Generalist predators may exert levels of predation on particular prey that result in, or contribute to, decline of that prey species. Bioenergetics models have been used to estimate the rates of consumption of Leach’s Storm Petrels Oceanodroma leucorhoa (45 g) and European Storm Petrels Hydrobates pelagicus (25 g) by Great Skuas Stercorarius skua on St Kilda (Western Isles, UK) and Hermaness (Shetland, UK). The models require estimates of the number of indigestible pellets produced from each individual storm petrel consumed, which have previously been determined by captive feeding trials or examination of pellets cast by free-living birds, but which have not discriminated between the two storm petrel species. Here we use information from dissection of 427 Great Skua pellets collected on Hirta (St Kilda, UK) and Mousa (Shetland, UK) to provide empirical estimates of the pellet:prey ratios for Leach’s and European Storm Petrels separately. We found that pellet:prey ratios were similar for collections of the ‘standing crop’ of pellets accumulated over the entire breeding season and samples of pellets cast within the preceding five days. However, pellet:prey ratios of both Leach’s and European Storm Petrel were considerably lower than reported previously. Furthermore, we found the pellet:prey ratio for European Storm Petrels consumed on St Kilda was 80% higher than the value for the same species on Mousa. Our study suggests that the use of a single generic value for the pellet:prey ratio for both species and all locations may lead to inaccuracies in estimation of consumption rates, and we recommend further work to understand the causes of such variation.

Bioenergetics models have been widely used for many years in seabird ecology to estimate rates of prey consumption at scales ranging from individual colonies to ocean basins (e.g. Guinet et al. 1996; Barrett et al. 2006). Such models can shed light on a range of processes from large-scale patterns of energy flux across ecosystems (Hunt et al. 2005) to the extent of competitive interactions between particular top-predator species and fishery activity (Bunce 2001). Here we focus on the application of bioenergetics models to assess rates of predation by seabirds on other seabird species. Quantifying rates of predation on specific prey types is important for understanding population dynamics, particularly where changes in predator or prey numbers are apparent or where conservation management may be required.

Numbers of Great Skuas Stercorarius skua have rapidly increased in Scotland during the last century, likely due to reduced persecution, increased availability of food from fishery discards and prey-switching, including direct predation of other seabirds (Mitchell et al. 2004; Votier et al. 2004a). Great Skuas are generalist predators and their diet includes fish, birds and invertebrates (Bayes et al. 1964; Furness 1987). Many of the seabird species that Great Skuas prey upon are declining in the UK (JNCC 2016) and the implementation of the Common Fisheries Policy discard ban is predicted to result in an increase in predation on seabirds as the availability of discarded fish decreases (Reeves & Furness 2002; Votier et al. 2004a; Bicknell et al. 2013).

Storm petrels (Hydrobatidae) are vulnerable to predation due to their small size and relative immobility on land. The breeding ecology of storm petrels is strongly influenced by predation risk: nest sites are located in crevices and burrows on islands free from introduced mammalian predators and adult birds are active at the colony only at night. Despite these adaptations, storm petrels remain vulnerable to avian predators such as gulls (Laridae) and skuas (Stercorariidae) (Watanuki 1986; Stenhouse & Montevecchi 1999; Stenhouse et al. 2000; Oro et al. 2005; Votier et al. 2006).

Traditionally, a range of methods have been used to study seabird diet, including identification of prey from feeding observations, pellets, prey remains, spontaneous regurgitates and stomach flushing (Votier et al. 2003). These techniques give broadly similar results, but there are biases associated with each (Votier et al. 2003). More recent advances include the extraction and identification of prey DNA from predator faeces or regurgitates (Bowser 2013) which may reduce bias, but are costly and time-consuming to implement at a large scale. In contrast, pellets of regurgitated, indigestible prey remains are easy to collect and can provide large sample sizes to determine the proportions of different prey types consumed (Votier et al. 2003). However, pellet analysis tends to overestimate indigestible material and any pellets produced away from the colony are not available for analysis, leading to uncertainty regarding the absolute quantities of prey consumed (Votier et al. 2003).

Pellet analysis can be combined with bioenergetics modelling to quantify consumption rates of different prey types without the need to estimate rates of pellet production. Such models have been used to calculate levels of storm petrel predation by Great Skuas at two large colonies in the UK (Phillips et al. 1999; Votier et al. 2004b; Miles 2010). The models firstly estimate the total energy requirement of the entire breeding and non-breeding population over the breeding season, then use the proportion, energy content and assimilation efficiency of each prey type to estimate its relative contribution to the total energy budget. The proportion of each prey type may be assessed by pellet analysis. Typically, each pellet comprises a single prey type, and the model requires for each prey type: (i) an estimate of the size (in prey mass or number of individuals) of an average ‘meal’ (i.e. the quantity of food present in a bird’s proventriculus on its return from a foraging trip, Phillips et al. 1999), and (ii) the number of pellets that are produced from a single meal. From these two quantities the number of pellets produced from each prey individual consumed (i.e. the pellet:prey ratio) is calculated, which is used as a so-called “correction factor” (Phillips et al. 1999) in the model. Although European Hydrobates pelagicus and Leach’s Oceanodroma leucorhoa Storm Petrels differ considerably in size (25 g and 45 g respectively), Phillips et al. (1999) considered that a single storm petrel of either species constituted a single meal, and “on the evidence of groups of pellets found together on breeding territories clearly consisting of combinations of wings, whole legs or body feathers, it was estimated that at least three pellets result from a meal of a single individual” of either species. Phillips et al. (1999) therefore used a correction factor of 3.0 pellets produced per storm petrel consumed, and concluded that in 1996 Great Skuas on St Kilda consumed 7,450 European Storm Petrels and 14,850 Leach’s Storm Petrels. Votier et al. (2004b) used a similar bioenergetics model to estimate the number of European Storm Petrels consumed annually by Great Skuas at Hermaness, Shetland, using a pellet:prey ratio of 2.5 pellets per European Storm Petrel consumed, as estimated by Votier et al. (2001). The ratio was obtained by feeding 11 storm petrel carcasses, as six separate meals, to captive, full-grown Great Skua fledglings (Votier et al. 2001, Tables 2 & 3, though note the methods section of that study incorrectly states that eight storm petrels were fed to the captive skuas). Since carcasses of European Storm Petrels were not available, a variety of larger-bodied storm petrel species from the austral Oceanitidae family were used (S. Votier pers. comm.). A total of 28 pellets were cast from the 11 storm petrels consumed, giving a pellet:prey ratio of 2.5 (Votier et al. 2001, Table 3). Pellets produced by Great Skuas held in captivity or produced from the consumption of large austral storm petrel species may not be entirely representative of pellets of European and Leach’s Storm Petrels produced by free-living Great Skuas. Votier et al. (2004b) concluded that Great Skuas at Hermaness consumed 215 European Storm Petrels in 2001.

Here we use dissection of Great Skua pellets collected at two colonies and containing remains of both European and Leach’s Storm Petrels to quantify the pellet:prey ratios for each prey species. For each sample of pellets, we calculated the minimum number of storm petrels consumed from the number of the most frequently occurring body part. For example, since each storm petrel has only one furcula, a sample of pellets that contains five storm petrel furculae represents the remains of a minimum of five storm petrels. We calculated pellet:prey ratios by dividing the number of pellets in a sample by the minimum number of storm petrels represented in that sample. For example, a sample of ten pellets that contained a total of five storm petrel furculae would give a pellet:prey ratio of 2:1, and a correction factor of two in a bioenergetics model. Specifically, we compare the pellet:prey ratios (i) for Leach’s and European Storm Petrels; (ii) for European Storm Petrels at two different colonies and (iii) for samples collected as the ‘standing crop’ of pellets accumulated over an extensive (and unknown) period of time with those collected from an area cleared of pellets five days previously.

We thank the operators of the Mousa ferry, Darron and Rodney Smith, for transport of equipment and personnel to the island. Permission to work on Mousa was granted by the RSPB, Scottish Natural Heritage and the Bell family. We thank Dr Will Miles and Dr Steve Votier for several helpful discussions about bioenergetics models of skua prey consumption and two anonymous reviewers for their helpful comments on the manuscript. ZD is supported by a NERC GW4+ Doctoral Training Partnership studentship from the Natural Environment Research Council [NE/L002434/1], supervised by Drs Frank Hailer, Renata Medeiros and Rob Thomas, who we thank for their support.

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