Yellow-eyed penguin

The yellow-eyed penguin is most easily identified by the band of pale yellow feathers surrounding its eyes and encircling the back of its head.[10] Its forehead, crown and the sides of its face are slate grey flecked with golden yellow.[11] Its eye is yellow.[2][10] The foreneck and sides of the head are light brown.[2] The back and tail are slate blue-black.[2][11] Its chest, stomach, thighs and the underside of its flippers are white in colour.[2]

It is the largest living penguin to breed on the mainland of New Zealand.[2] It stands 62–79 centimetres (24–31 in) tall and weighs 3–8.5 kilograms (6.6–18.7 lb).[2][12] Weight varies throughout the year, with penguins being heaviest just before moulting, during which they may lose 3-4 kilograms in weight.[13] Males (5.5 kg) are heavier than females (5.25 kg).[12][14]

Juvenile birds have a greyer head with no yellow band around their eyes.[10]

The yellow-eyed penguin may be long lived, with some individuals reaching 20 years of age. Males are generally longer lived than females, leading to a sex ratio of 2:1 around the age of 10–12 years.[15]

The yellow-eyed penguin is mostly silent.[2] It makes a shrill bray-like call at nest and breeding sites.[3]

Hoiho are found in two distinct populations, known as the northern and southern populations.[16]

The northern population extends along the southeast coast of the South Island of New Zealand, down to Stewart Island and Codfish Island.[2] It includes four main breeding areas in Banks Peninsula, North Otago, Otago Peninsula and the Catlins. It may also be referred to as the mainland population.[3]

The southern population includes the Subantarctic Auckland Islands and Campbell Island.[16]

There is little gene flow between the northern and southern populations as the large stretch of ocean between the South Island and Subantarctic region and the subtropical convergence act as a natural barrier.[17]

Around 90% of the yellow-eyed penguin's diet is made up of fish, chiefly demersal species that live near the seafloor (e.g. blue cod (Parapercis colias), red cod (Pseudophycis bachus), opalfish (Hemerocoetes monopterygius) [18]). Other species taken are New Zealand blueback sprat (Sprattus antipodum) and cephalopods such as arrow squid (Nototodarus sloanii). Recently, jellyfish were found to be targeted by the penguins. While initially thought that the birds would prey on jellyfish itself [19] deployments of camera loggers revealed that the penguins were going after juvenile fish and fish larvae associated with jellyfish.[20]

Breeding penguins usually undertake two kinds of foraging trips: day trips where the birds leave at dawn and return in the evening ranging up to 25 km from their colonies, and shorter evening trips during which the birds are seldom away from their nest longer than four hours or range farther than 7 km.[21] Yellow-eyed penguins are known to be an almost exclusive benthic forager that searches for prey along the seafloor. Accordingly, up to 90% of their dives are benthic dives.[21] This also means that their average dive depths are determined by the water depths within their home ranges.[22]

A family of yellow-eyed penguin at the Penguin Place Lodge at Otago Peninsula, Dunedin, New Zealand

Whether yellow-eyed penguins are colonial nesters has been an ongoing issue with zoologists in New Zealand. Most Antarctic penguin species nest in large high density aggregations of birds. For an example see the photo of nesting emperor penguin. In contrast yellow-eyed penguins do not nest within visual sight of each other. While they can be seen coming ashore in groups of four to six or more individuals then disperse along track to individual nests sites out of sight of each other. The consensus view of New Zealand penguin workers is that it is preferable to use habitat rather than colony to refer to areas where yellow-eyed penguins nest. Nest sites are selected in August and normally two eggs are laid in September. The incubation duties (lasting 39–51 days) are shared by both parents who may spend several days on the nest at a time. For the first six weeks after hatching, the chicks are guarded during the day by one parent while the other is at sea feeding. The foraging adult returns at least daily to feed the chicks and relieve the partner.

After the chicks are six weeks of age, both parents go to sea to supply food to their rapidly growing offspring. Chicks usually fledge in mid-February and are totally independent from then on. Chick fledge weights are generally between 5 and 6 kg.

First breeding occurs at three to four years of age and long-term partnerships are formed.

Calling at Curio Bay, New Zealand

In spring 2004, a previously undescribed disease killed off 60% of yellow-eyed penguin chicks on the Otago peninsula and in North Otago. The disease has been linked to an infection of Corynebacterium, a genus of bacteria that also causes diphtheria in humans. It has recently been described as diphtheritic stomatitis. However, it seems as if this is just a secondary infection. The primary pathogen remains unknown. A similar problem has affected the Stewart Island population.[29]