WINTERING CORMORANTS PHALACROCORAX-CARBO-CARBO IN THE YTHAN ESTUARY, SCOTLAND - NUMERICAL AND BEHAVIORAL-RESPONSES TO FLUCTUATING PREY AVAILABILITY

Throughout an entire winter season numbers of roosting and foraging Co rmorants were surveyed along the Ythan Estuary, Scotland. In this stud y both bird numbers present and proportion of foraging birds have been related to season, time of day, tidal cycle and section of estuary. A n earlier study had revealed Flounder to be the Cormorant's main prey species in the area. A sharp decrease in bird numbers from October to January coincides with lower Flounder abundance, while higher Cormoran t numbers in the mornings might reflect the fact that small 'catchable ' size classes of Flounder only visit intertidal mudflats at night. Fu rthermore, the only significant effect of the tidal cycle was that a h igher proportion of birds foraged during the ebbing tide. This too is considered to be a behavioural response to prey habits, since Flounder s are most likely to be detected by Cormorants at low water levels, wh ile moving from the mudflats into the mid-water channel.