The chicks stay in their colonies until they are seven to eleven weeks old, depending on their species, while they continue to rely on their parents for food. The penguin chicks, awaiting their parents return, recognize them based on the sound of their call.
But parents won’t give up food so easily – if chicks want to eat, they have to run for their food! Parents will run across the colonies away from their chicks before stopping to feed them, and often will run again once a chick has received its first bite. Some scientists think that this practice is a way for the adult penguins to make sure that both chicks are being fed the food that they need: penguin chicks that aren’t hungry won’t be as eager to chase their parents for food.
Penguins feed their young krill during the summer months, and supplement their diet with fish. While the penguin pairs are caring for their chicks, they will return from foraging every day to make sure that their chicks are well fed. This means that they must forage locally, since they do not have much time to travel long distances, and their survival depends on the availability of food close to the island where they are breeding. In years when krill are plentiful, penguins will stay near their breeding sites so that they can return quickly - but in years when krill are sparse, penguins are forced to travel longer distances to find food, and often their young suffer the consequence.
Adélie and chinstrap penguins that are not caring for chicks, for example during the two-to-ten day incubation stage, are known to spend the entire time foraging at sea. They travel farther and are able to cut down on the commuting costs of traveling back to their nest each day, which allows them to spend more time searching for remote food sources. Scientists also think that penguins that travel farther are able to reach richer prey sources.
While foraging, chinstrap penguins around Cape Shirreff have been found to dive up to 125m in search of food, but usually stay around 25 m. Gentoo penguins are the deepest divers of the three species, and have been known to dive up to 200 m at our research sites, although they normally only dive to 40 m.
The success of Adélie, chinstrap and gentoo penguin populations in the South Shetland Islands depends on the ability of the penguins to survive long enough to reproduce. Depending on the species, these penguins start to reproduce between two and four years of age. Each of these species may live up to twenty years (although most individuals live a much shorter time), and a breeding pair can produce two eggs every season.
However, few of the young produced by a breeding pair will survive. Juveniles that leave their nests for the first time to forage at sea are heavily targeted by leopard seals that rely on penguin breeding sites for a dependable food source. Those penguins that successfully leave their nests to forage at sea are faced with a harsh winter environment, where food is scarce and temperatures are so low that penguins spend a large portion of their energy keeping themselves warm.
Young gentoo penguins also have a unique opportunity among the penguins of the South Shetland Islands to forage alongside adult penguins while they are still being fed by their parents. Adélie and chinstrap penguins, on the other hand, leave their young to learn foraging skill on their own. It is possible that because gentoos learn to forage while still in their parents’ care, they have an advantage that increases their chance of survival.
Figure: Historic penguin population size in the South Shetland Islands.
Approximately 20% of chicks born in one year will survive to adulthood. After that, the survival of the adult penguins depends largely on their ability to survive subsequent winters and avoid predators.
Results from data collected by the Antarctic Ecosystem Research Division (AERD) indicate that Adélie and chinstrap penguin populations have fallen to about one-half of the population numbers seen in the 1970/80s. Evidence suggests that these population declines are due to low over-winter survival rates; for example, nearly 50% fewer juvenile Adélie penguins are surviving their first winters than survived a few decades ago. Since penguins are a long-lived species, their populations will shrink if penguins do not live long enough to reproduce several times.
Gentoo penguin populations have remained steady over the course of the AERD research program, and it is possible that they are even increasing. AERD scientists think that this is partly because they stay local throughout the year, and partly due to their generalist nature: gentoo penguins don’t look for specific nesting sites as Adélie penguins do, and they eat more types of prey than either chinstraps or Adélie penguins.
Little is known about the reasons why young penguins are not surviving to reproduce. AERD scientists have theorized that it could be because of a decrease in krill abundance. Krill depend on sea ice for their survival, and the warming ocean temperatures in the Antarctic Peninsula have resulted in less sea ice and less krill. AERD scientists are also considering whether penguins are competing with other species – whales, pinnipeds, and humans – for krill.
Jefferson Hinke is the leader of penguin research for the AMLR Program.