Several methods, each with specific advantages and disadvantages, are frequently used to obtain diet samples from seabirds. The collection of regurgitates (REG) as well as samples from the stomach water-off-load (WOL) or flushing technique are some of the most commonly used approaches. During the Austral breeding season of 2005/06 diet samples from Cape Petrel Daption capense and Snow Petrel Pagodroma nivea were collected at Signy Island, South Orkney Islands, Antarctica. Primarily, diet samples were obtained by stomach flushing but occasionally (8 Snow Petrels and 12 Cape Petrels) birds spontaneously regurgitated their stomach contents before flushing. These regurgitates were sampled completely and stored separately. Afterwards the remainder of the bird’s stomach contents was flushed. By doing so, a comparison could then be made between the REG samples and the total stomach contents (REG+WOL). In the REG samples of both species the fraction of fish was underestimated and the fraction of crustaceans overestimated compared to the REG+WOL samples. This study shows that methodology-specific outcomes are potentially to be expected when doing dietary research. Using REG samples is shown not to be suitable for quantitative assessments of the diets of petrels. The WOL technique, which collects the entire stomach content, suits this purpose better.

The fulmarine petrels (Fulmarinae) are a group of seabirds within the order of procellariids. Their diets and those of other seabirds have been extensively studied (e.g. Bierman & Voous 1950; Ainley et al. 1992; Soave et al. 1996; Hodum & Hobson 2000; van Franeker et al. 2001; Cherel et al. 2002) and with a variety of different diet sampling techniques (reviewed by Barrett et al. 2007). Two of the most widely used methods are the collection of regurgitates (REG) and stomach flushing or water-off-load technique (WOL; Wilson 1984) and both techniques have specific advantages and disadvantages (Votier et al. 2003). Although collecting regurgitates is less stressful for birds, the use of regurgitates to sample diets also has disadvantages. For example, meal size cannot be determined from regurgitates due to the likelihood of incomplete regurgitation (the entire stomach contents are not regurgitated), variation in the proportion of ingested food items, and the fact that some food types are easier to regurgitate than others (e.g. Votier et al. 2003; Barrett et al. 2007). The water-off-load technique may provide a more accurate picture of meal size and diet, but is likely to be more stressful for birds. Some bird species, such as auks, do not easily regurgitate and might be less suitable for this technique although it has been used successfully in this group (Wilson et al. 2004). As a quantitative approach is critical if we are to understand nutrient cycling and interactions between predators and their prey, choosing an adequate method based on knowledge of the limitations of the different methods is of utmost importance.

Van Franeker et al. (2001) compared different diet study methods in Antarctic fulmarine petrels and suggested a bias towards crustacean prey in regurgitated samples. However, these conclusions were based on REG and WOL samples from different individual birds. They proposed to resolve the question of such potential bias by future WOL studies that could separately store and analyse voluntary regurgitates of birds captured for WOL sampling. Such an approach became possible when we captured Cape Petrels Daption capense and Snow Petrels Pagodroma nivea to sample diets using the WOL technique, and some birds regurgitated part of their stomach contents on capture. These regurgitates were collected and analysed separately from subsequent WOL samples. By doing so we were able to study potential differences between REG and WOL samples of the stomach content within individual birds of two species of petrels.

The Netherlands Polar Program (NPP) of the Council for Earth and Life Sciences of the Netherlands Organisation for Scientific Research (ALW-NWO) and the foreign office of the VU University Amsterdam (Van Dittmer Fonds) co-funded the fieldwork of this study. Antarctic research by IMARES is commissioned by the Netherlands Ministry of Economic Affairs, Agriculture and Innovation under its Statutory Research Task Nature & Environment WOT-04-009-036 and was also supported by ALW-NWO under project nr. 851.20.011. This paper is a contribution to the BAS Ecosystem programme. We thank M. Dunn for his great help in the field and in the laboratory, R. Shreeve and G. Tarling for their help indentifying crustacean prey items, E. Bravo Rebolledo and M. Leopold for their help with otolith measurements, and R. Phillips for advice during the preparation of this project. We thank Bob Furness, Martin Heubeck and an anonymous reviewer for their input to improve the manuscript.

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