Feeding ecology of yellow-legged gulls Larus michahellis in the western Mediterranean: a comparative assessment using conventional and isotopic methods

Abstract

Accurate determination of feeding habits using only conventional dietary analysis is usually difficult, due to both biases caused by prey digestibility differences and to the discrete aspect of the sampling. However, combining conventional methods with stable isotope analysis provides an integrated view of the assimilated diet. Here, we measured stable isotope ratios of carbon (δ 13 C), nitrogen (δ 15 N) and sulphur (δ 34 S) in feathers of yellow-legged gull Larus michahellis nestlings from 4 colonies along the western Mediterranean coast, where the gulls consume different proportions of marine, terrestrial and garbage resources. We collected and analysed chicks' regurgitates in each colony, thereby determining the isotopic signatures in a significant sample of prey. By applying a mixing model to our isotopic values, we compared the information provided by the 2 methodologies. According to direct prey sampling and the particular conditions of the breeding areas, populations with more enriched δ 34 S values consumed a diet richer in marine prey, with lower δ 15 N signatures indicating garbage consumption. The main pattern resulting from the mixing model agreed with direct diet sampling. However, estimated proportions for small-sized prey were controversial, which indicates that small, soft prey items might be underestimated by regurgitate analysis. We conclude that stable isotope signatures and the use of mixing models are useful tools for the rapid assessment of feeding ecology in certain populations. Solving analytical biases should be considered in future feeding studies, thereby saving time and minimising bird disturbance by using an isotopic methodology.KEY WORDS: Carbon-13 · Mixing models · Nitrogen-15 · Regurgitate · Sulphur-34 Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 377: [289][290][291][292][293][294][295][296][297] 2009 because of the intrinsic characteristics of some food types (e.g. pieces of meat and garbage taken at refuse dumps). These methodologies provide only an episodic view of an individual's diet, since each sample represents only a specific feeding event (Votier et al. 2001). Thus, to obtain reliable information about a population's feeding habits, exhaustive monitoring over time is needed (Jordan 2005). However, conventional approaches are still required when we need to identify the taxonomic group of prey consumed. Regurgitated food appears as the least biased method for describing diet composition (González-Solís et al. 1997).The use of stable isotope analysis (SIA) in animal ecology has increased enormously in recent years and has become an important tool to study trophic ecology (Hodum & Hobson 2000, Post 2002. Although SIA does not provide the taxonomic detail achieved by conventional dietary analysis, it avoids prey digestibility biases because it only takes into account assimilated food. Moreover, it provides a ready-made integrated estimate because the assimilated diet for a certain period of time is summarised, dep...

Exploiting the closest productive area: geographical segregation of foraging grounds in a critically endangered seabird

“…Even if the patterns of movement of Balearic shearwaters are influenced by trawling activity (Bartumeus et al 2010), the spatial location of their foraging grounds might not change due to the overlap of marine productivity hotspots (including small pelagic fish) and main fishing grounds along the Iberian continental shelf (Demestre et al 1988, Estrada 1996. Second, isotopic values of small pelagic fishes change along the Iberian continental shelf (Ramos et al 2009), which may be explained by the natural geographic variation in the baseline isotopic values relating to water properties (e.g. Barnes et al 2009, Graham et al 2010.…”

Seasonal changes in the diet of a critically endangered seabird and the importance of trawling discards

“…We then created consumer end members for blood of shearwaters feeding exclusively on each of the . These values are derived from an isotopic study on the yellow-legged gull Larus michahellis from a breeding population that feed exclusively on pelagic Wsh (Ramos et al 2009). We applied the Iso-Error dual-isotope, 3-source mixing model of Phillips and Gregg (2001).…”

Stable Isotopes (δ13C, δ15N) Combined with Conventional Dietary Approaches Reveal Plasticity in Central-Place Foraging Behavior of Little Penguins Eudyptula minor

“…We combined data from nine seasons of stable isotope values (blood δ 13 C and δ 15 N, 2003-2011) with a longer dataset on 17 years of penguin stomach contents (1985-2012 not continuous) to unravel the feeding response of this seabird species to the various constraints imposed by its centralplace foraging behavior throughout the entire reproductive cycle. The combination of conventional diet analysis with more recent isotopic techniques has been proposed as the best way to address dietary reconstructions in seabirds (Karnovsky et al, 2008;Ramos et al, 2009;Chiaradia et al, 2012Chiaradia et al, , 2014. We explored trends in isotopic and conventional diet and body mass along the breeding season (prelaying, incubation, guard, and post-guard breeding stages) for years of "high, " "average, " and "low" breeding success, a proxy to food resource availability (Chiaradia and Nisbet, 2006).…”