by Mike Bingham
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Gentoo PenguinPygoscelis papua
Breeding Range: Subantarctic islands and Antarctic Peninsula
World Population: 320,000 breeding pairs
World-wide there are about 320,000 breeding pairs of Gentoo penguins, with breeding colonies on the Antarctic Peninsula, and the islands of Falklands, South Georgia, Kerguelen, Heard, South Orkney, Macquarie, Crozet, Prince Edward and South Sandwich.
Gentoo penguins have an average length of 80cm and an average weight of 5kg. They have a reddish orange bill, apart from the black culminicorn, and orange feet. White patches above each eye meet across the crown, with white speckling in the adjacent black plumage around the head. Females are slightly smaller than the males, but have similar markings.
Colonies rarely comprise of more than a few hundred breeding pairs, breaking up into smaller subcolonies when numbers exceed this. The preferred nesting sites are low coastal plains, fairly close to a sandy or shingle beach, which is used to gain access to the open ocean. A substantial amount of guano and waste accumulates around the nesting area during the breeding season, and colonies usually move a short distance onto fresh ground each season, retaining the same path to the sea.
Gentoo penguins are ground nesting birds, making rudimentary nests from stones, sticks, grass, feathers, or practically any material that they can find suitable for the purpose. Egg-laying is usually completed by late October, with two equally sized eggs of about 130g being laid. Incubation takes about 34 days, with both parents sharing incubation duties, and nest changes occurring every 1 to 3 days. Despite the two eggs being laid 4 days apart from each other, they both hatch within the space of 24 hours.
The young chicks remain in the nest until they grow their mesoptile plumage at about 3 to 4 weeks of age. During this period both parents brood the chicks alternately, feeding the chicks and changing over on a daily basis. Adults usually set out to forage in the early morning, returning later the same day, and foraging generally occurs within 20km of the breeding site. The time spent foraging increases as chicks get larger, and their demand for food gets greater.
After the brood period, chicks are able to leave the nest and form into large creches, allowing both parents to collect food to meet the ever increasing demand. The mesoptile plumage has similar markings to the adult plumage, except that the dark areas are a browny grey rather than black, and there is no white head patch.
Gentoo penguins put equal effort into raising both chicks, and have the ability to produce large numbers of chicks in seasons of high food availability. During such seasons of plenty, even deformed chicks which are unable to walk properly, may be reared to the point of fledging. By contrast, when food is scarce there is strong competition for food between chicks, and only the strongest survive. Adults are often observed running through the colony, closely pursued by one or two hungry chicks. This may well be part of the selection procedure, whereby the strongest, hungriest or most determined chick gets fed.
Chicks fledge at around 14 weeks of age, but may continue to be fed by the parents for several weeks after fledging. After completion of the breeding season, adults spend time at sea building up body fat reserves prior to undergoing their annual moult. The moult takes around 2 to 3 weeks, and during this time birds spend considerable amounts of time tending to their plumage. Gentoo penguins do not allopreen.
Gentoo penguin populations are characterised by large annual fluctuations in population size and breeding success, with the later ranging between 0.5 and 1.5 chicks fledged per breeding pair. Gentoo penguins are capable of breeding at just 2 years of age.
Because Gentoo penguins at most sites tend to move the colony a few metres each year, they do not retain the same nests from year to year. On occasions whole colonies that have remained at one site for years, will up and move to a new site many kilometres away, for no apparent reason. This may happen suddenly during a single year, or gradually over a number of years.
By comparison with other penguins, Gentoo pair-bonds are often long-lasting, despite annual nest changes. Many adults remain around the colony throughout the year, whilst others take the opportunity during the winter months to make longer foraging trips further afield.
Gentoo penguins generally forage close to shore at depths of 20 - 100m, although they have been recorded diving to depths of more than 200m. They may make as many as 450 dives during a single days foraging. Penguins all look clumsy on land, but in fact Gentoo penguins can out-run a man over short distances, and often situates its colony 1 or 2 kilometres from the sea.
Gentoo penguins are opportunistic feeders, and around the Falklands are known to take roughly equal proportions of fish (Patagonotothen sp., Thysanopsetta naresi, Micromesistius australis), crustaceans (Munida gregaria) and squid (Loligo gahi, Gonatus antarcticus, Moroteuthis ingens).
At sea, Gentoo penguins are subject to predation by sea lions, leopard seals and orcas. On occasions sea lions have been known to come inland after penguins, and even fur seals can disrupt breeding colonies on occasions. Nevertheless such incidents are rare, and Gentoo colonies are usually placed far enough inland to avoid such threats.
On land healthy adults have no natural predators, but skuas, gulls and birds of prey, such as caracaras, will steal eggs and small chicks if they get the opportunity. Chicks are also at risk from fluctuations in food supply and weather. Mesoptile plumage provides good insulation when dry, but if it becomes saturated by prolonged rain, chicks can die from hypothermia. By contrast in periods of very hot weather, chicks become too hot, and may die from heat stress.
The Falklands is the only Gentoo penguin breeding site with human habitation. Although farming has greatly modified the landscape around the Falklands, Gentoo penguins prefer open plains to breed, and consequently have not been greatly affected by the loss of the tall tussac grass. Gentoo penguins are also very tolerant of grazing animals, such as sheep, cattle and horses, which often wander around Gentoo colonies without causing alarm.
The expansion of roads throughout the Falklands, along with the increase in resident population and tourism, has greatly increased the level of disturbance at many Gentoo colonies. Nevertheless, studies of population numbers and breeding success show no evidence that Gentoo penguins are at risk from current levels of disturbance. Gentoo penguins become tolerant of human presence, and do not generally become alarmed unless people approach within about 15m of the nest site.
For many years the rural communities of the Falkland Islands took Gentoo eggs for food. Until recent years these eggs were an important supplement to the diet of many folk, but now with regular supplies of hen eggs the tradition is gradually dying out. Penguin eggs are always taken at the start of incubation, and the birds rapidly re-lay, so that colonies which have had eggs taken show little difference in productivity by the time chicks are ready to fledge.
Human impact at sea is more difficult to evaluate. There is considerable commercial fishing activity in Falklands waters for squid and finfish. Diet analysis shows that there is some overlap between those species being commercially harvested, and those which make up the diet of Gentoo penguins. Whilst it is true to say that the Falklands fisheries industry is well managed by international standards, the main aim of this management is to ensure that stocks are not over exploited commercially, rather than to consider the effects on wildlife.
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