AVIAN DIVING, RESPIRATORY PHYSIOLOGY AND THE MARGINAL VALUE THEOREM

Behavioural studies of diving birds have reported that the ratio of di ve duration to the duration of the subsequent period on the surface di splays a characteristic relation to dive duration. For short dives, th e dive to surface ratio increases with dive duration, whereafter the r elation peaks, and for longer dives decreases with increasing dive dur ation. Such a relationship is not a general prediction of existing mar ginal value models which have been used to predict optimal diving beha viour. This may be because the smooth curve used to describe the oxyge n gain rate of individuals after surfacing is not a good reflection of the respiratory physiology of birds. Here we argue that on physiologi cal grounds, the oxygen gain curve for avian divers will not be smooth , but will have two distinct regions (representing oxygen recovery in the respiratory tract, and in haemoglobin and myoglobin, respectively) . Modifying two of the classical diving models by incorporating such a kinked curve causes them to predict the humped relationship between d ive to surface ratio and dive duration under many circumstances. We al so present data on the duration of dives and surface periods from thre e species of diving seabirds: the shag, Phalacrocorax aristotelis, the black guillemot, Cepphus grylle and the common guillemot, Uria aalge. All three species showed a humped relationship for dive to surface ra tio as a function of dive duration. In line with the predictions of ou r model, when oxygen stores on surfacing were greatly depleted, the di ve to surface ratio peaked at short dive durations. (C) 1998 The Assoc iation for the Study of Animal Behaviour.