Alcids propel themselves by fl apping wings in air and water that have vastly diff erent densities. We hypothesized that alcids change wing kinematics and maintain Strouhal numbers ( St ϭ fA / U , where f is wingbeat frequency, A is the wingbeat amplitude, and U is forward speed) within a certain range, to achieve effi cient locomotion during both fl ying and swimming. We used acceleration and GPS loggers to measure the wingbeat frequency and forward speed of free-ranging rhinoceros auklets Cerorhinca monocerata during both fl ying and swimming. We also measured wingbeat amplitude from video footage taken in the wild. On average, wingbeat frequency, forward speed, and wingbeat amplitude were 8.9 Hz, 15.3 m s -1 , and 0.39 m, respectively, during fl ying, and 2.6 Hz, 1.3 m s -1 , and 0.18 m, respectively, during swimming. Th e smaller wingbeat amplitude during swimming was achieved by partially folding the wings, while maintaining the dorso-ventral wingbeat angle. Mean St was 0.23 during fl ying and 0.36 during swimming. Th e higher St value for swimming might be related to the higher thrust force required for propulsion in water. Our results suggest that rhinoceros auklets maintain St for both fl ying and swimming within the range (0.2 -0.4) that propulsive effi ciency is known to be high and St in both fl ying specialists and swimming specialists are known to converge.

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