Along with the Emperor penguin, the Adelie is the only other truly Antarctic penguin. Unable to tolerate air temperatures greater than 20˚C, the Adelie is restricted to colder waters, breeding further south than any other penguin.
The most abundant of all penguin populations, the Adelie numbers close to 2 1/2 million pairs throughout the Antarctic continent. Half of this number are found in the Ross Sea alone. Here at Cape Royds, only 20 miles from McMurdo, over 2000 penguins return each November to mate. Due to the short summers, their breeding cycle is brief, occurring only during the few weeks when temperatures are above freezing and food is abundant in open water found near by.
The annual mating period is extremely noisy as males compete by building rocky nests and calling to females who are searching for a chosen place to lay their eggs. Flipper waving, guttural gossip, and pebble thievery best characterize their annual courtship. Such socialization dies down quickly as eggs are laid by late November and are incubated for 32-37 days. The Adelie penguin will lay two eggs each, but only one can completely fit underneath the birds breast. Many times the outer egg is sacrificed to predators, such as the mightly Skua (the Antarctic sea gull), for the successful incubation of the better protected second egg. Fledging takes 41-64 days, and by three weeks a creche (or pack) of chicks form. By mid February, adults will leave their young and the chicks will gradually move sea. Mortality rate can be quite high, over 35% will not make it to open water. If they survive for the first two to three years, Adelies can live to be 10 - 12 years old.
Both males and females take turns laying on the eggs in between their frequent journeys to the sea for food.
Their travels to the sea ice edge can be tricky as Adelie penguins must negotiate the cracks that form between the sea ice that is fixed to shore and the annual sea ice. These fractures repeatedly open and freeze close from the changes in tides, swells, winds, and temperature.
Many times single penguins will break off from the rest of the pack to find their own way to the sea. Some penguins even have an inner urge to completely leave the rookery and venture to places that no one will never know.
The Adelie penguin's diet consists mainly of ice krill which lives in the shelf waters along the continental edge. Though the penguin can dive up to 450 feet deep, most of their food is caught close to the surface.
Adult Adelies reach a height of approximately 30 inches and weight close to 11 pounds. Male and female penguins share the same physical markings and can only be distinguished by their slight difference in size. Early explorers originally classified these penguins as fish, however, these birds are actually well adapted for their job, to fly in water. Their wings have been reduced to paddles, with the bones flattened. Their wrist and elbow joints fused, so their wings cannot be folded, thereby acting as powerful propellers in the water. Their cruising speed averages four and a half miles per hour. Their feathers are reduced in size and stiffened, with a downy insulating layer at the base which traps air over their thick blubber and skin. Their heads can also retract to make them amazingly hydrodynamic in shape. They use their legs, which are well set back on their body, to rudder their movements while underwater. On land, penguins waddle with a rolling gait and toboggan themselves on the ice and snow to rest their short legs.