Reproductive success in the Great Cormorant Phalacrocorax carbo carbo in relation to colony nest position and timing of nesting

Great Cormorant Phalacrocorax carbo carbo breeding success was studied at two colonies on small islands in Strangford Lough, Northern Ireland in 1993 and 1994. Mean clutch sizes were 3.71 on Bird Island and 3.76 on Black Rock in 1993, and 3.4 on Bird Island in 1994. Total productivity on Bird Island in 1993 was calculated as 1.79 young fledged per nesting attempt. Reproductive success was measured on Bird Island in 1993 using data collected from 121 marked nests, and included clutch size, the number of eggs hatched and the number of young fledged. These data were related to the timing of nesting and the position of the nest within the colony. Early nests tended to be located more centrally in the colony than later nests, which were progressively further from the centre. There was a progressive decline in mean values for each reproductive parameter between early and late nesting birds and with increasing distance from the colony centre.