Unravelling Diet Composition And Niche Segregation Of Colonial Waterbirds In A Mediterranean Wetland Using Stable Isotopes

Rice fields and waterbirds are an example of a synergy that can occur between agriculture and conservation. This connection is especially relevant during the farming period, when nesting waterbirds need to obtain resources to cover their energy requirements and those of their chicks in rice fields. However, new farming techniques may potentially put fulfilling this role at risk. Studies on how species use rice fields to feed during this critical period are essential to understand waterbird population dynamics and to optimize conservation measures. At present, several species of colonial birds in l'Albufera de Valencia (Spain) that depend on trophic resources available in rice fields have decling populations and decreases in productivity. We therefore assessed trophic niche segregation in the diet of chicks of a waterbird community composed of seven species through stable isotope analyses of carbon (delta C-13) and nitrogen (delta N-15) obtained from chick blood and tissue samples of potential prey. According to trophic niche widths, chicks of Black-headed Gull Chroicocephalus ridibundus and Gull-billed Tern Gelochelidon nilotica showed a similar diet, although Gull-billed Tern had a major trophic specialization. The Western Cattle Egret Bubulcus ibis fed its chicks in a wider range of different habitats, with organisms located at the base of the food chain and with a greater importance of terrestrial prey than other species. Conversely, the Glossy Ibis Plegadis falcinellus had a more differentiated diet, mostly in freshwater habitats and mainly consuming odonates. Grey Heron Ardea cinerea, Little Egret Egretta garzetta and Squacco Heron Ardeola ralloides showed overlap between their niches, largely explained by the presence and abundance of cyprinid fish and odonates in their diets. Our results suggest a less important role than expected of Red-swamp Crayfish Procambarus clarkii, and a more important role of odonates despite being a less energy-rich prey. In conclusion, the diet of generalistic species of waterbirds in this study suggests that their trophic niches can overlap when relative prey abundance is high.