J. Avian Biol. 40: 380387, 2009 doi: 10.1111/j.1600-048X.2008.04592.x # 2009 The Authors. J. Compilation # 2009 J. Avian Biol. Received 30 April 2008, accepted 14 October 2008 Underwater wingbeats extend depth and duration of plunge dives in northern gannets Morus bassanus Yan Ropert-Coudert, Francis Daunt, Akiko Kato, Peter G. Ryan, Sue Lewis, Kaori Kobayashi, Yoshihisa Mori, David Gre ´millet and Sarah Wanless Y. Ropert-Coudert (correspondence) and A. Kato, De´pt. Ecol., Physiol. et Ethol., Inst. Pluridscipl. Hubert Curien, CNRS-ULP, 23 rue Becquerel 67087 Strasbourg, France. Email: yan.ropert-coudert@c-strasbourg.fr.  F. Daunt, S. Lewis and S. Wanless, Centre Ecol. and Hydrol., UK.  P. G. Ryan, Percy FitzPatrick Inst., Univ. of Cape Town, Rondebosch, South Africa.  K. Kobayashi, Natl. Inst. of Polar Res., 1-9-10 Kaga Itabashi-Ku, Tokyo 173-8515 Japan.  Y. Mori, Dept. of Anim. Sci., Teikyo Univ. of Techn. and Sci., Japan.  D. Gre´millet, Centre d’Ecol. Fonction. et Evol., CNRS, Montpellier, France. Plunge-diving is a specialised hunting tactic used by some avian predators to overcome the high buoyancy encountered near the water surface and surprise prey. However, plunge-diving is effective only to a certain depth; to access deeper prey, birds need to use an additional method of propulsion, e.g. wings or feet. We used miniature accelerometers to record the details of the aerial and underwater phases of plunge dives by northern gannets Morus bassanus. Birds never reached depths 11 m using the momentum of the aerial part of the plunge dive and had to flap their wings underwater to gain additional depth. A biomechanical model demonstrates that little additional depth can be obtained from momentum alone when initiating a plunge from heights 40 m. Thus, the additional energy required to attain greater starting heights is not rewarded by reaching significantly greater depths. However, by using their wings underwater, gannets were able to more than double the depth attained (up to 24 m). It appears that prey may be captured by surprise in the first 10 m of the water column, whereas wing-propelled pursuit is required to catch prey at deeper depths, a strategy likely to be used only for prey of sufficient profitability to justify the cost of flapping the gannet’s large wings underwater. Our study demonstrates the importance of understanding the constraints placed on predators by the physical environment when interpreting predator-prey interactions. Predators have evolved a wide variety of hunting techniques to optimize prey detection, pursuit and capture (see Pfeffer 1989). Plunge-diving is an example of a highly specialised hunting strategy, where animals spot their prey from an elevated point and dive towards it at great speed to surprise and capture it. The technique is restricted to a few avian genera and plunge-diving species have evolved a series of features that enhance prey detection, minimise drag in air and water and provide protection from injury during rapid deceleration on impact with water (Nelson 1978, Elliot 1992). Among terrestrial birds, only some raptors (hawks, falcons, etc.) rely on aerial dives to capture their prey but plunge-diving is more widespread in water birds, such as gannets and boobies, terns, tropicbirds and kingfishers, especially those species that feed on mobile prey such as fish. Individuals plummet into the water from an elevated perch or in flight, adopting an arrow-like shape to minimise impact on entry (Ropert-Coudert et al. 2004a). This technique is particularly relevant to highly buoyant birds like gannets that are shaped for flight rather than under- water swimming (Wilson et al. 1992). Plunging from the air not only allows gannets to startle their prey (Nelson 1978), but also to travel efficiently through the first few metres of the water column, where the effects of buoyancy are greatest (Wilson et al. 1992). Intuitively, plunging birds can be expected to respond to a deeper distribution of prey by plunging from a higher elevation. However, studies have shown that northern gannets Morus bassanus, the deepest diving sulid species (maximum recorded depth 35 m; Brierley and Fernandes 2001) sometimes use their wings underwater to pursue and catch prey in addition to plunge-diving (Garthe et al. 2000, Ropert-Coudert et al. 2004a). Garthe et al. (2000) classified northern gannet dives into two categories based on their depth profiles: short, shallow dives without wing flapping to exploit prey near the surface, and longer, deeper dives with wing assisted propulsion to target deeper prey. Northern gannets may use wing flapping because it is a more energetically efficient method than plunging for pursuing prey at deeper depths. Alternatively it may be the only method of attaining deeper depths if there is a physical limit on depth attained from plunging alone. Quantifying the details of the switch between these two different locomotory modes is critically important in understanding the interac- tions between gannets and their prey. However, the data- loggers used by Garthe et al. (2000) did not have the 380