The occurrence of predation on Black Guillemots Cepphus grylle, of both adults and chicks, is an important consideration when assessing factors affecting breeding success. However, predators are often cryptic and confirmed interactions are difficult to identify. Through the use of camera traps, we recorded predation by mammalian and avian species on Black Guillemots on Stroma and North Ronaldsay in the 2016 and 2017 breeding seasons. Camera traps recorded two presumed instances of an Otter Lutra lutra predating chicks, one possible instance of an Otter predating an adult, one instance of a Hooded Crow Corvus cornix predating a chick, and the presence of several species of potential predators at nests. Camera traps were deployed concurrently during periods of visual observations, during which, sightings of predators were rare. We found the presence of camera traps to have no effect on breeding success between monitored (mean chicks fledged = 1.05, n = 52) and control (mean chicks fledged = 0.98, n = 98) nests. Here we highlight the potential role of camera traps in monitoring seabird nest success, and positively identifying sources of nest failure.

Seabirds encounter many factors which influence adult survival and breeding success. These include, but are not limited to: predation (Craik 1997; Miles et al. 2015; Buchadas & Hof 2017); shifts in prey availability or abundance related to climate change (Gaston & Elliott 2014; Divoky et al. 2015) and fisheries practices (Furness & Tasker 2000; Frederiksen et al. 2004). Though fewer investigations exist, additional sources of pressure may arise from marine renewable energy devices (Grecian et al. 2010; Bailey et al. 2014), fisheries bycatch (Žydelis et al. 2009) and plastic pollution (O’Hanlon et al. 2017). Nest surveys are often carried out to assess the breeding success of a colony, a measurement that may in turn be linked to external drivers influencing a population (Gjerdrum et al. 2003). However, positively identifying the cause of breeding failure can be difficult, but would be beneficial in the management of these impacts. Similarly, nest surveys are often carried out to assess the impacts of research activities on target study species, including the attachment of telemetry devices such as geolocators or GPS tags (Phillips & Croxall 2003; Hamel et al. 2004; Symons & Diamond 2019). Understanding additional factors affecting breeding success can improve the monitoring of tag effects, as certain biases may be influencing tagged vs. control birds.

Black Guillemots have been shown to be vulnerable to several species of predator including: American Mink Neovison vison (Craik 1995; Nordström 2002), Hooded Crow Corvus cornix (Hario 2001; Foster 2011), Great Skua Stercorarius skua (Furness 1987), and Otter Lutra lutra (Ewins 1985). While auks make up only a small proportion of the overall diet for Mink and Otter (Clode & Macdonald 1995), predator-linked disturbance and mortality can have marked impacts on Black Guillemot nesting locations (Ewins & Tasker 1985), and success (Ewins 1985). Previously, intensive nest monitoring was required to positively identify predation of chicks (Ewins 1985; Nelson & Hamer 1995). Motion-sensor cameras (herein referred to as ‘camera traps’) provide an opportunity for constant monitoring of nests to observe behaviour, diet and interactions with other species. Infra-red lighting provided by camera traps overcomes the issue encountered with visual observations at night, allowing the discovery of previously unseen predation activity (Dilley et al. 2013).

Camera traps are becoming more commonly used in the study of seabird demography (Black 2018; Hinke et al. 2018; Jones et al. 2018), and diet (Gaglio et al. 2017). The use of camera traps is already recognised as a suitable method for the identification of predation on cryptic nesting waders (Macdonald & Bolton 2008; Teunissen et al. 2008; Calladine et al. 2017). Regarding seabirds, they have notably been used in the study of endemic and invasive predators on Procellariiformes in the Southern Hemisphere (Dilley et al. 2013, 2015; Davies et al. 2015). Boulder-beach nesting Black Guillemots are potentially feasible to observe using camera traps, as nests are generally enclosed and adults enter through a single crevice. Camera traps have recently been used in the study of Black Guillemot diet, predator presence, and chick fledging (Hof et al. 2018). Camera traps could prove a useful tool in current research on Black Guillemots aimed at assessing colony impacts related to the interaction with marine renewable energy devices, i.e. tidal stream turbines (Furness et al. 2012; Johnston et al. 2018). They may be used to improve monitoring of tag effects in relation to the recent increase in GPS tracking studies (Owen 2015). Camera traps may also identify potential disturbance, by humans or invasive predators, to breeding individuals within the landward boundaries of six Marine Protected Areas (MPAs) allocated to Black Guillemots across Scotland (Scottish Natural Heritage 2014).

In this study we report the predation of Black Guillemot chicks and adults recorded by camera traps at nests on Stroma, Caithness; and North Ronaldsay, Orkney. Camera trapping was performed concurrently with a project GPS tagging breeding adults. Nest checking was carried out for both camera-trapped, and control nests to monitor effects of camera trap presence on breeding success. Nest checks additionally identified the outcome of predator interactions seen in photos. We discuss how identification of these interactions improved the assessment of Black Guillemot nest failure.

DTJ was funded through a Marine Alliance for Science and Technology for Scotland (MASTS) studentship supported by Scottish Natural Heritage (SNH). A special thank you is reserved for the volunteer camera trap analysts: Colin Mckenzie, Danny Allot, and Emily Kearl. We are grateful for the support given by the staff and volunteers of the North Ronaldsay Bird Observatory, and skipper William ‘Willy’ Simpson who accommodated fieldwork on Stroma. We would like to thank the anonymous reviewers of the manuscript.

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