I constructed a diving model for captive Tufted Duck Aythya fuligula foraging in shallow water in the manner of Little Grebe Tachybaptus ruficollis. This model was based on the assumption that part of the total oxygen uptake at the water surface is used for the bird's own metabolism during the diving interval and the remaining part stored in the lungs and air sacs is allocated to underwater diving. Oxygen utilization efficiency, η, is introduced and defined as the ratio of the oxygen used for diving to the total oxygen uptake. Here, η is formulated by using modified allometric equations relating avian respiratory variables, such as the tidal volume, VT1, and ventilation rate, Ves, in addition to the metabolic rate during the diving interval, EMR. On the basis of the oxygen utilization efficiency, η, thus derived, the diving interval, Tp, diving period, Td, maximum diving period, Tdmax, and average metabolic rate during diving interval and diving period, MRd, can be predicted. In estimating the diving period, Td, a total energy loss per unit time during diving is assumed to be a sum of the metabolic rate during the diving interval, EMR, and energy losses in relation to buoyancy, hydrodynamic drag, and forced convection heat transfer. Comparison of the estimated values of Tp, Td, Tdmax, and MRd with their respective measured values reported in literature is made to validate the modified allometric equations, the formula of η, and the assumed total energy loss per unit time during diving.
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