Assessment of Great Skua Stercorarius skua pellet composition to inform estimates of storm petrel consumption from bioenergetics models

Abstract

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.