Plastic debris is ubiquitous in the marine environment, and seabirds are among the most impacted marine vertebrates. Plastic ingestion was investigated in an internationally-important breeding population of Manx Shearwaters Puffinus puffinus on Skomer Island, Wales in 2018–19. Opportunistic necropsies were carried out on a total of 34 birds to collect contents of the gastrointestinal tract: 13 adults from April to July 2019 and 12 fledglings during September 2019, and a further nine opportunistic necropsies carried out from adults from July to September 2018. The presence, quantity, size and colour of plastic in the gastrointestinal tract was investigated. Plastic was found in 71% of Manx Shearwater stomachs (68% of adults, 75% of fledglings). Adults were found to have larger plastic pieces in their gastrointestinal tract than fledglings, and lighter birds had larger pieces of plastic in their tracts than heavier birds. This study shows that Manx Shearwaters on Skomer Island are vulnerable to plastic ingestion, and that adults are likely to pass plastic to their chicks.

Approximately 20 million tonnes of plastics enter aquatic ecosystems annually, with plastics continuing to accumulate as inputs exceed attempts at mitigation (Borrelle et al. 2020). The impacts of plastic pollution have been well- documented, affecting marine fauna via both entanglement and ingestion (Wilcox et al. 2015; O’Hanlon et al. 2017). Amongst marine vertebrates, seabirds are most affected by plastic (Lavers et al. 2014), being known to forage in convergence zones (Gould et al. 1997; Pichel et al. 2007) with high concentrations of ocean plastic (Wilcox et al. 2015). Therefore, plastic pollution in the marine environment may be contributing to the global decline of seabirds, a decline which is occurring more quickly than in any other bird group (Lavers et al. 2014).

Plastic ingestion in seabirds was first noted in the 1960s, in the Laysan Albatross Phoebastria immutabilis (Moser & Lee 1992). In the 1970s, Northern Fulmars Fulmarus glacialis in the North Sea were found to have 1–2 plastic pieces per stomach, but by the 1980s this had risen to an average of 10 pieces per stomach (van Franeker, 1985; van Franeker & Law 2015). Since then, it has been reported that more than 100 different seabird species have ingested plastic (Provencher et al. 2018; Kühn & van Franeker 2020), exceeding 90% of individuals in some studies (van Fraeneker et al. 2011; Codina-García et al. 2013; Roman et al. 2016). By 2050 it is predicted that 99% of seabirds will ingest plastic regularly (Wilcox et al. 2015; Provencher et al. 2018). Entanglement in plastic litter is also a threat, with 36% of seabird species known to have been affected in this way. Furthermore, of the 265 bird species entangled globally, 83% were caught up in fishing gear (Ryan 2018). Additionally, many seabirds use plastic as nesting material, including Double-crested Cormorants Phalacrocorax auritus (Podolsky & Kress 1989) and Northern Gannets Morus bassanus (Montevecchi 1991; Votier et al. 2011; Rodríguez et al. 2013), increasing the risk of plastic entanglement and ingestion for both breeding adults and their chicks (Podolsky & Kress 1989).

Seabirds may ingest plastic either directly, often mistaking plastic for potential prey items (Azzarello & van Vleet 1987; Moser & Lee 1992; Shaw & Day 1994; Derraik 2002; van Franeker et al. 2011), or indirectly, via lower trophic levels, including filter-feeding organisms and other prey species (Graham & Thompson 2009; van Franeker et al. 2011; Ryan 2015; O’Hanlon et al. 2017). Plastic ingestion may increase mortality rates in seabirds through a number of mechanisms: shredding the stomach lining, causing blockages that lead to starvation, taking up stomach space that results in poor energy return (Petry et al. 2008), suppressing appetite, or decreasing fat deposition (Auman et al. 1997). Additionally, chemicals, such as alkylphenols, phthalates and organophosphates, are used in the production of plastic (Lahens et al. 2018; Rhodes 2018) and, once ingested, can leach into an organism’s tissues and potentially disrupt endocrine systems (Oehlmann et al. 2009), with adverse effects on development and reproduction (Oehlmann et al. 2009; Lusher et al. 2015). In water, plastic can also absorb persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs), persistent bioaccumu- lative and toxic substances (PBTs) and trace metals, posing additional threats if they are ingested by organisms (Rochman et al. 2013; Lahens et al. 2018).

Although plastic is not biodegradable, wave action and ultraviolet light causes macroplastics to fragment, producing microplastics (ter Halle et al. 2016; Weinstein et al. 2016; Ostle et al. 2019). This is problematic as microplastics are harder to remove from the environment and are more easily ingested than larger plastic items (Auta et al. 2017).

Olfactory foraging, using dimethyl sulphide (DMS) as a cue to find productive areas of ocean and thus prey, can lead seabirds to confuse plastic for food items, because plastics in the marine environment also emit a DMS signature (Savoca et al. 2016). The surface, shallow-diving foraging habit of Procellariiformes compounds this threat by bringing birds into contact with buoyant plastics at the surface (e.g. Hammer et al. 2016). Furthermore, Procellariiformes have a constriction in the digestive system between the gizzard and proventriculus, along with a small gizzard, making it difficult for ingested plastic pieces to be regurgitated (Azzarello & van Vleet 1987); for example, ingested balloons present a particularly high risk of mortality to seabirds (Roman et al. 2019a). Indeed, Manx Shearwaters Puffinus puffinus (one of the most widely studied Procellariiformes) are known to ingest plastics: a study in Brazil found that 83% of the items found within Manx Shearwater stomachs (N = 25) were plastic (Colabuono et al. 2009). Interestingly, Manx Shearwater fledglings may have an increased risk of plastic ingestion through intergenerational transfer, as adults regurgitate plastic during chick feeding, whereas chicks regurgitate less frequently, causing plastic to accumulate more within their bodies (Ryan 1988; Carey 2011). This accumulation can lead to increased pollutant concentrations in tissues, consequently reducing fledgling body condition (Day et al. 1985; Sileo et al. 1990; van Franeker & Meijboom 2002; Hutton et al. 2008; Colabuono et al. 2009; Carey 2011; Acampora et al. 2016; Lavers & Bond, 2016).

In this study, the presence, quantity, colour and size of plastic found in the gastroin- testinal tract of Manx Shearwaters was investigated to discern: 1) differences in plastic loads between adult and fledgling birds, 2) associations between body weight and plastic ingestion, and 3) patterns of ingested plastic colour, as previous studies have found preferential selection due to plastics being mistaken for prey (e.g. Azzarello & van Vleet 1987; Verlis et al. 2013). This study is the first to investigate plastic ingestion in Manx Shearwaters on their breeding grounds in the United Kingdom, a population in the northeastern Atlantic; an area poorly studied in terms of plastic pollution (O’Hanlon et al. 2017).

This study was conducted with the permission of the Wildlife Trust for South and West Wales, with approval from the Skokholm and Skomer Islands Conservation Advisory Committee.We thank the wardens for logistical support and advice during the field study and for arranging laboratory facilities on Skomer Island, volunteers for collecting carcasses, and Dale Sailing for boat transfers.

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