Richner H. (1995) Wintering Cormorants Phalacrocorax carbo carbo in the Ythan estuary, Scotland: Numerical and behavioural responses to fluctuating prey availability. ARDEA 83 (1): 193-197

Throughout an entire winter season numbers of roosting and foraging Cormorants were surveyed along the Ythan Estuary, Scotland. In this study both bird numbers present and proportion of foraging birds have been related to season, time of day, tidal cycle and section of estuary. An 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 Cormorant numbers in the mornings might reflect the fact that small 'catchable' size classes of Flounder only visit intertidal mudflats at night. Furthermore, the only significant effect of the tidal cycle was that a higher proportion of birds foraged during the ebbing tide. This too is considered to be a behavioural response to prey habits, since Flounders are most likely to be detected by Cormorants at low water levels, while moving from the mudflats into the mid-water channel.