Studies of predator-prey interactions, and quantification of predation frequencies can be crucial to understanding prey population declines. We present a study of gull Larus spp. predation on Leach’s Storm-petrels Hydrobates leucorhous and European Storm petrels H. pelagicus, relative to other prey types. We studied a population of approximately 160 gulls in mixed colonies of Herring Gulls L. argentatus and Lesser Blackbacked Gulls L. fuscus, on Elliðaey Island, Iceland. We dissected 191 pellets and found gulls fed mostly on avian prey, insects and molluscs. We estimate that the total consumption of Leach’s Storm-petrels by all gulls in the colonies amounted to approximately two individuals per day, over the course of the study. European Stormpetrels were not preyed upon in high quantity during the study period, with a minimum of one individual depredated within a four-day study period, potentially reflecting the later breeding season of this species. We also provide a correction factor of pellets produced per storm-petrel consumed, which could be used to quantify the number of individual storm-petrels consumed from counts of pellets in future studies.

Studies of trophic interactions are crucial to understanding how species respond to changes in prey availability, and changes to predator populations. Consideration of predator-prey dynamics is important where predator or prey species are of conservation concern, where predators are undergoing rapid population increases, where predators are breeding in areas they have recently colonised, and where prey availability is changing (Rodríguez et al. 2019; Church et al. 2018). For example, population dynamics of generalist avian top-predators can be relatively independent of availability of single prey types and this can lead to potential negative impacts on particular prey species, even if they comprise a small proportion of the total diet (Miles et al. 2013). Such predator species, including some large gulls and skuas Stercorarius spp., can switch prey or vary their foraging strategies to allow population resilience; in some populations, successful dietary switching and exploitation of abundant resources has led to locally-increased populations (e.g. Furness et al. 1992; Mitchell et al. 2004; Duhem et al. 2008). There is evidence that increased populations of generalist avian top-predators, and the development of individual dietary-specialisations within these populations, is having negative impacts on other avian populations through predation (e.g. Phillips et al. 1999; Matias & Catry 2010). For example, Great Skua S. skua predation on seabirds has shown a general increasing trend since the 1970s, implicated to be a consequence of changing environmental and anthropogenic factors (Church et al. 2018). Large gulls have undergone similar local expansions in some areas (e.g. Duhem et al. 2008; Lisnizer et al. 2011), despite ongoing global population declines.

Leach’s Storm-petrel (c. 45 g) and European Storm-petrel (c. 25 g) are highly pelagic petrel species (Snow & Perrins 1998). Generally, they come to land only to breed and activity at the colony is restricted to the hours of darkness. They breed in colonies between May and November, in burrows and crevices on islands free of invasive, non-native predators such as rodents. Their burrowing ecology and nocturnal behaviour are adaptations to predator avoidance (Watanuki 1986; Miles et al. 2013). However, Leach’s and European Storm-petrels are still vulnerable to predation by avian predators such as gulls, skuas and owls (e.g. Williams & Frank 1979; Phillips et al. 1999; Stenhouse & Montevecchi 1999; Votier et al. 2006; Miles 2010; Pollet & Shutler 2019). The global population of Leach’s Storm-petrel is estimated at approximately 7 million breeding pairs, though numbers are declining; the species is therefore listed as ‘Vulnerable’ by the International Union for Conservation of Nature (IUCN), having been upgraded from ‘Least Concern’ in 2016 (BirdLife International 2017a). A recent population study estimated that the local population of Elliðaey Island, in the archipelago of Vestmannaeyjar, south Iceland, has undergone a substantial decline since 1991 (Deakin et al. pers. comm.). Despite this, there is little understanding of the causes of population declines of Leach’s Storm-petrels. European Storm-petrels are classified as ‘Least Concern’ by the IUCN, with an estimated global population of 430,000–519,999 breeding pairs, however their population trends are unknown (BirdLife International 2017b). The population size of Vestmannaeyjar is also unknown, but is thought to be relatively small (<20,000 pairs), so is therefore potentially vulnerable to population-level impacts of predation.

The interaction between generalist avian top-predators and storm-petrels at breeding colonies is relatively well studied. For example, Phillips et al. (1999) estimated that a population of just 229 breeding pairs of Great Skuas consumed 36,000 seabirds in the breeding season of 1996 on St Kilda, including almost 13,000 Leach’s Storm-petrels and 6,500 European Storm-petrels. A more recent estimate by Miles (2010) found storm-petrels to be depredated in similarly high numbers by 175–229 breeding pairs of Great Skuas on St Kilda, with approximately 21,000 Leach’s Storm-petrels depredated annually, as well as approximately 8,500 European Storm-petrels. Less research has focused on gulls as predators of Leach’s Storm-petrels (but see Watanuki 1986). However, multiple studies have focused on Yellow-legged Gulls L. michahellis feeding on European Storm-petrels. Oro et al. (2005) found that a population of over 400 pairs of Yellow-legged Gulls consumed around 115 European Storm-petrels annually. Sanz-Aguilar et al. (2009) showed that the majority of predation upon stormpetrels was being carried out by specialist individuals within the gull population. Additionally, studies have found that Herring Gull predation on Leach’s Storm petrels on Great Island, Newfoundland was high until Capelin Mallotus villosus arrival (Stenhouse & Montevecchi 1999; Stenhouse et al. 2000). Only stormpetrel- specialist gulls continued predation beyond that point, implying that alternative prey availability could be an important factor in predation rates upon storm-petrels. The results of Stenhouse et al. (2000) suggest that breeding birdspecialist Herring Gulls were capable of consuming more than five Leach’s Stormpetrels per pair per day. Therefore, predation rates could be relatively unpredictable and prone to variation between populations dependent on relative proportions and absolute quantities of avian specialist predators.

Herring Gulls and Lesser Black-backed Gulls are large gulls (adults < 1.4 kg and < 1 kg, respectively). Both species have wide breeding ranges across Europe, breeding in colonies between April and July (Harris 1964; MacRoberts & MacRoberts 1972). Herring Gulls will breed in a range of habitats but show some preference for cliffs and rocky shores (Snow & Perrins 1998). Lesser Black-backed Gulls will also use a wide range of breeding habitats but show preference for short vegetation and levelground. They exhibit a highly generalist diet at the population level, feeding on a wide range of marine and terrestrial prey, in addition to anthropogenic waste. Both gull species are listed as ‘Least Concern’ by the IUCN, though Herring Gull populations are decreasing (and Red-listed within Europe, BirdLife International 2015) whilst Lesser Black-backed Gulls are increasing (BirdLife International 2018a; 2018b). Within Iceland, population trends are unknown, but previous estimates have suggested 5,000–10,000 Herring Gull pairs in 1990 and 40,000–50,000 pairs of Lesser Black-backed Gulls in 2004 (Guðmundsson & Skarphéðinsson 2012). Both species are presumed to have colonised Elliðaey Island since 2008, when human presence on the island declined following a hiatus on Atlantic Puffin Fratercula arctica hunting, though the local gull population was not previously estimated.

The aim of this study was to assess the consumption of Leach’s and European Storm-petrels relative to other prey types by Herring and Lesser Black-backed Gulls resident on Elliðaey Island from pellet analysis. We also provide an estimate of how many pellets are produced by Larus spp. for every Leach’s and European Stormpetrel consumed, for use in future bioenergetics models to estimate rates of consumption of these species (e.g. Phillips et al. 1999; Deakin et al. 2018). No previous studies have been carried out into the diet of gulls, or their impact on storm-petrel populations, at the study site. Given the apparent decline of Leach’s Storm-petrels on this island, and the importance of the Vestmannaeyjar archipelago to the local storm-petrel population, understanding the role of predation in these areas is crucial to storm-petrel conservation.

We thank members of the RSPB sabbatical team, Richard Barnard, Daniel Trotman and Anne-Marie McDevitt, who assisted with pellet collection. Thanks to Zoe Deakin for assistance with pellet collection and identification. Thanks to Marino for use of accommodation facilities on Elliðaey.

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