Rockhoppers are distinguished from other crested penguins by their smaller size and by having only a thin yellow superscilium. The feather plumes are yellow, not orange as in Macaroni Penguin, and thinner than in the remaining Eudyptes species. The red eye is distinctive. Southern Rockhopper Penguins differ from their Northern counterparts in having a narrower supercilium and shorter plumes, which reach just over the black throat. Their vocalisations are also different. The eastern form mainly differs from the nominate subspecies in having a pink line of fleshy skin along the lower mandible which is black in the nominate subspecies. Immature birds have only a narrow supercilium and a pale mottled grey chin. Identification of juveniles is difficult. Shape of the supercilium, bill shape, body size and underwing pattern can aid identification. Separation of juvenile Southern and Northern Rockhopper Penguins in the field is probably impossible.
These gregarious marine birds are among the world's smallest penguins, standing about 20 inches (50 centimeters) tall. They have blood-red eyes, a red-orange beak, and pink webbed feet. During annual breeding times, rockhoppers gather in vast, noisy colonies, often numbering in the hundreds of thousands, to construct burrows in the tall tussock grasses near shore. They return to the same breeding ground, and often to the same nest, each year, and usually seek out their previous year's mate. Both parents take turns incubating the eggs, aggressively pecking at anything, big or small, that may stray too close. Rockhoppers ply the frigid waters of their range using strong, narrow, flipper-like wings for propulsion. They usually stick to the shallows, but are capable of diving up to 330 feet (100 meters) in pursuit of fish, crustaceans, squid, and krill.
These penguins are among the most numerous on the planet, but their population is in rapid decline. Colonies on the Falkland Islands were once the largest anywhere, but commercial overfishing, pollution, and other factors have cut the penguins' numbers by 90 percent. Breeding colonies on other islands are in trouble as well, and some estimates say rockhopper penguins have declined by more than 30 percent over the past 30 years. They are listed as vulnerable by the IUCN, and if declines continue, they are likely to be uplisted to endangered in the near future.