‘Central place forager’ is a term used to describe the behaviour of a foraging animal that repeatedly returns to the same place with food (Orians and Pearson, 1979; Lessells and Stephens, 1983). The close analogy between a bird returning to its nest with food (Kacelnik, 1984) and a diving animal returning to the surface to breathe means that the concept of central place foraging can be applied to surfacing divers (Houston and McNamara, 1985). In this case, the surface of the water is the ‘central place’. Thus, models that examine the behaviour of central place foragers can be transposed to examine the behaviour of a diving animal. During foraging bouts, assuming that a diver wants to be under the water acquiring food for as much time as possible, repeatedly returning to the surface for oxygen is (generally) in direct competition with food gain (Ydenberg and Clark, 1989). Optimal foraging models, which are based on the marginal value theorem (Charnov, 1976), have been developed to determine the optimal surface time for oxygen loading that maximises the proportion of time spent foraging (Kramer, 1988; Houston and Carbone, 1992; Carbone and Houston, 1996). These models are based on the physiological gains and losses of oxygen over the dive cycle and therefore the post- dive loading curve is an integral, but as yet unquantified, part of many theoretical studies of optimal time allocation (Thompson et al., 1993; Carbone and Houston, 1994, 1996; Lea et al., 1996; Walton et al., 1998; Mori, 1998, 1999). Because the mammalian lung system rapidly collapses on immersion (Kooyman and Ponganis, 1998), the majority of the oxygen stores of marine mammals consist of haemoglobin and myoglobin. Thus it is likely that the generic shape for the oxygen loading curve proposed and used by Kramer (1988) in his optimal breathing model (Fig. 1) may be correct for most diving mammals. However, Walton et al. (1998) have argued that this shape is not accurate for diving birds. The respiratory system of birds differs from that of mammals and consists mainly of large distended air sacs and a lung, which is comparatively small and rigid (Scheid, 1979). However, the estimated contribution of the air sacs to the total body oxygen stores of diving birds ranges from 23% to 64%, e.g. tufted duck (Keijer and Butler, 1982), thick-billed murre (Croll et al., 1992), lesser scaup (Stephenson, 1995), Adélie penguin (Kooyman and Ponganis, 1998), king penguin (Ponganis et al., 1999). Therefore, it has been suggested that the respiratory system will have a profound influence on the dynamics of post-dive oxygen loading. The oxygen stored within the air sacs of tufted ducks during a dive is made available for consumption through locomotion, and associated abdominal activity. This produces pressure differentials 3945 The Journal of Experimental Biology 205, 3945–3954 (2002) Printed in Great Britain © The Company of Biologists Limited JEB4342 The rate of oxygen uptake at the surface between dives was measured for four tufted ducks, Aythya fuligula, during bouts of foraging dives to a depth of 1.8 m. The ducks surfaced into a respirometer box after each dive so that the rate of oxygen uptake (V . O ∑) could be measured. V . O ∑ decreased over time at the surface and there was a particularly rapid phase of oxygen uptake for approximately the first 3 s. The specific shape of the oxygen uptake curve is dependent upon the duration of the preceding dive. The uptake curve after longer dives was significantly steeper during the first 3 s at the surface than after shorter dives, although V . O ∑ after the first 3 s was not significantly different between these two dive duration bins. Thus, the mean total oxygen uptake (V O ∑) was higher after surface periods following longer dives. Due to the high V . O ∑ during the initial part of the surface period, the curve associated with longer dives was statistically biphasic, with the point of inflection at 3.3 s. The curve for shorter dives was not statistically biphasic. The birds may increase their respiratory frequency during the first 3 s after longer dives, producing the increased V . O ∑, which would enable the birds to resaturate their oxygen stores more rapidly in response to the increased oxygen depletion of the longer submergence time. Key words: tufted duck, Aythya fuligula, diving, oxygen uptake, optimal foraging, model. Summary Introduction Oxygen uptake during post dive recovery in a diving bird Aythya fuligula: implications for optimal foraging models Roland Parkes 1 , Lewis G. Halsey 1, *, Anthony J. Woakes 1 , Roger L. Holder 2 and Patrick J. Butler 1 1 School of Biosciences, and 2 School of Mathematics and Statistics, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK *Author for correspondence (e-mail:lgh013@bham.ac.uk) Accepted 16 September 2002