Buoyancy and body drag were measured in lesser scaup (Aythya affinis) and the data were used to estimate average power output during diving and surface swimming. Buoyant force (mean±s.D.) of fully submerged ducks was 2.89±0.17 N (body mass 0.623±0.089kg; body volume 918±88cm3; N=11). Buoyancy was decreased by 6.2° by artificial compression of the feathers during full immersion, but was reduced by 42° when the ducks were allowed to breathe during head-out immersion. Therefore, voluntary compression of the plumage by the duck appears to have relatively small effects on buoyancy and hence dive costs, whereas alteration of respiratory volume (e.g. by pre-dive expiration) could substantiallyalter buoyancy and power requirements.

Surface and subsurface body drag (DSUR and DSUB respectively, in newtons) of frozen duck carcasses increased with velocity (U, ms−1) as follows:

DSUR= 0.239-1.292U+2.027U2 (r2 = 0.965),

DSUB = - 0.144+0.562U + 0.622U2 (r2 = 0.980).

Work required to overcome body drag is greater for a lesser scaup during diving than during surface swimming at average velocities normally attained during these activities (less than 0.7 ms−1). However, the drag force curves merge at 0.8-l.0ms−1.

It is calculated that the average power output during diving ranges from 1.003 to 1.695 W and that in ducks at least 95° of the work done during a dive is required to overcome buoyancy. Comparison of these biomechanical estimates with aerobic metabolic power input (Voo2) data reported by Woakes and Butler (1983) indicates that, for freely diving ducks, aerobic efficiency (ηa=average power output/total aerobic power input) is 0.088–0.149 and net aerobic efficiency [ηnet=average power output/(total aerobic power input minus resting aerobic power input)] is 0.124–0.209. These values are significantly greater than those during surface swimming at the same velocities (ηa=0.004–0.037, ηnet=0.039–0.063).

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