In waterbirds, formation of mixed-species aggregations has been attributed to social facilitation and might also allow birds to exploit the ability of other species to make prey more accessible, to reduce their own time or effort spent in foraging and correspondingly increase foraging efficiency. However, some participants in mixed-species flocks may not benefit from the association. Dalmatian Pelicans often exhibit mixed-flock fishing with Great Cormorants, but there are no quantitative studies about the pattern and efficacy of mixed-flock fishing in Dalmatian Pelicans. Therefore, we studied the foraging behaviour of 328 Dalmatian Pelicans at the Lakes Mikri and Megali Prespa (Greece), during August 1993 and 2016. We examined the foraging patterns and the foraging success of Dalmatian Pelicans in relation to Cormorants' presence, water depth, Pelicans' age and flock size. Mixed-flock fishing with Cormorants was the dominant foraging behaviour of Pelicans, especially in deep waters and among adult Pelicans. Foraging effort was higher in adult Pelicans, in shallow waters, when Cormorants were absent and when occurring in larger flocks. Moreover, the number of successful dips per minute was higher in adults and when Cormorants were present. Finally, the number of successful prey captures per dip was higher in adults, in deeper waters and when Cormorants were present. However, in all cases interactions between the examined variables affected the observed differences between sexes and foraging conditions. These findings suggest that Dalmatian Pelicans can adjust their feeding tactics to the local habitat characteristics. We advocate that it is a case either of commensalism or of mutualism between the two species.