Space utilization by a cormorant (Phalacrocorax carbo L.) colony in a multi-wetland complex in relation to feeding strategies

In this study, we investigated the response of inland breeding cormorants Phalacrocorax carbo to a complex spatial configuration of feeding habitats in relation to social and individual feeding strategies. The numbers of feeding trips outside the colony site (Lake Grand-Lieu, western France), where only solitary fishing is used by cormorants, and the number of birds fishing on the lake where social fishing predominates were investigated during the breeding season and compared with the fledging period. From the investigation of feeding trip traffic, we identified three major habitats used by cormorants in the vicinity of the colony site (< 25 km around the colony site) that accounted for 94.1 of the IN flights and 92.0% of the OUT flights (n = 1745 arrivals and 2404 departures respectively), and notably one area that accounted for 58% of total flights although it is the furthest away. No fundamental change in the relative significance of these feeding grounds for solitary fishing cormorants was found throughout the breeding season, even in a between-years comparison (1996-2001), in contrast to what has often been found elsewhere. Although the peak of foraging activity in the surrounding habitats and also within the lake waters largely coincided with the time when the majority of young had fledged, the index of cormorant numbers (ratio between bird numbers at a given time and that for a baseline date) on the lake remained at a high level until late August compared to movements outside the lake, as a result of regular social fishing (84.9 +/- 4.0% of fishing numbers). From these findings, we discuss factors governing the selection of feeding grounds throughout the breeding season in relation to energy considerations, feeding strategies and food resources.