Rapid acceleration is the key to a successful escape manoeuvre and has attracted considerable research attention in a wide array of taxa. I recorded take-offs of least auklets Aethia pusilla and crested auklets Aethia cristatella with digital video (60 frames per second). To smooth time–location data derived from video, I used predicted mean square error quintic splines, which have been shown to be good predictors of true acceleration. Repeated recordings of the same individual bird allowed me to measure repeatability of take-off acceleration and velocity to find the most robust and biologically meaningful measure. The most repeatable take-off parameters were power at time t=0.17 s after take-off (r=75%) and acceleration at t=0.17 s (r=72%). The horizontal component of velocity at t=0.32 s was least affected by the slope of the take-off trajectory. The mean acceleration of both species is close to expected values based on body mass, even though all previously studied species had considerably lower body mass. Within least auklets, however, I did not find a significant relationship of velocity or acceleration with mass. This would be expected if the observed drop in mass after hatching was an adaptation to reduce the risk of predation. I conclude that acceleration and exerted power at a certain time after take-off is repeatable and the most suitable measure of performance for both inter- and intra-specific comparisons.
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