Unravelling diet composition and niche segregation of colonial waterbirds in a Mediterranean wetland using stable isotopes

Abstract

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 València (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 (δ13C) and nitrogen (δ15N) 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.

Effects of Rice Field Water Cycles on the Breeding Biology of Grey Heron (Ardea cinerea) and Conservation Implications

“…L'Albufera de Vàlencia covers an area of 21,120 ha, of which 2,800 ha correspond to a shallow lagoon and 290 ha of marshland vegetation, which is fed by crop returns and water from the fluvial systems of the Júcar and Turia rivers. The rice fields cover 14,000 ha and represent an important habitat for the waterfowl community, both resident and migratory, especially during the breeding season, as they are the main feeding area for the latter (Fasola and Ruiz 1996, Fasola et al 1996, Dies and Dies 1998, Prosper 2000, Toral and Figuerola 2010, Antón-Tello et al 2021, Forti et al 2021b). The hydrological dynamics of the paddy fields depend on the amount of water that enters the system twice a year.…”

“…However, in recent decades, rice field management has varied in terms of the start and duration of flooding periods during the annual period and in agronomic practices during the growing season, with flooding starting later and for shorter periods, among others. These changes have affected negatively both the quality of the feeding habitat for herons and other birds, as well as the reproductive success of some species such as the Little Egret ( Egretta garzetta ) or Cattle Egret ( Bulbucus ibis ) (Antón-Tello et al 2021, Forti et al 2021b). According to these studies, it is to be expected that the reproductive biology of the Grey Heron in L'Abufera de Vàlencia could have been affected, both in phenology and reproductive success, due to the effect of new practices in rice cultivation, which could also be related to the population declines that have been reported for the area (Generalitat Valenciana 2021, Forti et al 2021a).…”

Diversity and abundance of migratory waterbird communities in natural and artificial agricultural wastewater habitats of New Valley Governorate, Egypt

“…Even within species, waterbirds exhibit significant variation in their dietary choices, and their dietary patterns vary based on patterns of land use (Adhurya et al, 2020;Rajpar et al, 2022;Mott et al, 2023). In addition, waterbirds could alter their dietary composition based on the season and their location (Almeida et al, 2020;Antón-Tello et al, 2021) Red knot feeds on hard shelled mollusks during the winter and soft-bodied arthropods during the summer (Davis & Smith, 2001).…”

High trophic niche overlap in mixed‐species colonies using artificial nests

“…These include Lesser Kestrels, Common Kestrels Falco tinnunculus, European Rollers Coracias garrulus, Western Barn Owls Tyto alba, Little Owls Athene noctua, Western Jackdaws Corvus monedula, Spotless Starling Sturnus unicolor and Feral Pigeons Columba livia . Excluding Feral Pigeons and Western Jackdaws, the dietary habits of these species, mostly comprising large arthropods and small rodents, suggest that their use of trophic resources may overlap to a large extent (Jaksi c et al 1982, Motis et al 1997, Tom e et al 2008, Catry et al 2016a, Orihuela-Torres et al 2017, especially during the breeding season, when competition for food should peak as parents need to obtain resources to fulfil their own energy requirements and those of their offspring (Orłowski et al 2014, Ant on-Tello et al 2021. High densities of breeding pairs could lead to prey depletion in the vicinity of the colonies, further increasing inter-and intraspecific competition (sensu Ashmole's halo, Birt et al 1987, Dehnhard et al 2020, Jenkins & Davoren, 2020.…”

“…2014, Antón‐Tello et al . 2021). High densities of breeding pairs could lead to prey depletion in the vicinity of the colonies, further increasing inter‐ and intraspecific competition ( sensu Ashmole’s halo, Birt et al .…”