Seabird Group Seabird Group

Plastic ingestion in adult and fledgling Manx Shearwaters Puffinus puffinus on Skomer Island, Wales

Clare L. Alley1, Shanice Arkless2, Ellie Ames3, Mark Abrahams4 ORCID logo, Louise K. Gentle1 ORCID logo and Matt J. Wood5* ORCID logo

1 School of Animal, Rural and Environmental Sciences, Nottingham Trent University, Southwell, Nottinghamshire, NG25 0QF, UK;

2Department of Applied Sciences, University of the West of England, Bristol, BS16 1QY, UK;

3Skomer Island,The Wildlife Trust of South and West Wales, Martinshaven, Haverfordwest, SA62 3BJ, UK;

4Field Conservation and Science Department, Bristol Zoological Society, Bristol, BS8 3HA, UK;

5School of Natural and Social Sciences, University of Gloucestershire, Cheltenham, GL50 4AZ, UK.

Full paper


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.


Acampora, H., Lyashevska, O., van Franeker, J. A. & O’Connor, I. 2016. The use of beached bird surveys for marine plastic litter monitoring in Ireland. Marine Environmental Research 120: 122–29. [Crossref]

Andrady, A. L. 2000. Plastics and their impacts in the marine environment. In: Shomura, R.S. & Godfrey, M.L. (eds.), International Marine Debris Conference on Derelict Fishing Gear and the Ocean Environment: 848–869.

Auman, H. J., Ludwig, J. P., Giesy, J. P. & Colburn, T. 1997. Plastic ingestion by Laysan Albatross chicks on Sand Island, Midway Atoll, in 1994 and 1995. In: Robinson, G., Gales, R. (eds.), Albatross Biology and Conservation: 239–244, Surrey Beatty & Sons, Chipping Norton, New South Wales.

Auta, H. S., Emenike, C. U. & Fauziah, S.H. 2017. Distribution and importance of microplastics in the marine environment: A review of the sources, fate, effects, and potential solutions. Environment International 102: 165–176. [Crossref]

Azzarello, M. Y. & van Vleet, E. S. 1987. Marine birds and plastic pollution. Marine Ecology 37: 295–303. [Crossref]

Barnes, D. K. A., Galgani, F., Thompson, R. C. & Barlaz, M. 2009. Accumulation and fragmentation of plastic debris in global environments. Philosophical Transactions of the Royal Society B: Biological Sciences 364: 1985–1998. [Crossref]

Bond, A. L., Provencher, J. F., Daoust, P. Y. & Lucas, Z. N. 2014. Plastic ingestion by fulmars and shearwaters at Sable Island, Nova Scotia, Canada. Marine Pollution Bulletin 87: 68–75. [Crossref]

Borrelle, S. B., Ringma, J., Law, K. L., Monnahan, C. C., Lebreton, L., McGivern, A., Murphy, E., Jambeck, J., Leonard, G. H., Hilleary, M. A. & Eriksen, M. 2020. Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution. Science 369: 1515–1518. [Crossref]

Carey, M. J. 2011. Intergenerational transfer of plastic debris by Short-tailed Shearwaters (Ardenna tenuirostris). Emu-Austral Ornithology 111: 229–234. [Crossref]

Codina-García, M., Militão, T., Moreno, J. & González-Solís. 2013. Plastic debris in Mediterranean seabirds. Marine Pollution Bulletin 77: 220–226. [Crossref]

Colabuono, F. I., Barquette, V., Domingues, B. S. & Montone, R. C. 2009. Plastic ingestion by Procellariiformes in southern Brazil. Marine Pollution Bulletin 58: 93–96. [Crossref]

Day, R. H., Wehle, D. H. S. & Coleman, F. C. 1985. Ingestion of plastic pollutants by marine birds. In: Shomura, R.S. & Yoshida, H.O. (eds.) Proceedings of the Workshop on the Fate and Impact of Marine Debris, 26–29 November 1984. NOAA Technical Memorandum, Honolulu, Hawaii, USA: 344–386.

Derraik, J. G. B. 2002. The pollution of the marine environment by plastic debris: a review. Marine Pollution Bulletin 44: 842–852. [Crossref]

Gould, P., Walker, W. & Ostrom, P. 1997. Foods of Northern Fulmars Associated with High-aSeas Drift Nets in the Transitional Region of the North Pacific. Northwest Nature 78: 57–61. [Crossref]

Graham, E.R. & Thompson, J. T. 2009. Deposit- and suspension-feeding sea cucumbers (Echinodermata) ingest plastic fragments. Journal of Experimental Marine Biology and Ecology 368: 22–29. [Crossref]

Guilford, T. G., Meade, J., Willis, J., Phillips, R. A., Boyle, D., Roberts, S., Collett, M., Freeman, R. & Perrins, C. M. 2009. Migration and stopover in a small pelagic seabird, the Manx shearwater Puffinus puffinus: insights from machine learning. Proceedings of the Royal Society: Biological Sciences 276: 1215–1223. [Crossref]

Hammer, S., Nager, R. G., Johnson, P. C. D., Furness, R. W. & Provencher, J. F. 2016. Plastic debris in great skua (Stercorarius skua) pellets corresponds to seabird prey species. Marine Pollution Bulletin 103: 206–210. [Crossref]

Hutton, I., Carlile, N. & Priddel, D. 2008. Plastic ingestion by Flesh-footed Shearwaters, Puffinus carneipes, and Wedge-tailed Shearwaters, Puffinus pacificus. Papers and Proceedings of the Royal Society of Tasmania 142: 67–72. [Crossref]

Kühn, S. & van Franeker, J. A. 2020. Quantitative overview of marine debris ingested by marine megafauna. Marine Pollution Bulletin 151: 110858. [Crossref]

Lahens, L., Strady, E., Kieu-Le, T-C., Dris, R., Boukerma, K., Rinnert, E., Gasperi, J. & Tassin, B. 2018. Macroplastic and microplastic contamination assessment of a tropical river (Saigon River, Vietnam) transversed by a developing megacity. Environmental Pollution 236: 661–671. [Crossref]

Lavers, J. L. & Bond, A. L. 2016. Ingested plastic as a route for trace metals in Laysan Albatross (Phoebastria immutabilis) and Bonin Petrel (Pterodroma hypoleuca) from Midway Atoll. Marine Pollution Bulletin 110: 493–500. [Crossref]

Lavers, J. L., Bond, A. L. & Hutton, I. 2014. Plastic ingestion by the Flesh-footed Shearwaters (Puffinus carneipes): Implications for fledgling body condition and the accumulation of plastic-derived chemicals. Environmental Pollution 187: 124–129. [Crossref]

Lusher, A. L., Hernandez-Milian, G., O’Brein, J., Berrow, S., O’Connor, I. & Officer, R. 2015. Microplastic and macroplastic ingestion by deep diving oceanic cetacean: The True’s beaked whale Mesoplodon mirus. Environmental Pollution 199: 185–191. [Crossref]

Montevecchi, W. A. 1991. Incidence and types of plastic in gannets’ nests in the northwest Atlantic. Canadian Journal of Zoology 69: 295–297. [Crossref]

Moser, M. L. & Lee, D. S. 1992. A fourteen-year survey of plastic ingestion by Western North Atlantic seabirds. Colonial Waterbirds 15: 83–94. [Crossref]

Norris, C. 2019. Ingestion of marine debris by Manx shearwater (Puffinus puffinus) fledglings on Skúvoy Island, Faroe Islands, during 2003–2018. Unpublished MSc Thesis,University of York.

Nuelle, M.-T., Dekiff, J. H., Remy, D. & Fries, E. 2014. A new analytical approach for monitoring microplastics in marine sediments. Environmental Pollution 184: 161–169. [Crossref]

Oehlmann, J., Schulte-Oehlmann, U., Kloas, W., Jagnytsch, O., Lutz, I., Kusk, K. O., Wollenberger, L., Santos, E. M., Paull, G.C., van Look, K. J. W. & Tyler, C. R. 2009. A critical analysis of the biological impacts of plasticizers on wildlife. Philosophical Transactions of the Royal Society B: Biological Sciences 364: 2047–2062. [Crossref]

O’Hanlon, N. J., James, N. A., Masden, E. A. & Bond, A. L. 2017. Seabirds and marine plastic debris in the northeastern Atlantic: A synthesis and recommendations for monitoring and research. Environmental Pollution 231: 1291–1301. [Crossref]

Ostle, C., Thompson, R. C., Broughton, D., Gregory, L., Wootton, M. & Johns, D. G. 2019. The rise in ocean plastics evidenced from a 60-year time series. Nature Communications 10: 1–6. [Crossref]

Perrins, C. M., Wood, M. J., Garroway, C. J., Boyle, D., Oakes, N., Revera, R., Collins, P. & Taylor, C. 2012. A whole-island census of the Manx Shearwaters Puffinus puffinus breeding on Skomer Island in 2011. Seabird 25: 1–13. [Crossref]

Perrins, C. M., Padget, O., O’Connell, M., Brown, R., Büche, B., Eagle, G., Roden, J., Stubbings, E. & Wood, M. J. 2019. A census of breeding Manx Shearwaters Puffinus puffinus on the Pembrokeshire Islands of Skomer, Skokholm and Midland in 2018. Seabird 32: 106–118. [Crossref]

Petry, M. V., da Silva Fonseca, V. S., Krüger-Garcia, L., da Cruz Piuco, R. & Brummelhaus, J. 2008.Shearwater diet during migration along the coast of Rio Grande do Sol, Brazil. Marine Biology 154: 613–621. [Crossref]

Pichel, W. G., Churnside, J. H., Veenstra, T. S., Foley, D. G., Friedman, K. S., Brainard, R. E., Nicoll, J. B., Zheng, Q. & Clemente-Colon, P. 2007. Marine debris collects within the North Pacific Subtropical Convergence Zone. Marine Pollution Bulletin 54: 1207–1211. [Crossref]

Pierce, K. E., Harris, R. J., Larned, L. S. & Pokras, M. A. 2004. Obstruction and starvation associated with plastic ingestion in a Northern Gannet Morus bassanus and a Greater Shearwater Puffinus gravis. Marine Ornithology 32: 187–189.

Podolsky, R. G. & Kress, S. W. 1989. Plastic Debris Incorporated into Double-Crested Cormorant Nests in the Gulf of Maine. Journal of Field Ornithology 60: 248–250.

Provencher, J. F., Bond, A. L., Avery-Gomm, S., Borrelle, S. B., Rebolledo, E. L. B., Hammer, S., Kühn, S., Lavers, J. L., Mallory, M. L., Trevail, A. & van Franeker, J. A. 2017. Quantifying ingested debris in marine megafauna: a review and recommendations for standardization. Analytical Methods 9: 1454–1469. [Crossref]

Provencher, J. F., Bond, A. L., Hedd, A., Montevecchi, W. A., Muzaffar, S. B., Courchesne, S. J., Gilchrist, H. G., Jamieson, S. E., Merkel, F. R., Falk, K. & Durinck, J. 2014. Prevalence of marine debris in marine birds from the North Atlantic. Marine Pollution Bulletin 84: 411–417. [Crossref]

Provencher, J. F., Bond, A. L. & Mallory, M. L. 2015. Marine birds and plastic debris in Canada: a national synthesis and a way forward. Environmental Review 23: 1–13. [Crossref]

Provencher, J. F., Borrelle, S. B., Bond, A. L., Lavers, J. L., van Franeker, J., Kühn, S., Hammer, S., Avery-Gomm, S. & Mallory, M. L. 2019. Recommended best practices for plastic and litter ingestion studies in marine birds: Collection, processing and reporting. FACETS 4: 111–130. [Crossref]

Provencher, J. F., Vermaire, J. C., Avery-Gomm, S., Braune, B. M. & Mallory, M. L. 2018. Garbage in guano? Microplastic debris found in faecal precursors of seabirds known to ingest plastic. Science of the Total Environment 644: 1477–1484. [Crossref]

Rhodes, C. J. 2018. Plastic pollution and possible solutions. Science Progress 101: 207–260. [Crossref]

Rochman, C. M., Hoh, E., Kurobe, T. & Teh, S. J. 2013. Ingested plastic transfers hazardous chemicals to fish and induces hepatic stress. Scientific Reports 3: 3263.


Rodríguez, A., Rodríguez, B. & Carrasco, M. N. 2012. High prevalence of parental delivery of plastic debris in Cory’s shearwaters (Calonectris diomeda). Marine Pollution Bulletin 64: 2219–2223. [Crossref]

Rodríguez, B., Bécares, J., Rodríguez, A. & Arcos, J. M. 2013. Incidence of entanglements with marine debris by northern gannets (Morus bassanus) in the non-breeding grounds. Marine Pollution Bulletin 75: 259–263. [Crossref]

Roman, L., Schuyler, Q. A., Hardesty, B. D. & Townsend, K. A., 2016. Anthropogenic debris ingestion by avifauna in eastern Australia. PLoS ONE 11: e0158343. [Crossref]

Roman, L., Hardesty, B. D., Hindell, M. A. & Wilcox, C. 2019a. A quantitative analysis linking seabird mortality and marine debris ingestion. Scientific Reports 9: 1–7. [Crossref]

Roman, L., Bell, E., Wilcox, C., Hardesty, B. D. & Hindell, M. 2019b. Ecological drivers of marine debris ingestion in Procellariiform Seabirds. Scientific Reports 9: 1–8. [Crossref]

Ryan, P. G. 1988. Intraspecific Variation in Plastic Ingestion by Seabirds and the Flux of Plastic Through Seabird Populations. The Condor 90: 446–452. [Crossref]

Ryan, P. G. 1990. The effects of ingested plastic and other marine debris on seabirds. In: Shomura, R.S. & Godfrey, M.L. (eds.) 1989. Proceedings of the Second International Conference on Marine Debris, Honolulu, Hawaii, 2–7 April 1989. USA: NOAA Technical Memorandum. 623–634.

Ryan, P. G. 2015. A Brief History of Marine Litter Research. In: Bergmann, M., Gutow, L. & Klages, M. (eds.) Marine Anthropogenic Litter. Springer, London: 1–25. [Crossref]

Ryan, P. G. 2018. Entanglement of birds in plastics and other synthetic materials. Marine Pollution Bulletin 135: 159–164. [Crossref]

Savoca, M. S., Wholfeil, M. E., Ebeler, S. E. & Nevitt, G. A. 2016. Marine plastic debris emits a keystone marine infochemical for olfactory foraging seabirds. Science Advances 2: e1600395. [Crossref]

Shaw, D. G. & Day, R. H. 1994. Colour-and form-dependent loss of plastic micro-debris from the North Pacific Ocean. Marine Pollution Bulletin 28: 39–43. [Crossref]

Sileo, L., Sievert, P. R., Samuel, M. D. & Fefer, S. I. 1990. Prevalence and characteristics of plastic ingested by Hawaiian seabirds. In: Shomura, R. S. & Godfrey, M. L. (eds.) Proceedings of the Second International Conference on Marine Debris, Honolulu, Hawaii, 2–7 April 1989. Washington, DC: US Department of Commerce. NOAA Technical Memorandum. Honolulu, Hawaii: 665–681.

Spear, L. B., Ainley, D. G. & Ribic, C. A. 1995. Incidence of plastic in seabirds from the tropical pacific 1984–91: Relation with distribution of species, sex, age, season, year and body weight. Marine Environmental Research 40: 123–146. [Crossref]

ter Halle, A., Ladirat, L., Gendre, X., Goudounèche, D., Pusineri, C., Routaboul, C., Tenailleau, C., Duployer, B. & Perez, E. 2016. Understanding the fragmentation pattern of marine plastic debris. Environmental Science & Technology 50: 5668–5675. [Crossref]

van Franeker, J. A. 1985. Plastic ingestion in the Northern Atlantic fulmar. Marine Pollution Bulletin, 16: 367–369. [Crossref]

van Franeker, J. A. 2004.Save the North Sea Fulmar-Litter-EcoQO Manual Part 1: Collection and dissection procedure. Alterra-rapport 672: 1–38.

van Franeker, J. A., Blaize, C., Danielsen, J., Fairclough, K., Gollan, J., Guse, N., Hansen, P. L., Heubeck, M., Jensen, J. K., Le Guillou, G. & Olsen, B. 2011. Monitoring plastic ingestion by the northern fulmar Fulmarus glacialis in the North Sea. Environmental Pollution 159: 2609–2615. [Crossref]

van Franeker, J. A. & Law, K. L. 2015. Seabirds, gyres and global trends in plastic pollution. Environmental Pollution 203: 89–96. [Crossref]

van Franeker, J. A. & Meijboom, A. 2002. Marine litter monitoring by Northern Fulmars (a pilot study). Alterra-rapport 401: 1–72.

Verlis, K. M., Campbell, M. L. & Wilson, S. P. 2013. Ingestion of marine debris plastic by the wedge-tailed shearwater Ardenna pacifica in the Great Barrier Reef, Australia. Marine Pollution Bulletin 75: 244–249. [Crossref]

Votier, S. C., Archibald, K., Morgan, G. & Morgan, L. 2011. The use of plastic debris as nesting material by a colonial seabird and associated entanglement mortality. Marine Pollution Bulletin 62: 168–172. [Crossref]

Weinstein, J. E., Crocker, B. K. & Gray, A. D. 2016. From macroplastic to microplastic: degradation of high-density polyethene, polypropylene, and polystyrene in a salt marsh habitat. Environment Toxicology and Chemistry 35: 1632–1640. [Crossref]

Wilcox, C., Sebille, E.V. & Hardesty, B. D. 2015. Threat of plastic pollution to seabirds is global, persuasive and increasing. Proceedings of the National Academy of Sciences of the United States of America 112: 11899–11904. [Crossref]

Wischnewski, S., Arneill, G. E., Bennison, A. W., Dillane, E., Poupart, T. A., Hinde, C. A., Jessopp, M. J. & Quinn, J. L. 2019. Variation in foraging strategies over a large spatial scale reduces parent–offspring conflict in Manx shearwaters. Animal Behaviour 151: 165–176. [Crossref]