoming rigid and motionless [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. Chlamydia species can remain active in the tissues, feces, discharges, and even plumage of affected birds, and can be transmitted between individuals by direct contact. The only known way to control the spread of the disease in wild birds is by the removal of dead or affected birds from colonies [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF].

Although waterfowl, pigeons, herons and parrots are the most commonly infected wild birds, chlamydiosis has been reported in over 150 species belonging to 20 orders. The most common wild seabirds infected by the disease include gulls and fulmars (Charadriiformes and Procellariformes; [START_REF] Franson | Probable epizootic chlamydiosis in wild California (Larus californicus) and ring-billed (Larus delawarensis) gulls in North Dakota[END_REF]Hermann et al. 2006). Studies have also isolated C. psittaci from Red-footed (Sula sula) and Nazca Boobies (Sula granti; [START_REF] Padilla | Survey for Haemoproteus, Trichomonas gallinae, Chlamydophila psittaci, and Salmonella in Galapagos Islands columbiformes[END_REF]).

However, since chlamydiosis in wild birds is often inapparent, the effect of the disease on these populations has not been discussed in any of the studies we found. In outbreaks recorded in wild gulls (species unidentified), chlamydiosis has been fatal for adults, though fledglings reportedly died at a much higher rate [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF].

Haemosporidia

Haemosporidia, or blood parasites, are vector-borne parasites that have been wellstudied (e.g., [START_REF] Padilla | Survey for Haemoproteus, Trichomonas gallinae, Chlamydophila psittaci, and Salmonella in Galapagos Islands columbiformes[END_REF][START_REF] Valkiunas | Avian malaria parasites and other haemosporidia[END_REF][START_REF] Vanstreels | Do blood parasites infect Magellanic penguins (Spheniscus magellanicus) in the wild? Prospective investigation and climatogeographic considerations[END_REF]. They are transmitted to wild birds primarily by midges (Culicoides spp.), mosquitoes (Culex and Aedes spp.) and flies (Simulium and Hippoboscidae spp.; [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF][START_REF] Vanstreels | Do blood parasites infect Magellanic penguins (Spheniscus magellanicus) in the wild? Prospective investigation and climatogeographic considerations[END_REF]). There are currently four primary types of haemosporidia identified in seabirds, each carried by a different type of vector. Plasmodium species are carried by mosquitoes, and are associated with diseases such as malaria in humans [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF][START_REF] Vanstreels | Do blood parasites infect Magellanic penguins (Spheniscus magellanicus) in the wild? Prospective investigation and climatogeographic considerations[END_REF]. Haemoproteus species are carried by midges, and Leucocytozoon species are carried by flies [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. It is unconfirmed what vectors carry Babesia spp., but it is assumed to be ticks (Earle et al. 1992;[START_REF] Yabsley | Description of novel Babesia species and associated lesions from common murres (Uria aalge) from California[END_REF]. Although many groups of birds are known hosts of various haemosporidian parasites, often with high prevalence in populations (e.g., passerines, waterfowl), in detailed surveys of North America birds, Greiner et al. (1962[START_REF] Greiner | Distribution of the avian hematozoa of North America[END_REF] considered seabirds to be "nearly hematozoan-free".

Haemosporidia have been found in wild seabirds belonging to the orders Charadriiformes, Suliformes, Sphenisciformes, and Procellariformes, and in some cases certain parasites (e.g., Plasmodium relictum) have been associated with high mortality (e.g., wild penguins; [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF]. However, in the majority of studies, there is little evidence of mortality of wild seabirds caused by haemosporidia [START_REF] Padilla | Survey for Haemoproteus, Trichomonas gallinae, Chlamydophila psittaci, and Salmonella in Galapagos Islands columbiformes[END_REF][START_REF] Parsons | Occurrence of blood parasites in seabirds admitted for rehabilitation in the Western Cape, South Africa, 2001-2013[END_REF][START_REF] Peirce | Babesia ugwidiensis, a new species of avian piroplasm from Phalacrocoracidae in South Africa[END_REF][START_REF] Yabsley | Description of novel Babesia species and associated lesions from common murres (Uria aalge) from California[END_REF]). Temperature, migration, and local climatic conditions supporting ectoparasites are key considerations in determining the spread of haemosporidia amongst wild seabirds [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF][START_REF] Van Riper | Epizootiology and effect of avian pox on Hawaiian forest birds[END_REF], and help explain regional prevalence levels (e.g., Grenier et al. 1975;[START_REF] Bennett | Scarcity of haematozoa in birds breeding on the arctic tundra of North America[END_REF]. For example, [START_REF] Vanstreels | Do blood parasites infect Magellanic penguins (Spheniscus magellanicus) in the wild? Prospective investigation and climatogeographic considerations[END_REF] found no evidence of haemosporidia in Magellanic Penguins in the Sub-Antarctic region of Argentina, and [START_REF] Mallory | Breeding status, contaminant burden and endoparasites of northern fulmars (Fulmarus glacialis) from the Canadian high arctic[END_REF] also found no haemosporidia in Norhern Fulmars from the high Arctic. It is also thought that many haemosporidia are host species specific. For example, Babesia shortii is the only Babesia spp. known to cause pathogenicity in birds (Peirce, 2000). However, it is best studied in Falconidae, and experimental transmissions of the parasite into other terrestrial species have failed in producing pathogenic responses (Peirce, 2000).

Discussion

While our review focused on eight orders of seabirds (Procellariformes, Phaethontiformes, Suliformes, Charadriiformes, Sphenisciformes, Pelicaniformes, and select Gaviiformes, and Anseriformes), we found that species belonging to the order Charadriiformes were the most represented in the parasite and pathogen literature (Table 2). Although

Anseriformes have been studied more extensively in terms of parasites, our focus on parasites in seaducks resulted in far fewer study results. The research done in the eight orders is generally representative of the number of species within each order; Charadriiformes and Procellariformes have the most numerous species and resulted in the more parasite studies found in the literature, while Phaethontiformes and Gaviiformes have the fewest species, and we found the fewest parasite studies (Table 2).

The focus on parasites and pathogens in colonial nesting seabirds, such as those belonging to Charadriiformes and Procellariformes, may be because they are easier to study due to their high population densities during the breeding season. This colonial nature lends these species to parasite related studies as larger sample sizes can be accessed relatively easily.

On the other hand, species belonging to Gaviiformes are found in lower densities on the landscape, making population-wide analyses more resource-and time-intensive. Important to note is that the colonial nature of some species of birds may also make them more vulnerable to higher levels of parasites and pathogens as compared with birds that breed in isolation because of the contact between individuals at bird colonies (e.g. [START_REF] Boulinier | Dispersal and parasitism[END_REF][START_REF] Boulinier | Migration, prospecting, dispersal? What host movement matters for infectious agent circulation?[END_REF][START_REF] Mccoy | The role of seabirds of the Iles Eparses as reservoirs and disseminators of parasites and pathogens[END_REF]. For example, in some cliff-nesting seabird species where there is little to no vegetation cover individuals may literally be covered in the faeces of those nesting above them (Figure 5); these faeces are a potential vector for a number of parasites.

The impact of ectoparasites on seabirds are studied through two main metrics; the impact on seabird populations through direct parasitism, and the potential for seabird ectoparasites to act as disease vectors. The impacts of direct parasitism on wild seabirds can dependent on the age, sex, and body condition of the host (e.g., [START_REF] Granroth-Wilding | Parasitism in early life: environmental conditions shape within-brood variation in responses to infection[END_REF][START_REF] Provencher | Direct and indirect causes of sex differences in mercury concentrations and parasitic infections in a marine bird[END_REF]. Although adult mortality can be caused by ectoparasites (e.g., [START_REF] Gaston | Effects of climate-induced changes in parasitism, predation and predator-predator interactions on reproduction and survival of an Arctic marine bird[END_REF], this occurs most often in cases of hyper-infestations. The most commonly reported impact of ectoparasites on adult seabirds is that of reduced parental care due to parasitism (e.g., [START_REF] Duffy | The ecology of tick parasitism on densely nesting Peruvian seabirds[END_REF][START_REF] Dietrich | Worldwide distribution and diversity of seabird ticks: implications for the ecology and epidemiology of tick-borne pathogens[END_REF]. Nest abandonment and inconsistent/slower offspring development are the most well-known outcomes. Mortality in fledglings due to parasitism is much more likely, and has been reported in species belonging to six of the eight orders studied here (Table 2).

The effect of endoparasites on seabirds is much more diverse (Table 2). Flaviviruses, Chlamydophillia spp., haemosporidia and Borrelia burgdorferi s.l. seem to be asymptomatic in seabirds, which makes understanding their relationships with their hosts difficult. In contrast, highly pathogenic avian influenza and avian cholera can cause mass mortality and colony collapses (e.g., [START_REF] Allison | Cyclic avian mass mortality in the northeastern United States is associated with a novel orthomyxovirus[END_REF][START_REF] Descamps | Avian cholera, a threat to the viability of an Arctic seabird colony?[END_REF]. Except for these exceptional cases when such massive die-offs are reported, the physiological and behavioral effects of viral and bacterial infections in seabirds remain largely unknown, at both the individual and population levels. This limitation is mostly associated with the inherent properties of viruses and other microparasites. In particular, field investigation of infectious status and prevalence in populations usually requires strict conservation procedures of biological material. Laboratorybased techniques (e.g. molecular biology, virus isolation) in sometimes high security laboratories such as for highly pathogenic avian influenza and West-Nile viruses is needed, and certain biological safety precautions are required by law in some regions (e.g., [START_REF] Allison | Cyclic avian mass mortality in the northeastern United States is associated with a novel orthomyxovirus[END_REF][START_REF] Lebarbenchon | Persistence of highly pathogenic avian influenza viruses in natural ecosystems[END_REF][START_REF] Lebarbenchon | Influenza A virus on oceanic islands: host and viral diversity in seabirds in the Western Indian Ocean[END_REF].

Effects of certain parasites and pathogens may also be subtle and difficult to measure with respect to the impact of other environmental factors, and choosing the appropriate traits to measure requires clear information on the biological system (Sanchez et al. 2018). For example, the effects of a parasite may only be detectable during times of physiological stress (e.g. breeding or migration), but not during other parts of the annual cycle [START_REF] Provencher | Antiparasite treatment, but not mercury burdens, influence nesting propensity dependent on arrival time or body condition in a marine bird[END_REF]. Identifying the links between such basic epidemiological information and host physiological and behavioural traits, in order to assess the precise effects of infection on host fitness, therefore remains very challenging. Experimental approaches in the field can also be somewhat limited; ethics and biosafety considerations naturally preclude the manipulation of the infectious status of natural migratory hosts with parasites (although this may be easier for helminths than viruses and bacteria).

Understanding the interactions between different parasite types affecting seabirds is also lacking, but has been shown to be essential in other animal groups (e.g., [START_REF] Graham | Ecological rules governing helminth-microparasite coinfection[END_REF].

Additional observational and experimental studies from a larger range of seabirds aimed at specific parasitic groups would greatly enhance our understanding of these interactions. New methods for investigating the diversity of endoparasites using metabarcoding analyses of faeces may facilitate our understanding of the diversity and distribution of these organisms at different spatial scales (e.g., [START_REF] Mcinnes | Optimised scat collection protocols for dietary DNA metabarcoding in vertebrates[END_REF], and may enable us to incorporate parasite dynamics into long-term population survey data. Although there are limitations to metabarcoding approaches to studying effects of parasites as metrics beyond presence and absence will be difficult to quantify. Control measures for the spread of parasites are also rudimentary and resource/time intensive. One way to control their spread in seabirds is by the selective removal of afflicted or dead birds from colonies (e.g., [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF].

Vaccination has also been recently proposed as an effective measure to protect endemic albatrosses from avian cholera [START_REF] Bourret | Vaccination protects endangered albatross chicks against avian cholera[END_REF], and may be an important conservation consideration for seabirds of particular conservation concern, but is not likely a viable solution for more metropolitan species.

Due to the biological connection between many endo-and ectoparasites, it is important to note that the factors influencing their spread are intrinsically connected. Research shows that abiotic conditions such as heat and humidity, and biotic conditions such as host body condition and colony density, are key factors determining in the spread of parasites within seabird colonies (Caillouet et al. 2013;[START_REF] Monticelli | Juvenile survival in a tropical population of roseate terns: interannual variation and effect of tick parasitism[END_REF][START_REF] Proctor | Feather mites (Acari: Astigmata): ecology, behavior, and evolution[END_REF][START_REF] Ramos | Activity patterns and effect of ticks on growth and survival of tropical Roseate Tern nestlings[END_REF].

Discussing the spread of these parasites on a global scale is more complex, however. Factors such as migration, climate change, and range extension/change for seabirds and parasites alike need to be taken into consideration. Seabirds are often long-lived and highly migratory, so it should come as no surprise that they can act as effective parasite reservoirs and transporters.

There is already evidence showcasing range and species expansions for parasites, although the exact factors involved are as yet unknown (de Souza Peterson et al. 2017;[START_REF] Kane | Avian pox in Magellanic penguins (Spheniscus magellanicus)[END_REF].

Changing seasonal temperatures and water levels due to climate change can also extend the time period that seabirds can be parasitized by ectoparasites, potentially resulting in increased local abundances and subsequently higher chances of pathogenicity. Increasing anthropogenic pollution and higher water levels due to climate change can further contribute to the spread of devastating diseases such as botulism and cholera [START_REF] Friend | Field manual of wildlife diseases. General field procedures and diseases of birds[END_REF][START_REF] Garamszegi | Climate change increases the risk of malaria in birds[END_REF][START_REF] Parkinson | Potential impacts of climate change on infectious diseases in the Arctic[END_REF]. The interaction between different environmental stressors and parasitism is only recently starting to be considered for understanding the overall impact on seabird populations.

There is clearly a bias toward research interest in seabird parasites that have the capacity to parasitize or infect human beings (i.e., zoonoses). Evidence of this comes from the comparatively extensive research on West Nile virus and Lyme disease, which can be transmitted to humans through vectors such as ticks and mosquitoes. Although direct transmission of pathogens from birds to humans is considered uncommon, little work has been done to assess seabird pathogens in humans and thus this relationship is likely underestimated.

Vectors carried by migratory birds can play a role in the transmission of at least 10 different human pathogens [START_REF] Tsiodras | Human infections associated with wild birds[END_REF] Beyond their zoonotic implications, the impact of parasites on seabirds and human beings have economic and cultural implications. Commonly harvested birds such as Common Eiders and murres have experienced pathogenicity, mortality, and even annual colony collapse due to increased parasitism [START_REF] Gaston | Heat and mosquitoes cause breeding failures and adult mortality in an Arctic-nesting seabird[END_REF][START_REF] Gaston | Effects of climate-induced changes in parasitism, predation and predator-predator interactions on reproduction and survival of an Arctic marine bird[END_REF]Kristjansson et al. 2016;[START_REF] Lovvorn | Modeling marine protected areas for threatened eiders in a climatically changing Bering Sea[END_REF]. Anthropogenic activities such as overfishing and oceanic pollution are also evolving factors contributing to reduced health in seabirds [START_REF] Braune | Elevated mercury levels in a declining population of ivory gulls in the Canadian Arctic[END_REF][START_REF] Sydeman | Best practices for assessing forage fish fisheries-seabird resource competition[END_REF].

The long-term impacts of these changes can contribute to increased, additive mortality caused by parasites in seabird colonies. Changing climatic conditions and food availability have also resulted in seabirds being preyed upon by novel predators (Iverson et al. 2014;[START_REF] Smith | Has early ice clearance increased predation on breeding birds by polar bears?[END_REF]). More research on cumulative effects from parasitism, reduced food availability due to anthropogenic activity, and changing predatory interactions is needed to understand the collective impact these factors can have on parasite-host relationships in seabird colonies [START_REF] Dey | Forecasting the outcome of multiple effects of climate change on northern common eiders[END_REF]Iverson et al. 2014;[START_REF] Ramos | Productivity of white-tailed tropicbird on Aride Island, Seychelles[END_REF]. Understanding these interrelations becomes even more crucial when we consider endemic or naïve species of seabirds that may be experiencing increases in multiple stressors due to changing conditions in their habitats. For example, increasing numbers of non-native species in Hawaii have introduced parasites that have led to mass mortality in native songbirds [START_REF] Van Riper | Epizootiology and effect of avian pox on Hawaiian forest birds[END_REF]. Expanding ranges for parasites and disease vectors have raised similar concerns in Antarctic seabird populations (Grimaldi et al. 2015).

It is clear that the colonial nature of seabirds makes them effective reservoirs and hosts for parasites. Not all seabirds come in direct contact with each other in these colonies, however, because of strong territoriality. More research is needed to understand the impact of localized separation in colonial nesting seabirds on the spread of parasites. Colonial nesting might indeed yield some benefits to hosts against certain parasites. For example, [START_REF] Douglas | Chemical odorant of colonial seabird repels mosquitoes[END_REF] showed evidence of aldehydes created by Crested Auklets (Aethia cristatella) can effectively repel ectoparasites. There are also studies to suggest that certain species of wild birds can slow the spread of pathogens like West Nile virus [START_REF] Levine | Supersuppression: Reservoir competency and timing of mosquito host shifts combine to reduce spillover of West Nile virus[END_REF]. The role of seabird diversity and abundance in suppressing zoonotic disease is a worthwhile avenue for further study.

Concurrently, most ecosystems are warming under climate change [START_REF] Walther | Ecological responses to recent climate change[END_REF], and evidence suggests that increased parasitism is linked with warmer weather [START_REF] Bellard | Impacts of climate change on the future of biodiversity[END_REF]Kutz et al. 2008). Therefore, there is a need to understand current level of parasites in seabirds to understand changing patterns and trends in the future. In particular, we stress that studies examining parasitism in bird populations should publish results, regardless of the outcome (e.g., no infection found in seabirds). These baseline studies are essential for documenting the effect of shifting environmental conditions on bird-parasite relationships and teasing apart the questions 'is this new?' or 'new to us?'; a critical question to understanding how environmental changes will impact biota.

  , but a much better understanding of this phenomenon is still required. Recent work by Fossadal et al. (2018) has shown a positive association between handling C. psittaci infected juvenile Northern Fulmars -a species hunted for human consumption -and human psittacosis disease in the Fulmar Islands. It is possible that other commonly harvested species, such as eiders and murres may present similar correlations in future works. The aforementioned control measures for diseases such as cholera and influenza also expose human beings to these parasites. With the increased risk of outbreaks in seabirds due to climate change and range expansions, pathogen spillover events will likely become more frequent.

Figure 1 :Figure 2 :Figure 4 :GFigure 5 :

 1245 FIGURES

  found that Kerguelen Shags (Phalacrocorax verrucosus) nesting approximately 25 km from each other had significantly different prevalence and infection loads of the nematode Contracaecum rudolphii. The authors attributed this result to differences in diet between the two colonies, highlighting how local diet can be a significant factor for parasitism in seabirds. Sex bias was also identified by[START_REF] Robinson | Helminth pathogen cathepsin proteases: it'sa family affair[END_REF] in Double-crested Cormorants in Canada, where males had significantly higher levels of the nematode Contracaecum spp. when compared with females. Experimental removals of gastrointestinal nematode parasites was found to alter resource allocation in European Shags in relation to local breeding conditions[START_REF] Granroth-Wilding | Indirect effects of parasitism: costs of infection to other individuals can be greater than direct costs borne by the host[END_REF], 2014; Reed et al. 2008),

highlighting the importance of the interaction between parasites and local environmental factors in determining parasite impacts

[START_REF] Granroth-Wilding | Parasitism in early life: environmental conditions shape within-brood variation in responses to infection[END_REF]

).

  West Nile virus has been well studied in some bird populations, it is important to note that the most common flaviviruses isolated from seabirds do not include West Nile

virus. Most research on West Nile virus in birds has focused on relatively common terrestrial species such as American Crows (Corvus brachyrhynchos), Common Grackles (Quiscalus quiscula), Blue Jays (Cyanocitta cristata), and House Sparrows (Passer domesticus;

Rappole and Hubalek, 2008;[START_REF] Ladeau | West Nile virus emergence and largescale declines of North American bird populations[END_REF]

. In wild seabird populations, West Nile virus has been identified in post-mortem analyses of American White Pelicans (Pelicanus erythrorhynchos),

Acknowledgements

We thank the many researchers and teams that have and continue to explore and study parasties and seabirds globally. Thanks Grant Gilchrist and Thierry Boulinier for allowing us to use their photographs. Thanks to ** anonymous reviewers for their constructive feedback.