SUMMARY Large and medium-sized alcids have a very intense wing molt wherein many flight feathers are shed in rapid succession and wing surface area is reduced by as much as 40%. Although these birds are rendered flightless during wing molt, they must still use their wings to propel themselves underwater. A molt-induced loss of wing area could simply reduce wing propulsion such that more muscular work would be required to maintain a given speed. Alternatively,molt could reduce drag on the wings, making a bird more penguin-like and actually enhancing diving ability. I addressed this issue by filming captive common guillemots Uria aalge and tufted puffins Fratercula cirrhata using an array of video cameras to plot the birds' movements in three dimensions. From these coordinate data I calculated swimming velocities,angles of descent and absolute depths. These values allowed me to estimate the forces due to drag and buoyancy that must be counteracted by flapping, which in turn yielded estimates of the amount of work generated during each flap as well as the average power and cost of transport. Within-bird comparisons of diving performance when wings were intact and during several stages of wing molt indicated that molt is associated with more frequent flapping, reduced displacement during the flap cycle, and possibly reduced work per flap. These negative effects on diving may explain why primary and secondary molts were offset in the birds I studied such that the period during which all of the flight feathers are effectively missing is minimized.