Patrick Butler University of Birmingham, UK Title To what extent is foraging behaviour of aquatic birds constrained by their physiology? Abstract Aquatic birds have access to a limited amount of usable O2 when they forage (dive) underwater, so the major constraint is the need periodically to visit the water surface to replenish these stores and remove accumulated CO2 . The size and the rate at which the O2 stores are used (VO2) are the ultimate determinants of the duration that aquatic birds can remain underwater at the food source, without resorting to anaerobic metabolism. However, the assumption that the decision to terminate a dive is governed solely by the level of a bird’s O2 stores is not always valid. The VO2 of tufted ducks during diving is about 3 times the resting value, but they dive well within their calculated aerobic dive limit (cADL, total usable O2 stores/VO2 during diving). Furthermore, the rates of CO2 production during dives and removal during surface periods, seem to influence the duck’s foraging behaviour. Nevertheless, behaviour is still consistent with the predictions of diving models based on the balancing of oxygen stores during the dive cycle. Some species, such as macaroni and emperor penguins, dive for durations closer to their cADL and may organize their diving to maximize the proportion of a diving bout spent at the food source. In contrast, many species, such as gentoo and king penguins, appear to remain submerged for excessive durations, with about 20% of their dives exceeding the cADL, even if VO2 during diving is assumed to be the same as that of birds resting in water. Biography Patrick Butler is Mason Professor and Professor of Comparative Physiology at the School of Biosciences, University of Birmingham. He published 143 articles in peer refereed journals and 48 review articles. In 1982 he received the Scientific Medal of the British Zoological Society. He is on the editorial board of the Proceedings of the Royal Society B, Physiological and Biochemical Zoology, and the Journal of Comparative Physiology B. Selected recent publications Butler, P.J. 2004. Metabolic regulation in diving birds and mammals. Resp. Physiol. Neurobiol. 141, 297-315. Butler, P.J., Green, J.A., Boyd, I.L. and Speakman, J.R. 2004. Measuring metabolic rate in the field: the pros and cons of the doubly-labelled water and heart rate methods. Func. Ecol. 18, 168-183. Froget, G., Butler, P.J., Woakes, A.J., Fahlman, A., Le Maho, Y. and Handrich, Y. 2004. Heart rate and energetics of free-ranging king penguins (Aptenodytes patagonicus). J. exp. Biol (in press). Fahlman, A., Handrich, Y., Woakes, A.J., Bost, C-A., Holder, R., Duchamp, C. and Butler, P.J. 2004. The effect of fasting on the and ƒH relationship in king penguins, Aptenodytes patagonicus. Am. J. Physiol. 287, R870-R877. Butler, P.J. and Jones, D.R. 1997. The physiology of diving of birds and mammals. Physiol. Rev. 77, 837-899.