Penguins rely on vision to travel and hunt at sea.
Vision in marine predators, particularly those hunting phototactic prey under a broad range of light intensities, must be better understood to realize how these species respond to changes in their environment. We studied the effects of daily cycles in light intensity on visual predators by examining the duration and timing of chinstrap penguins' Pygoscelis antarctica foraging trips and the size, composition, and timing of their meals. We used radio telemetry and stomach-contents sampling to study adult penguins that were provisioning chicks during the summers of 1993 and 1994 at Seal Island, Antarctica. The penguins rarely initiated or terminated foraging trips at night, but otherwise varied the timing and duration of trips to sea. Diurnal foraging was most common in 1993, whereas overnight foraging predominated in 1994.