Do the barrier islands of the Po Delta constitute an ecological trap for colonising Slender-billed Gulls Larus genei?

Abstract The Slender-billed Gull (Larus genei) breeds with a scattered distribution on an extensive nesting area, ranging from India and Afghanistan in the East to the Iberian Peninsula in the West, including the Mediterranean and the Black Sea. A number of habitats are used for breeding, such as sand-spits and beaches along coasts and islands of land-locked seas, steppe lakes, but also brackish or freshwater lagoons near river deltas. Sea level rise dramatically affects coastal sites, thus being the greatest threat to the survival of many seabird species, including gulls. This note describes habitat selection, breeding success and causes of failure of Slender-billed Gulls during their colonisation of the northern Po Delta (NE Italy) during 2018–2022. Slender-billed Gulls colonised the northern Po Delta in 2018. Six colonies were found during the study period. Birds used both natural barrier islands and artificial dredge islands. Productivity in the first five years after the colonisation event was zero, mostly due to colony sites being flooded by high tides and storms. If Slender-billed Gulls will switch to nest in nearby fish farms, these could provide plenty of suitable breeding sites, safe from tidal flooding and with very low predation pressure, allowing sufficient productivity. At the moment, Slender-billed Gulls are unsuccessful in colonising the barrier islands of the Po Delta.