SHIFTS IN THE DIET OF THE GREY HERON (ARDEA-CINEREA) IN THE DUERO BASIN, CENTRAL-WEST SPAIN, FOLLOWING THE INTRODUCTION OF EXOTIC FISH SPECIES

The diet of the grey heron (Ardea cinerea) in the Duero basin (west-ce ntral Iberian peninsula) was studied through analysis of pellets colle cted at two nesting localities during 1990 and 1991. Fishes were the d ominant food item (62.9% in frequency occurrence and 90.1% by fresh we ight), with carp Cyprinus carpio, at 38% and bocage barbel Barbus boca gei at 10%. A comparison of the heron's diet during 1990-91 and 1986-8 7 in the same basin, revealed that the proportions of American crayfis h (Procambarus clarkii) and pumpkinseed (Lepomis gibbosus), both intro duced species to the River Duero, were higher in 1990-91. Thus, the di et of the grey heron in the Duero basin has changed in only 2-3 years.