Résumé
We combined radiotelemetry techniques with an automatic nest balance system to obtain detailed, concurrent data on foraging activity, prey consumption and foraging performance in four pairs of Shags Phalacrocorax aristotelis during chick rearing on the Isle of May, Scotland. During the study period the location of the main feeding area changed abruptly from c. 10 km away from the colony (long-range trips, LRT) with return flight times 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) and probably also in the size of prey taken. There were significant differences between trip types in almost every aspect of feeding activity recorded. LRT were characterised by high diving efficiency (the proportion of the dive cycle spent foraging) and low feeding rates (mass of prey caught per second foraging). In contrast, diving efficiency was lower and feeding rates were higher for SRT. The gross intake rate (the product of diving efficiency and feeding rate) was significantly higher in SRT than LRT. We suggest that the observed switch in preferred feeding area was caused by the sudden appearance of large numbers of small lesser sandeels Ammodytes marinus in the waters just off the Isle of May. The higher feeding rates attained by birds on SRT could have been 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.