H. Richner, WINTERING CORMORANTS PHALACROCORAX-CARBO-CARBO IN THE YTHAN ESTUARY, SCOTLAND - NUMERICAL AND BEHAVIORAL-RESPONSES TO FLUCTUATING PREY AVAILABILITY, Ardea, 83(1), 1995, pp. 193-197
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.