Between air and water:: the plunge dive of the Cape Gannet Morus capensis

Gannets have one of the most spectacular prey-capture behaviours of all marine predators, plummeting from up to 30 m into the water, where they seize fish with their razor-sharp beaks. However, there is little detailed information on this brief behaviour. In January and December 2002, we monitored the biomechanics of plunge diving in 25 free-ranging Cape Gannets Morus capensis using a recently developed, rapid-sampling acceleration and depth recorder. Our data provide the first detailed description of this highly specialized foraging technique. We recorded no or a very low deceleration when Gannets entered the water, which underlines the remarkable streamlining of this large bird. Birds use their momentum to travel underwater at an average descent rate of 2.87 m/s (sd = +/-1.53) before actively braking once they attain the desired depth (range 0.3-9.7 m). We show that Gannets sometimes used either their wings or feet for underwater propulsion during the course of 9.4% of the dives that had undulations in their depth profiles. After chasing prey, birds developed an upward momentum before gliding passively back to the surface, making use of their buoyancy to complete the dive at the lowest possible energy cost.