CapsuleThe winter diet of Common Kingfishers Alcedo atthis using two rivers in central Spain was 94.9% fish, all of which were non-native species.AimsTo determine the composition Common Kingfisher diet in an area recently colonized by non-native fish species, and to compare estimated prey size during the winter to that of fish fed to nestlings during the breeding season.MethodsThe diet was studied by analysis of pellets collected from the River Duero (lentic, still water) and River Pisuerga (lotic, riverine) in central Spain. Fish size (total length and biomass) calculated from paired remains contained in each pellet.ResultsPrey items were almost exclusively fish (94.9% of all prey), with a small proportion of non-native Red Swamp Crayfish Procambarus clarkii (4.9%) and aquatic insects (0.2%). The diet consisted of only five species of fish, all of which were non-native. The Common Bleak Alburnus alburnus and Eastern Mosquitofish Gambusia holbrooki were the prey captured most often in the River Pisuerga and River Duero, respectively. Estimated length of fish prey ranged from 1.5 to 10.5 cm, but 53.6% of individuals were between 3 and 5 cm. Mean biomass of the fish consumed was also small (1.08 g per fish). Mean values of both fish total length and biomass were lower than the measurements reported elsewhere in Europe during the breeding season.ConclusionNative fish species have disappeared from the diet of Common Kingfishers in our study area, probably because the non-native species are more common.