Factores que influyen en la conducta del consumidor

species are known to make use of several methods (Schreiber & Burger, 2001). Camphuysen (2011), for example, reported that northern gannets tended to adopt deep or shallow plunge diving strategies when feeding alone, but actively switched to a scooping strategy when feeding in association with multi-species assemblages, most likely a consequence of increased availability of prey in the surface waters. Conversely, some species are highly specialised, adopting single foraging techniques to access prey (Furness & Tasker, 2000). Water clarity may also exert selective pressure on seabird foraging modes and has been shown to influence the foraging and distribution of seabirds (Ainley, 1977; Camphuysen, 2011) with plunge diving strategies more common in clear waters and pursuit diving positively correlated with increasingly turbid waters. Such responses most likely reflect plunge divers requiring relatively clear waters to keep a visual fix on their prey, while pursuit divers rely more on surprise to catch prey (Wanless & Harris, 1997).

1.2.4 Analytical approaches for monitoring the diet and trophic status of seabirds

Analysing the diet and trophic position of a top-level marine predator is important to the understanding of marine ecology food webs (Ruiz-Cooley, 2004). In recent years, attention has focused on the possible role of climate change on sandeel distribution and abundance in the declines of specific seabird species around the UK coast (Montevecchi & Myers, 1997; Durant et al. 2003; Sandvik et al. 2005, Wanless et al. 2007). Evaluating hypotheses such as these requires specific information on the diet of seabirds in the areas. However, the application of traditional methods (i.e., regurgitates, faeces, stomach contents, observations) for studying diet or foraging behaviour of animals has known biases and limitations (Burns et al. 1997; Kelly, 2000; Hamond & Wilson, 2016; Wilson & Hammond, 2016). Observational data, particularly for seabirds, are typically limited to breeding colonies during spring and summer months and reveal little about the foraging habits of these animals at other

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times of year (Dunnet et al. 1990). Moreover, often the assumption is made that nestling and adult diets are similar, but studies have shown that adults feed on prey of different sizes and species than they deliver to their chicks (Wanless et al. 1993; Shealer 1998) and that their foraging behaviour differs between self-feeding and chick provisioning (Forero & Hobson, 2003).

The analysis of stomach contents and regurgitates/faecal pellets can also provide useful information about the diets of many seabirds, but are often biased in favour of the remains of durable hard parts due to differential rates of digestion (Hobson and Clark, 1992; Lajtha and Michner, 1994; Hilton et a. 2000b; Phillips et al., 2005). For example, Votier et al. (2001) reported an overestimation of the proportion of birds in the diet of great skuas Catharacta skua when using pellet contents. Hobson & Welch (1992) found that seabirds in Barrow Strait and Lancaster Sound consumed more lower trophic level invertebrates than previously suggested through conventional dietary analysis highlighting the importance of more systematic, controlled studies to calibrate sampling techniques (Brown & Ewin, 1996). Furthermore, the use of temporal dietary information is often difficult to quantify, leading Dalerum et al. (2005) to argue that many of these methods provide only a “snapshot” of the diet at a point in time, and may not be representative of the typical long-term diet of the animal.

To avoid some of these difficulties, researchers have made inferences about diet in marine mammals and seabirds based on diving behaviour, foraging location and the behaviour of potential prey (Wanless et al. 1999; Harris et al. 2005). However, observed errors and biases with both these and conventional dietary analysis have created a need to employ additional dietary methods that may more accurately reflect the long term diet and that can be obtained from sampling free-living animals less intrusively (Herman et al. 2005). Analysis of stable isotope ratios has recently emerged as a powerful technique and has been increasingly used to assess the dietary preferences and trophic position of a diversity of seabirds and marine mammals (Forero et al. 2002; Weimerskirch et al. 2005; Bearhop, 2006; Bird et al. 2008; Bond & Jones, 2009).

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1.3 Stable Isotope Analysis

You are what you eat …… or are you? Natural variation in stable isotope ratios of elements has long been a tool used by geochemists and palaeo-oceanographers (Lajtha & Michner, 1994). Since the landmark publications of DeNiro & Epstein (1978, 1981), their application in ecological and environmental studies has been widespread with stable isotopes increasingly used to improve our understanding about dietary patterns of animals (Gannes et al. 1997 & 1998; Thompson et al. 1995). Stable isotopes have also been utilised to discriminate between animals living in different biomes (Furness et al. 2006, Inger et al. 2006), to trace the migration patterns of birds (Forero & Hobson, 2003), to unravel trophic relationships in food webs (Hobson & Montevecchi, 1991, Sydeman et al. 1997) and more recently to understand the behaviour of contaminants since isotopic measures may allow insights into dietary sources and trophic levels of individuals (Elliott & Scheuhammer, 1997; Das et al. 2003; Krahn et al. 2007). The utility of stable isotopes in such ecological studies is based on the following principles. First, the isotopic composition of a consumer reflects that of its diet in a predictable manner (DeNiro & Epstein, 1978; Hobson & Cark, 1992a). Second, because tissues of a consumer turnover at different rates, they integrate information on diet over different temporal scales (Tieszen et al. 1983). Third, the isotopic signatures of potential dietary sources are often isotopically distinct and hence are distinguishable from one another (Gannes et al. 1997).