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.