Emperor penguins, like many seabirds, have evolved a life strategy that is characterised by two main features — first, the birds are long-lived (up to at least 40 years), and second they have a low annual reproductive output. Each breeding pair can produce only one chick per year. Emperor penguins breed during the Antarctic winter and only the male incubates the single egg. Should the egg be lost or the chick die, there is no chance for a pair to breed again in that season. Chick mortality tends to be high once the youngsters leave the colony and head for the ocean. They have to learn very quickly about predators and how and what to hunt. It is not surprising that maybe only a third of each year’s cohort may live to their first birthday.
Every environment is subject to changes that vary from season to season and from year to year. Even in Antarctica where it is always cold, the variations can be quite marked. For example, the number and intensity of storms and blizzards can vary from year to year, as can the time at which the sea-ice forms in autumn or breaks out in summer. These environmental changes are likely to have an influence on the animals that live there. In a year during which the fast-ice extends a long way north it may mean that Emperor penguins have to walk longer distances across the ice before they reach the pack-ice where they can forage.
Any environmental change can also influence the abundance and distribution of prey. Thus, the effort the penguins have to put into gathering food may vary. When studying Emperor penguins we are trying to determine how these changes in the environment affect the penguin populations. Say, for example, we have an especially cold year during which the ice-edge is a long way from the colony, the penguins must travel maybe 80–100km before they can enter the open water. Once they are there the birds may find it much harder to hunt the fish and squid which means that they have to spend more time at sea to get what they need for themselves, as well as for their chicks. In a year like that the chicks have to wait much longer between feeds. Some may be thin and many may not even live long enough to fledge (shed their down, grow real feathers and become waterproof).
Emperor penguins depend on the fast-ice for their long-term survival. Without a breeding platform they have nowhere to go. But it is also important that this platform is available for the duration of the chick rearing period. If global warming alters the patterns of ice break-out or stability, it may be that the fast-ice disappears before the chicks are ready to go to sea. Such a scenario puts the further survival of Emperor penguins at risk.
Using science to determine population change
Our study of Emperor penguins attempts to find out how variable breeding success is from year to year, and whether a population changes over time or remains stable. We do this by counting Emperor penguins twice a year at the colony at Taylor Glacier. The location is ideal because it is not very large and because we can sneak up on the birds from behind a ridge and photograph them without being spotted by the penguins. Pictures are taken around mid-winter (21 June) when the males incubate. That gives us a reasonable idea about the number of pairs that attempted to breed that year. A second round of photographs are taken in late November/early December just before the chicks leave the colony. The number of chicks provides a good indication about how successful the penguins at that colony were in a given year.