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Knowledge of a species' diet and foraging habitat is crucial for understanding its general ecology. Slenderbilled Gulls Chroicocephalus genei mainly occur in the Mediterranean and Black Sea region, in West, Southwest and South Asia, and in West Africa (Del Hoyo et al. 1996). The world population is estimated at c. 100,000 pairs and the species is categorized as 'Least Concern' (Birdlife International 2017). However, the West African breeding population is relatively small, comprising some 7500 breeding pairs (Wetlands International 2006). Part of the Mediterranean population migrates to West Africa in winter, as has been shown by Spanish colour-ringed individuals (B. Piot pers. comm.). However, so far there are no indications that birds from both populations mix during the breeding season. At present, more than 98% of the West African population breeds at three sites: the Saloum Delta and the Estuary of the Senegal River in Senegal, and the Banc d'Arguin

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A frequently used method to estimate diet composition is based on the identification of fish otoliths present in pellets and feces. However, whether pellets and feces provide similar unbiased estimates of the diet remains poorly understood. The diet of African Royal Terns (Thalasseus maximus albididorsalis) breeding in the Parc National du Delta du Saloum, Senegal, was studied. Prey species composition based on otoliths in freshly regurgitated pellets and a mixture of pellets and feces (excrement) accumulated near nests during the incubation period were compared. Altogether, 59 fish species were identified. Pellets contained far less prey species than excrement. Maximum diet overlap between excrement and pellets varied between 0.34 and 0.43 (mean = 0.36). Differences between minimum and maximum overlap between both sample types were small in all years. Pellets contained almost exclusively large otoliths (widths 3.0-8.5 mm), whereas excrement contained two fractions: large sized ones, identical to those present in the pellets and smaller-sized ones (0.5-3.0 mm) originating from feces. It is hypothesized that large otoliths cannot pass the intestinal tracts of the birds and are therefore regurgitated. Differences in prey species composition in pellets and excrement could potentially be explained by a combination of seasonal changes in availability of prey species and size of otoliths. Neither pellets nor feces alone give an unbiased picture of the diet of African Royal Terns.