BIAS ASSOCIATED WITH DIET SAMPLES IN AUDOUINS GULLS

We analyzed five different types of food samples from Audouin's Gull ( Larus audouinii), collected during the breeding seasons of 1994 and 19 95 at its two main breeding colonies, the Ebro Delta and the Chafarina s Islands. These food samples included spontaneous regurgitates, dry b oli containing partially digested food, food remains, pellets, and pre y identified during direct observations of chick provisioning. We comp ared estimates of biomass, levels of taxonomic determination allowed b y each kind of food sample, and the associated potential biases to ass ess which sampling method provides the best estimate of diet in gulls. Regurgitates allowed identification of most prey to species level and reliable biomass estimates, but their collection was time-consuming a nd invasive. Dry boli provided almost the same information as regurgit ates at order level and were easy to collect. However, both underestim ated soft-bodied prey and prey with large, hard parts. Food remains pr ovided an estimate of diet composition that was highly biased towards prey with large distinctive hard parts. However, food remains were a g ood complement to dry boli, enhancing biomass estimates for food items that had a good relationship of weight and linear measurements of pre y-hard parts; Direct observation allowed identification of prey only t o upper taxonomic categories, and is useful when only a broad categori zation of prey types is required. Pellets showed important biases towa rds fish with robust otoliths, and inaccurate conversion to biomass, b ut they can be useful for monitoring variations in the consumption of certain prey items. Several factors such as time spent collecting, sam ple availability, disturbance to animals, and the status of the specie s studied need to be considered when deciding on a method of diet asse ssment sampling.