RESULTS AND DISCUSSION

The Cape Washington colony, 10 times larger and 7° further south than Pointe Géologie, is not ice bound despite its more southerly location. Instead, the ice edge is routinely within 5 km of the colony.

Based on our observations from our visit on 28 May 1998 at the Cape Washington colony, many, if not most, of the females had not yet laid eggs. Many of the birds were moving about as pairs, and there was much trumpeting by both males and females. The observations are limited, as they usually are for winter studies of emperor penguins.

The body masses of the four captured birds ranged between 25.2 and 28.6 kg. Table 1 summarizes the movement of the satellite transmitter-tagged birds. They remained in the colony from 9 to 23 days. All departed in June may have been females after laying, or non-breeding males and females. Assuming the birds did not feed after tagging and lost mass at 137 g day⁻¹ (Ancel et al., 1997), they would have had a body mass ranging from 23.7 to 25.4 kg. Compared with birds nurturing a chick and foraging from Coulman Island (73.3°S, 169.6°E, 190 km north of Cape Washington) in early November, the average mass of five birds was 26.6 kg (Kooyman and Kooyman, 1995). The transmissions ceased from late July to late August. During the winter darkness, the birds travelled extensively from the Cape (Fig. 1). Bird 6286 travelled the least and was only 56 km from the colony on its last transmission on 25 July. Bird 6286 spent all of its time diving in darkness. The other three birds, 6255, 6256 and 6287, also departed in the June darkness. Their tracks lasted from 11 to 31 August, by which time there were 4–9 h of daylight, depending on their latitude. Although penguins may have an aversion to diving in the dark, they will do so if necessary (Ainley and Ballard, 2012). Our observations indicate that they will dive in the dark in numbers during the autumn and winter when high-latitude colonies are near the ice edge. It is assumed that all transmissions ceased because of battery depletion.

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Fig. 1.

Movement of the four tagged birds from the departure date to the last transmission. Each bird is represented by a different panel, A–D; bird identifier number is in the upper left-hand corner in each case. Locations were calculated from the state/space model (Jonsen et al., 2005). All birds were tagged at Cape Washington as labelled in A. Date labels in A–D are similar: the date of departure is the earliest date label; the last date label is the date of the last transmission. Compass direction is shown in the upper right-hand corner of A, and the scale bar is given in the lower right-hand corner. Hydrographic contours are in the blue section of A–D. There are no depth labels. Location points are indicated by colour-filled circles. Green is 6255, yellow 6256, pink 6286 and red 6287.

The importance of understanding emperor penguin behaviour in colonies located near an ice edge lies in the potential for expanded breeding success in birds with greater foraging opportunity. This possibility is transformative in our knowledge of this species. If the birds can successfully hunt in the dark, it makes sense that they would feed during the prenuptial and pre-laying period. A shortened fast, from 120 to ∼70 days, provides greater assurance that males could successfully endure the entire incubation period without abandoning the egg. Moreover, a female that has recently fed is in better condition for laying, and there is no vigorous effort after laying for the short walk to an ice edge. She also has the challenge of travelling and hunting to forage initially in the dark, moonlight and/or civil twilight. To her advantage, the open water beyond the edge of Cape Washington may form a polynya that in winter extends much further north. If so, the watery path route to the north would enable females to travel faster by swimming, to a zone of longer duration, higher light levels, and possibly richer foraging than exists near the colony. Based on the extensive tracks in the area, the birds seemed in no hurry to travel north.

The number and concentration of emperor penguin colonies in the high latitudes of the Ross Sea are some of the greatest in the Antarctic (Fretwell et al., 2012). The possibility of birds feeding in the winter may help to explain the robustness of some of these colonies compared with those found at lower latitudes along Antarctica's continental shores. In the face of continued warming in the Antarctic, the success of these higher latitude colonies may be vital. As sea ice loss around the northern perimeter of the continent proceeds, the situation may not be so dire for the sea ice dependence of this species (Jenouvrier et al., 2014) as northern birds may shift towards the south. At present, such an emigration may already be occurring as indicated by the sharp rise in the adult population of emperor penguins at Cape Colbeck from 1994 to 2012 (Kooyman and Ponganis, 2016). At nearly 78°S, this is one of the most southerly and presently one of the largest of all emperor penguin colonies.