Strategies for efficient foraging in a deep-diving bird, the imperial shag (Phalacrocorax atriceps)

• Main Author: Shephard, Emily Laura Cairns
• Published: Swansea University 2009
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.752172

Predators are frequently involved in an arms race with their prey, with improved abilities on one side demanding compensatory improvements on the other. Those that breathe air but forage underwater are faced with the additional challenge of capturing prey in a medium where their own capacity to remain is limited. This thesis examines some of the strategies used by my model organism, the imperial shag (Phalacrocorax atriceps) to enhance its foraging efficiency. I did this by using recent developments in animal-attached technology to measure the patterns and costs of bird behaviour during foraging at a fine-scale. Time appeared to be of the essence for these birds, as their movements were consistent with a strategy to maximise the rate of energy gain. Male and female shags were found to forage at depths where their foraging efficiency was maximised, which manifested itself in the horizontal segregation of male and female foraging areas. Analysis of the mechanical power used underwater suggested that these birds may be limited in the burst speeds they can produce at shallow depths; as the greater the power required to counteract their buoyancy the less is available for prey pursuit. Finally, analysis of the fine-scale tortuosity in the foraging movements of imperial shags revealed that the distribution of their prey was not aggregated at the scales over which they forage. Nevertheless, tortuosity was a good indicator of prey ingestion rates and revealed that shags adjusted their movements to recent prey encounter within both prey-searching and resting phases. This work indicates that imperial shags have an extensive armoury of strategies by which they may increase their efficiency as underwater predators, and methods used and refined in this thesis mean that users are now well-equipped to investigate them.