THE FORAGING ECOLOGY OF FEMALE EMPEROR PENGUINS IN WINTER

The foraging location, diving behavior, dietary composition, and feedi ng rates of female Emperor Penguins (Aptenodytes forsteri) from the Au ster and Taylor Glacier colonies in Antarctica were investigated durin g the 1993 austral winter. The study was conducted between late May an d early August, when male emperors remain at the colonies to incubate eggs and females forage at sea for self-maintenance. During winter, tw o satellite-tracked penguins from Auster foraged approximate to 100 km northeast of the colony in open water 200-500 m deep, over the outer continental shelf and shelf slope. Ten Auster and four Taylor Glacier penguins that carried time-depth recorders took approximate to 8 d to reach the ice edge, spent 50-60 d at sea foraging, and took 4 d to ret urn across the fast ice to the colony. The females occasionally huddle d together to minimize heat loss while in transit to the ice edge and between foraging days. The penguins foraged on 93.2% of their days at sea and rested for the remainder. On each foraging day, penguins usual ly entered the water just after dawn and averaged 4.71 h in the water before exiting at dusk. The hourly dive rate was constant throughout w inter, but the daily dive rate increased as day length increased, sugg esting that day length is a primary determinant of hunting effort. Pen guins exhibited behavior indicative of foraging on 47% of their dives, the remainder being travel or search dives. Penguins made, on average , 26 foraging dives/d. Females from Auster targeted prey at water dept hs of 20-70 m and 100-150 m, whereas Taylor Glacier birds targeted pre y at 10-70 m, 250-300 m, and 330-400 m, suggesting between-colony diff erences in prey distribution. The stomach contents of 17 females retur ning to Auster to brood their chicks were dominated by pelagic prey sp ecies: Antarctic krill (Euphausia superba, 70% by mass) and Antarctic silverfish (Pleuragramma antarcticum, 13% by mass). Food consumption r ates during winter for five penguins from both colonies were similar a nd averaged 62.6 +/- 5.8 g.kg(-1).d(-1) (1.8 +/- 0.1 kg/d for a 28.8 k g female), which equated to a metabolizable energy intake of 236.6 +/- 22.0 kJ/kg for each day they foraged (all, values are mean +/- 1 SD). This enabled the birds to gain approximate to 6.1 kg for a trip spann ing approximate to 70 d. Based on mean prey masses and the penguins' d ive rates, the penguins consumed approximate to 115 x 0.6 g krill or 1 6 x 4.3 g Antarctic silverfish per foraging dive, or some combination of both. In their winter trip, each breeding female consumed approxima te to 100 bg of prey. The female populations at Auster and Taylor Glac ier consumed an estimated 1350 Mg and 250 Mg of prey, respectively, in winter.