The locomotory muscles of the tufted duck, Aythya fuligula (L.), were analysed for mass, aerobic and anaerobic enzyme activities, fibre-type proportions, capillarity, mitochondrial and myoglobin content. The estimated aerobic capacity of the muscles correlated well with the muscles' maximal oxygen uptake both when measured during swimming and when predicted for steady-state flight. The results suggest that exercise performance in birds cannot be predicted purely on the basis of muscle mass (see Butler & Woakes, 1985); the specific enzyme complement of each muscle must also be taken into account. The delivery of oxygen to mitochondria is facilitated by the dense capillarity and high myoglobin content of the muscles.

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