Predicted seabird distributions in the North Sea: the consequences of being hungry

This paper describes a model of the physiology of feeding of a generic seabird. The bird is represented as two types of tissue - structural tissue and reserve tissue - and the ingestion of the food is controlled by a Holling type II functional response. The food passes into the gut, from where it is cleared by a process of exponential decay and is incorporated into the reserve tissue. The reserve tissue supplies the energy required for maintenance and movement, modelled as a power function of the total body mass, with parameters appropriate to the current activity such as flying or surface feeding. A behavioural rule is added to the physiological model. This causes the bird to remain feeding as long as the mass of reserve tissue is increasing. When the bird ceases to feed, it chooses an adjacent feeding area at random and moves to it. This rule can be shown formally to generate foraging behaviour that converges to that predicted by optimal foraging theory. Currently, the model represents the behaviour of an adult seabird in the phase of life before it begins to breed. This paper presents the model parameterized for the kittiwake, foraging over the North Sea, the matrix of patches of potential food being represented by 220 cells, the ICES statistical rectangles, and describes the expected distribution of kittiwakes that would arise if the population fed exclusively on sandeels, Ammodytes spp. The main difference between the predicted distributions and the observed distributions is that the observed kittiwakes are more closely associated with the coast during the breeding season. (C) 1997 International Council for the Exploration of the Sea.