West African Slender-billed Gulls Chroicocephalus genei are considered vulnerable due to a very restricted breeding range. However, little information on their ecology is available to support conservation action. We describe diet and foraging range of Slender-billed Gulls breeding in the Saloum Delta National Park in Senegal. Diet was analysed on the basis of fish otoliths in excretion (mixture of pellets and faeces) collected near nests at the end of the incubation period between 2000 and 2015. Gulls mainly consumed fish of the families Cichlidae (25-93%), Clupeidae (0-54%) and Mugilidae (0-34%). The log-ratio proportion of Mugilidae increased significantly between 2000 and 2015, but there was no significant trend for other prey taxa. In 2014, UvA-BITS GPS-trackers were fitted to three Slender-billed Gulls to study movement and foraging range. During daytime, these gulls spent 27% of their time incubating the eggs, 10% elsewhere in the colony and 63% outside the colony on foraging trips. Foraging trips lasted on average 18 +/- 9.5 (+/- SD) hours. Total distance covered was on average 96 +/- 39 km with a maximum distance to the colony of 37 +/- 13 km. There were marked and consistent individual differences in the flight paths of the gulls. In two birds, foraging mainly took place in mangrove-bordered lagoons, salt pans, creeks, rivers and a complex of abandoned rice fields. The third bird almost exclusively explored the Atlantic coast near a fishing village in The Gambia. The home range and the foraging area of the three birds together measured 2400 and 1800 km(2), respectively. The diet components found near the nests of the three birds could only partly be related to their foraging areas.