FORAGING MODES OF CHINSTRAP PENGUINS - CONTRASTS BETWEEN DAY AND NIGHT

Penguins rely on vision to travel and hunt at sea. Vision in marine pr edators, particularly those hunting phototactic prey under a broad ran ge of light intensities, must be better understood to realize how thes e species respond to changes in their environment. We studied the effe cts of daily cycles in light intensity on visual predators by examinin g 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 p enguins that were provisioning chicks during the summers of 1993 and 1 994 at Seal Island, Antarctica. The penguins rarely initiated or termi nated foraging trips at night, but otherwise varied the timing and dur ation of trips to sea. Cluster analyses using departure and arrival ti mes revealed 5 distinct modes of foraging: 3 were strictly diurnal (ea rly, mid-, and late) and 2 were partly nocturnal (overnight and extend ed). Durations of diurnal trips (4 to 11 h) were shorter than overnigh t (13 to 14 h) and extended trips (18 to 22 h). Early and mid-diurnal trips and extended trips were significantly shorter in 1993 than in 19 94; late diurnal and overnight trip durations did not differ between y ears. Diurnal foraging was most common in 1993, whereas overnight fora ging predominated in 1994. Shortened diurnal foraging in 1993 appears to have increased the frequency of diurnal foraging by allowing more p arent birds to alternate diurnal trips within a single day and by redu cing the incidence of birds extending diurnal foraging through the nig ht. That penguins foraged more frequently by day when permitted by sho rter trip durations tin 1993) suggests that they opted to forage diurn ally whenever possible. Returning diurnal and overnight foragers had g reater than 99 and 74% Antarctic krill Euphausia superba by weight in their stomachs, respectively. However, overnight foragers also returne d with significant amounts of highly digested remains of pelagic fish, suggesting birds were in offshore waters taking fish during the night . In contrast, only 1 out of 40 diurnal foragers from both years combi ned had evidence of fish. Thus, the daily light cycle affected both th e timing and duration of chinstrap penguin foraging as well as the typ e of prey consumed during trips to sea.