The slender-billed gull Chroicocephalus genei is a typical saltpan and brackish lagoon inshore forager. In 2005, there were two breeding colonies in the Ebro Delta (western Mediterranean): one south of the delta on a large saltpan, and the other one in the north, where typical foraging habitats are scarce. Aerial surveys showed that gulls foraged at sea throughout the study period, but were mainly restricted to the northern part of the delta from June to August. We compared 6 different approaches to model the gulls" distribution at sea: Poisson generalized linear model (GLM), zero inflated Poisson GLM, spatial Poisson generalized linear mixed model (GLMM), non-spatial Poisson GLMM, negative binomial GLM and zero inflated negative bi -nomial GLM. The best approach, simple negative binomial GLM, suggested that the gulls" distribution was mainly influenced by the proximity to the colony, tern density and water temperature. The water temperature was indicative of river runoff, which resulted in an anchovy population explosion, explaining the gulls" extensive foraging at sea in summer. Slender-billed gulls are capable of changing their foraging strategy to exploit resources at sea when occupying an area with limited terrestrial but abundant marine resources. Population growth in the western Mediterranean may lead to the occupancy of non-typical habitats elsewhere. Because slender-billed gulls are rare and have an unfavorable conservation status, conservation policies affecting this species should be extended to the sea.