Causes of change

The recent decline in numbers may, in part, have been due to low productivity that occurred in some northern colonies between 2003 and 2007, which coincided with a decline in sandeel abundance [Burnell et al. 2023]. The commercial fishing industry can also negatively impact Guillemot populations through both bycatch, particularly in gillnets [Northridge et al. 2023], and overfishing of important prey species, which is likely to increase foraging pressure and decrease breeding productivity [Nettleship et al. 2018a]. Offshore wind farms are an additional pressure on Guillemots, through potential displacement of birds from foraging grounds [Peschko et al. 2020].

Increases in the sea surface temperature are causing changes in the abundance and distribution of Guillemot prey, such as sandeels [Erikstad et al. 2013; Heath et al. 2009; Régnier et al. 2017; Riordan & Birkhead 2018; Wanless et al. 2005], potentially resulting in a switch to less energy-rich alternative prey items [Heubeck 2009] which can result in a lower breeding success in areas where Guillemots are heavily dependent on sandeels in the breeding season. Guillemot adults, eggs and chicks are vulnerable to predation during the breeding season by a range of species, including Hooded Crows, Carrion Crows and Herring Gulls [Booth Jones 2020], a threat that increases in smaller auk colonies, which offer less protection for individual birds [Gilchrist 1999]. Guillemots are particularly susceptible to severe winter storms, which reduce foraging opportunities and can in extreme cases lead to starvation. Climate change is increasing the frequency of these extreme weather events (Field et al. 2012), which can lead to many dead birds being washed up in seabird wreck. In the winter of 2013/14, large wrecks occurred along British coasts, and post-mortems suggested starvation and some oil contamination as likely causes (Jessop 2014; Sellers 2014).