FORAGING ACTIVITY AND PERFORMANCE OF SHAGS PHALACROCORAX-ARISTOTELIS IN RELATION TO ENVIRONMENTAL CHARACTERISTICS

We combined radiotelemetry techniques with an automatic nest balance s ystem to obtain detailed, concurrent data on foraging activity, prey c onsumption and foraging performance in four pairs of Shags Phalacrocor ax aristotelis during chick rearing on the Isle of May, Scotland. Duri ng the study period the location of the main feeding area changed abru ptly from c. 10 km away From the colony (long-range trips, LRT) with r eturn flight limes from the feeding area to the colony of 7-14 min to c. 0.8 km away (short-range trips (SRT), return flight times <3 min). The two feeding areas differed in water depth (LRT dives shallower) an d probably also in the size of prey taken. There were significant diff erences between trip types in almost every aspect of feeding activity recorded. LRT were characterised by high diving efficiency (the propor tion of the dive cycle spent foraging) and low feeding rates (mass of prey caught per second foraging). In contrast, diving efficiency was l ower and feeding rates were higher for SRT. The gross intake rate (the product of diving efficiency and feeding rate) was significantly high er in SRT than LRT. We suggest that the observed switch in preferred f eeding area was caused by the sudden appearance of large numbers of sm all lesser sandeels Ammodytes marinus in the waters just off the Isle of May. The higher feeding rates attained by birds on SRT could have b een due to higher concentrations of prey and/or reduced foraging costs associated with unsuitable habitat for sandeels in this area. Results from this study suggest that the distribution of available resources influences the choice of foraging areas in Shags.