|Specific Name: Pygoscelis papua|
|Adult Height: 75cm|
|Adult Weight: 5-5.5kg|
|Adult Flipper Length: 19-25cm|
|Estimated Population: 300000 breeding pairs|
General Notes / Distribution:
Gentoo Penguins, along with Adelie and Chinstrap Penguins, belong to the Pygoscelis (brush-tailed) family of penguins. As can be seen on the map, Gentoo Penguins have a wide geographical range, spanning a number of temperate and subpolar islands and including the colder polar region of the antarctic peninsula. A small colony is also found on Islas de los Estados (Argentina). Numbers are nevertheless fairly low and individual colonies rarely number more than a couple of thousand breeding pairs. The 2005/06 population census on the Falklands (Huin 2006) revealed a max. colony size of about 7000 breeding pairs with a mean size of about 800, although this is often split into several subcolonies. A mean colony size of under 600 breeding pairs is found in the Antarctic Peninsula (see Ainley et al., 1995. Auk 112(1), p.171-182).
Two subspecies are currently distinguished, the Northern (P.p.papua) and the Southern (P.p.ellsworthi). The Southern subspecies, which is found on the antarctic peninsula, S. Shetland, S. Orkney and S. Sandwich Isl. is about 10% smaller and generally has a slightly shorter, narrower and darker orange bill (Stonehouse 1970. Ibis 112, p.52-57). The latter may however merely be the result of the higher proportion of krill in their diet. Further, it has a thicker plumage, and reduced relative flipper width.
Subantarctic populations have largely been in steady decline over the last decades, whilst the populations of the antarctic peninsula have generally increased slightly, although this depends on the exact locality (Woehler and Croxall 1997, Mar. Ornithol. 25, p.43-66; Woehler et al., 2001. SCAR Report). Recent figures from Kerguelen (30% decline from 1987-2004) and from other sites such as S. Georgia, Falklands, Marion, Heard, all confirm the downward trend in subantarctic regions, with only Macquarie populations appearing to be increasing slightly (see Lescroel and Bost, 2006. Antarct. Sci. 18(2), p.171-174)).
The largest population concentrations are on the Falklands (about 65,000 breeding pairs in 2005 (Huin 2006)), South Georgia (90,000 bp (Williams 1995. The Penguins. Oxford Acad. Press)), Kerguelen (40,000 bp (Bost and Jouventin 1990. Penguin Biol., Davis & Darby Eds., S. Diego, p.85-112)), Antarctic Peninsula (24,000 bp Poncet and Poncet 1987. Brit. Antarct. Surv. Bull. 68, p.71-81). It is noted that these figures may have changed significantly due to ongoing population trends and the large interannual fluctuations generally observed in Gentoo populations.
Gentoo Penguins are considered largely sedentary and tend to forage diurnally in inshore waters near their colonies. Foraging may be either pelagic or benthic, depending on local oceanographic and topographic (bathymetric) conditions and season. Maximum foraging dive depths of over 200 meters have been reported. Migration may occur during the winter season, yet distances covered are small compared to other penguins. The penguins may have to walk significant distances from their colonies to the sea before embarking on their feeding trips. The penguins usually do this in small groups.
Gentoo Penguins, like many other penguins, prey essentially opportunistically on a variety of crustaceans (i.e. krill), fish and cephalopods. Significant geographical, seasonal and interannual variations are observed. Fish are generally more important in the diet at more northerly locations with krill becoming increasingly important at more southerly locations. Due to its relatively large size, the Gentoo can consume fish of over 40 cm in length with weights of more than 1 kg, although most fish consumed averaged 22 g (ice-fish) or 130 g (antarctic cod) in a 1985/86 study at S. Georgia (Croxall et al., 1988. Condor 90, p.157-167). Krill ranged in size from 28-64 mm, with the largest krill available generally being taken.
Numerous studies of Gentoo diet have been performed at the Falkland Islands. Studies at various colonies on the W coast of the Falklands in 2000 revealed that birds breeding near shallow sheltered waters predominantly fed on demersal species (in particular lobster krill (Munida sp.)), whereas those near deeper exposed waters preyed more on pelagic fish species such as Falklands Herring (Sprattus fuegensis) and squid (Moroteuthis ingens and Loligo gahi)). It was concluded that fish and squid are preferred over crustaceans, thus suggesting that the penguins are not entirely opportunistic feeders (Clausen et al., 2005. Polar Biol. 28, p.653-662). On average the diet consisted of 44.4% crustaceans, 34.6% fish and 20.8% squid.
Studies covering a 12 year period and various breeding sites in the Falklands from 1986-1999 revealed that average Falkland Gentoo diet consisted mainly of squid, followed by crustaceans and fish (Pütz et al., 2001. Polar Biol. 24, p.793-807). A trend towards more fish, possibly due to reduced availability of squid (which has been commercially overexploited) and crustaceans was observed. Further, regional differences were apparent, with Gentoos along the N coast consuming proportionally more fish with those on the S coast consuming fish and crustaceans but hardly any squid. As in the aforementioned study, the diet of W coast Gentoos included a mixture of fish, crustaceans and squid.
At Marion Island, during a year long period spanning 1984/85, the Gentoo diet consisted (by mass) of 7.8% (15.2% during summer) Nototheniid fish, 34.7% (37.8%) Myctophid fish, 10.3% (22.6%) Channichthydiae fish, 22.2% (15.6%) Euphausiid crustaceans, 21.8% (0%) Natantia crustaceans. The remainder of the prey consisted of small numbers of cephalopods (Adams and Brown 1989. Mar. Ecol. Prog. Ser. 57, p.249-258). The intraannual fluctuations probably result from differing prey availability as well as greater foraging ranges outside of the breeding season. Breeding birds transited at 7.9 km/h and reached ranges of 14 km from the colony.
At Kerguelen, a 2 year study from 1987-89 at various sites revealed several trends (Lescroel et al., 2004. Polar Biol. 27, p.206-216). As in the Falklands, diet at colonies facing the open sea (38-95% fish by mass) differed from a site in sheltered waters (84% crustaceans). Further, the proportion of fish (largely Nototheniid spp. or Channichthydiae gunnari) consumed was highest in winter, with the proportion of crustaceans (largely E. vallentini) increasing in summer. Interestingly, Annelids (segmented marine worms) were found for the first time in the diet of Gentoo Penguins, making up to 20% of diet by mass at one colony.
At Macquarie, studies during the 1993/94 breeding season revealed a diet (by mass) of 91.6% fish (especially Gymnoscopelus spp. and Paranotothenia magellanica) and 8.3% squid (Robinson and Hindell 1996. Ibis 138(4), p.722-731). A study in the period before the breeding season in 1985 also revealed 92% of the diet to be constituted by fish, largely juvenile Krefftichthys anderssoni, E. carlsbergi and Paranotothenia magellanica. Cephalopods and crustaceans made up the remainder (Hindell 1989. Emu 89(2), p. 71-78).
At the South Shetland Islands, studies revealed that breeding Gentoos primarily feed on antarctic krill (E. superba), although the diet included 15.4% fish, largely Pleurogramma antarcticum (Volkman et al.,1980. Condor 82, p.373-378). Interestingly, males consumed about 3-fold more fish than females and the proportion of fish in diets of other eudyptid penguins breeding nearby was negligible. The krill taken was larger than that found in other Pygoscelid penguins breeding nearby, possibly as Gentoos can dive deeper than their conspecifics, thus allowing them to feed on adult krill which is generally found deeper than adolescents. The concentration on larger krill is consistent with the results of further studies covering the 1998-2006 seasons at the S. Shetlands which compared krill in the diet with that caught in net trawls (Miller and Trivelpiece 2007. Polar Biol. 30, p.1615-1623). Gentoos did not generally feed on krill shorter than 30 mm and generally fed on krill 1-3 mm longer on average than that caught in the nets. This is attributed to the fact that penguins are visual predators and that large krill is easier to locate. Also, it is clearly more efficient to concentrate on larger and thus more nutritious specimens.
Unlike the other Pygoscelid penguins at the S. Shetlands, Gentoo Penguins do not migrate following the breeding season and thus remain in the vicinity of their natal colonies (Tanton et al., 2004. Polar Biol. 27, p.299-303). It is thought that this fact accounts for the fact that in the S. Shetlands, juvenile survival is higher than that of migratory Adelie and Chinstrap Penguins, whose wintering grounds appear to have been more severely affected by recent krill shortages (Hinke et al., 2007. Oecologia 153, p.845-855). The higher juvenile survival appears to account for stability in Gentoo numbers at the site, in contrast to falling numbers of the other Pygoscelids.
On S. Georgia, a number of studies have been performed at Bird Island. One study compared diet between the 1986 and 1994 breeding seasons (Croxall et al., 1999. Mar Ecol. Prog. Ser. 177, p.115-131). Breeding Gentoo Penguins in the 1986 breeding season consumed about 70% krill (by mass), with the remainder of the diet consisting mainly (27%) of Champsocephalus gunnari - icefish. In 1994, when krill biomass was 4-fold lower, fish made up 86% of the diet, yet whilst the proportion was much higher, the actual amount consumed was hardly increased over 1986, as fish does not appear to be available in sufficient quantities at this time in the inshore feeding grounds. As a result, meal sizes delivered to chicks were reduced by 90% and only few pairs successfully raised chicks. This demonstrates that the S. Georgia population is effectively dependent on krill availability during the breeding season. During the winter season, the Gentoos also feed on krill or fish. In the winter of 1988, krill was almost exclusively consumed, yet in 1987, fish predominated in the diet in some months. In July 1987, male diets consisted of 83% fish, with females however only consuming 8% fish (Williams 1991. Ibis 133, p.3-13). Stomach flush samples were highly digested and small at this time, suggesting that the penguins were foraging at slightly increased ranges due to a local lack of prey availability. Commercial fisheries data suggest that krill was available but patchy around S. Georgia in winter 1987. It thus seems that the penguins did not extend their ranges significantly enough to locate offshore krill patches, but rather chose to remain in their usual feeding areas.
Due to their large size, Gentoo Penguins are able to dive to over 200 m, only being surpassed by the even larger King and Emperor Penguins. Most feeding is however performed at shallower depths. The penguins usually porpoise as they rapidly leave or return to land, whilst once further out at sea, transit behaviour involves shallow swimming dives of about 50 secs with pauses of about 12 secs between (Trivelpiece et al. 1986. Auk 103, p.777-781). Travelling speeds of 4.2 km/h are reached on average taking into account pause times. Between the porpoising and underwater swimming phases, a bathing phase is usually observed on outward journeys. Bathing is presumably delayed till the birds are nearly a km from land due to the higher density of predators near the coastline.
Studies at Bird Island, South Georgia, have investigated foraging during the breeding and winter season. This is possible in Gentoo Penguins, due to the fact that they largely remain at their colonies all year round, unlike most other penguin species. Gentoo Penguins mainly forage diurnally during the breeding season, thus returning to their colonies at night.
Breeding birds could be distinguished into two groups, those predominantly eating krill (E. superba) and a smaller group of birds that largely ate juvenile fish (Antarctic Cod (Notothenia sp.); Ice Fish (C. gunnari)). Birds of the respective groups had stomach contents comprising by weight 91% krill or 87% fish, respectively. Those birds that had been feeding on fish tended to have been foraging at deeper depths, with 59% of foraging dives exceeding 54 m, compared to only 23% for birds concentrating on krill (Croxall et al., 1988. Condor 90, p.157-167). It is possible that those birds concentrating on fish had discovered a shoal and repeatedly fed on this. This is suggested by the fact that fish-feeding birds can be subgrouped into those containing largely Notothenia or largely Champsocephalus in their stomachs, with significant mixture of the two types of fish being rare. Foraging trip durations were about 10 hours in both cases with approx. 100 foraging dives. The mean stomach mass was 880 g when the birds returned to the colony. The interesting grouping of birds into fish and krill feeders had also been previously observed at the South Shetland Islands (Jablonski 1985. Acta. Zool. Cracov. 29, p.117-186). The fact that birds may harvest extensively from a single shoal is further supported by the fact that another study of breeding birds at S. Georgia found that 56% of trips consisted of a single diving bout (i.e. series of successive deep dives), with the remainder consisting of 2-4 bouts (Williams et al., 1992. J. Zool. 227(2), p.211-230).
Wintering birds at S. Georgia also forage diurnally, with slightly shorter trip durations of about 7 hours on average (Williams et al., 1992. Auk 109(2), p.223-234). The birds departed after sunrise and nearly 80% of trips were completed before sunset, with an average of 0.8 trips/day. The birds dived deepest at midday, reaching an average of about 80 m during foraging dives at this time, whilst average depths of around 20 m were observed around dawn or dusk. This is explained by the fact that the krill upon which these birds were feeding is found largely at depths of 80-120 m during daylight but may be found at shallower depths as light levels fall. Dives can be distinguished into searching dives, which are generally shallow (about 6 m on average) and short (about 1 min average) with relatively long surface intervals in between, and deep foraging dives (mean 90 m / 2.8 min) which are performed in rapid succession with short surface intervals once food has been located. The searching dives were virtually all (97%) V-shaped, as were 86% of deep dives, with the rest being U- or W-shaped. About 25% of foraging dives exceeded 100 m with a maximum depth of 166 m. The Gentoos stayed within a range of 10 km of the colony and spent just over 50% of time at sea under the water. All of the data in this study corresponded quite closely to data for breeding birds collected at the same site by the same author (Williams et al., 1992. J. Zool. 227(2), p.211-230). However, a number of minor differences are notable. Foraging ranges during brooding and creche were only 2.3 and 4 km, respectively. Trips were daily and averaged 6 hours during the brood period, and nearly daily (96%) but with a longer average of 10 h during the creche period.
Dive parameters were also studies at Bird Island during the 1992/93 breeding season (Mori and Boyd 2004. Mar. Ecol. Prog. Ser. 275, p.241-249). Whilst about 35% of prey patches (krill swarms) were preyed on at a depth of 0-10 m, 46% were in the 40-90 m range. The Gentoos appeared to be able to dive aerobically for at least 150 secs and were observed diving for up to 280 secs. If the aerobic limit is exceeded, birds generally require a longer surface time to recover.
Whilst studies at S. Georgia do not show a winter dispersal or migration of Gentoos from their colony, studies at other sites do reveal extended foraging trips and limited migration. Gentoo Penguins breeding at Ardley Island, S. Shetlands, remained within 16 km of their colony when attending chicks, yet ranged for over 50 km during incubation and up to a maximum of 268 km during the winter (Wilson et al., 1998. Pol. Biol 19, p.407-413). Wintering birds spent up to 10 days at sea foraging before returning to land at various sites. The birds may be absent from their own breeding colonies for several months.
Two Falkland Island Gentoo Penguins from the Kidney Cove colony were satellite tracked during the winter of 2000. This also revealed extensive movement away from the birds “home” colonies. Both birds travelled extensively in coastal waters around the Falklands, making numerous stops at land, often at known Gentoo colonies. Foraging trips included several short trips from a single point, as well as numerous longer trips with extended periods at sea. One bird travelled 276 km off-shore to the NE of the Falklands in a trip lasting 12 days. Mean travelling speed was about 2.5 km/h, although one bird covered 82 km in a single day.
At Crozet Island, the maximum dive depths of Gentoos feeding on E. vallentini were determined using capillary depth gauges. A maximum of 210 m was recorded, although the mean was only 77 m (Bost et al., 1994. Mar. Ornithol. 22, p.237-244). No significant difference between male and female birds could be detected. The sea-floor around Crozet is steeply inclined so penguins have to dive deeper at this site to forage benthically. Data from a single bird with a continuous dive logger revealed a pattern of flat-bottomed dives suggesting benthic feeding. The depth increased as the day progressed, presumably as the bird gradually moved further from land into deeper water.
At Marion Island, a maximum depth of 70 m was recorded using capillary depth gauges (Adams and Brown 1983. Condor 85, p.503-504).
Recent technological advances have allowed the deployment of camera loggers on diving penguins. Gentoo Penguins were studied using these devices in combination with depth loggers on King George Island, Antarctica (Takahashi et al., 2008. Pol. Biol. 31, p.1291-1294). Transit times of about an hour were recorded before krill (E. superba) appeared on images. During 25% of foraging dives krill was visible. Other penguins were only visible in 6.7% of the foraging dives which is lower than in Adelie or Chinstrap Penguins, and no cooperation was evident. Interestingly, it appears that during benthic foraging, the penguins swim at the top of the krill swarm, effectively trapping the krill between themselves and the sea floor. Mean depth dives during the study were about 35 m with a mean maximum dive depth of 80 m and absolute maximum of all birds studied of just over 100 m.
Male birds appear more able to switch their diet to fish, based on their significantly higher fish consumption than female birds at certain times (e.g. Williams 1991. Ibis 133, p.3-13). It is suggested that the larger size and flipper length of the males may convey advantages when hunting fish, which are significantly more mobile than krill.
When nesting with or near other Eudyptid penguin species, i.e. Adelie and Chinstrap penguins, the larger Gentoos generally forage deeper but closer to shore (Trivelpiece et al., 1987. Ecol. 68, p.351-361). This allows the different species to occupy slightly different ecological niches even when breeding close to each other and feeding on the same prey species. Similar segregation of foraging was observed when comparing Gentoo Penguins with Macaroni Penguins at S. Georgia (Mori and Boyd 2004. Mar. Ecol. Prog. Ser. 275, p.241-249). This was attributed to the different diving capacities of the two species (e.g. Gentoos can dive aerobically for 150 secs, whereas Macaronis for only 120) which meant that given the prey distribution at the study site Macaroni Penguins energy intake per dive cycle would be optimal at depths of 30-40 m, whilst Gentoos were optimally foraging at below 40 m, with a peak depth frequency of about 70 m.
It has been hypothesized that breeding colonies need to be distributed and limited in size so that each has access to sufficient prey during the breeding season. However, at the antarctic peninsula, colony size could not be correlated to the presence of other colonies nearby. This may be due to superabundance of prey during the breeding season in antarctic waters (Ainley et al., 1995. Auk 112(1), p.171-182). The hypothesis remains to be examined at other sites.
Nest & Partner Selection
Gentoo Penguins generally nest in relatively small colonies. Studies at the Antarctic Peninsula reveal colony location preferences. Gentoos occupy relatively flat terraces, preferably slightly above the surrounding ground. These elevated areas are most quickly free of snow and safe from flooding. Availability of small pebbles for nest-building and North-orientation at sloping sites are also preferred, since these provide nesting material and more sun to provide warmth in the harsh environment (Quintana 2001. Mar. Ornithol. 29, p.109-112). Whilst Gentoo penguins may share rookeries with Adelie and Chinstrap penguins, the actual nesting sites within the rookeries are still segregated by species, in part reflecting the abovementioned preferences. Gentoos use more but smaller stones than the other Pygoscelid penguins for nest-building in these mixed rookeries (Volkman and Trivelpiece 1981. Wilson Bull. 93(2), p.243-248).
At more moderate locations such as Macquarie Island, Gentoo penguins may use plant material instead of stones to build their nests (Reilly and Kerle 1981. Notornis 28, p.189-202).
Prior to the breeding season most birds are absent from the colony during the day. The season begins when male birds attend the nest site, usually to be followed 1-2 days later by the females. Pairing occurs within days following a series of courtship displays / rituals and a first egg is generally laid about 1 week later. The birds then each forage briefly, with one bird always remaining at the nest. The female returns to land as the laying time approaches. Sexual maturity is reached at 2 years of age, much earlier than in most other penguins species, although young breeders are less successful.
The pre-breeding period is short and in poor years the penguins may delay their arrival and / or only sporadically attend the nest after arriving due to continued foraging during the day. This is in stark contrast to e.g. Macaroni Penguins which remain at the nest site during pre-breeding. This behaviour may account for the large fluctuations in mate fidelity observed during a several year study at Bird Island, S. Georgia since it may reduce the probability of the partners reuniting and reestablishing a pair bond via the usual courtship rituals (Williams and Rodwell 1992. Condor 94, p.636-645). Whilst in normal years mate fidelity reaches nearly 90%, in 1987, when food was scarce, none of the 13 study pairs that mated in the previous season did so again. In 38% of these pairs both birds were present, yet divorce took place, whereas in the remaining cases at least one partner was absent, probably dead. In fact, only 20% of the previous years breeders appeared to return to breed in 1987. Usually, this figure can reach 80%. Nest fidelity was high in all years at Bird Island, with between 89 and 100% of returning birds using the same nest as in the previous season. Where divorce occurred, the male tended to stay at the nest site.
Previous data from the S. Shetlands also had found 90% mate fidelity, but at Crozet mate fidelity only appears to be about 50%. This is probably the result of the unusual extended breeding season at this site which involves reduced synchrony at the onset of the breeding season.
Timing of Breeding
Whilst most Gentoo populations show a relatively synchronized onset of breeding and generally make a single breeding attempt each year, the northern populations at e.g. Crozet start to breed several months earlier and may make a second breeding attempt if the first fails. This subpopulation of Gentoo penguins breeds over an extended period of 5 months and is unique amongst Eudyptid penguins in this respect.
In the antarctic, the breeding season usually commences in November, with first eggs hatching in mid-late December. On the Falklands the timing of the breeding season is similar but starts slightly earlier. Taking studies at the Volunteer Beach colony in 2001/02 (Otley et al., 2004. Mar. Ornithol. 32, p.167-171) as an example, breeding started in October and eggs were laid between 24. October and 8. November. After an incubation period of about 37 days, first eggs hatched on 29 Nov. The brood period averaged 26 days, before chicks started to form creches. At the end of January, the first chicks fledged.
Extended Breeding / Relaying
The breeding season starts in the winter on Crozet, Marion and Kerguelen Islands. This extended breeding is presumably an adaptation to the different temporal availability of food and moderate climate at these northerly sites. It is thought that the antarctic convergence shifted southwards after the last ice age, leaving the Gentoo populations at Crozet and Marion stranded in less productive waters (Williams 1980. Gerfaut 70(3), p.283-296). Also, the earlier breeding avoids direct competition with the resident Macaroni Penguins, is more aligned with periods of greatest food availability and is at least partially at a time before arrival of Skuas at the breeding site.
The breeding of Gentoo Penguins was studied extensively on Crozet from 1983-1989 (Bost and Jouventin 1991. Ibis 133, p.14-25). Early breeders started laying eggs from the end of June to mid-August, whilst late breeders, most of which had experienced failed early breeding attempts, laid from September to mid-November. The interval between breeding failure and relaying is at least 25 days.
The mean laying date for the first eggs in 1983-85 was from 26 July to 7 August, with most eggs being laid within a week of these dates. In 94% of nests, two eggs were laid with a mean interval of 3.2 days between. About 5% of nests had only one egg, with 1% having more than two due to pairing of two females with one male. In 71% of clutches the first egg was marginally larger and heavier. This was also the case for replacement clutches laid later in the year, in which both eggs were about 7% smaller. Single egg clutches were more common in late breeders (12%). Single eggs are often much smaller and may predominantly come from young, first-time breeders.
At Crozet, it is generally sufficient for one egg to hatch, since in only 1% of 2-chick nests both chicks survive to fledging. Whilst there is no direct evidence of favouritism by parents, in most cases (98.5%) the first-hatched chick survives. This is because it generally receives the first meal and grows faster, thus making it more effective at obtaining food from the parent. The difference in mass increases rapidly and eventually the second chick starves, usually before the end of the brood period. First chicks were found to gain weight as fast as single chicks.
The mean age of emancipation and creching of early chicks was 25 and 29 days (range 20-37), respectively, and fledging occurred after about 90 days, before dispersal after 103. In late breeders, emancipation occurred at a similar time, yet creching was delayed by 2-3 days. Fledging was at a younger age in late birds and they dispersed after only 85 days on average, with a mass 10% lower than early birds. The reduced length of the rearing period of late breeders probably result from the declining food resources at the end of the breeding season and need for adult birds to depart for pre-moult feeding. A cost of late breeding may be poor condition of birds in the following season. It has been shown that late breeders are more likely not to breed or to breed later in the following year (Bost and Jouventin 1990. Ornis Scand. 21(19, p.63-70). This may also partially account for the reduced synchronization of the first laying attempt in northern Gentoo populations.
The core study period from 1983-85 included one year with poor breeding success (1985) and one (1984) with good breeding success. Chicks fledged per pair ranged from 0.29-0.65 for early breeders, and only 0-0.3 for late breeders.
Several reasons may explain the low success of late breeders. The birds relaying may be poor breeders since only those birds that failed earlier relay. Further, the previous breeding attempt is likely to have had a negative impact on the birds condition. The proportion of unpaired females and / or infertile eggs is higher (51% v. 32%). Further, foraging trip durations for late breeding birds were generally longer and meals consequently less frequent during the brood/guard phase, probably due to declining food availability. This results in reduced growth rates and relatively smaller sizes of late chicks. As a consequence, especially in poor years such as 1985 when both early and late chicks were reduced in weight by a mean of 18% compared to normal years, the late chicks are extremely small and unable to defend themselves at the end of the guard period. As a result, 87% of late chicks fell victim to skuas in 1985. Nevertheless, in most years, it appears that late breeding can make a significant contribution to overall breeding success.
Laying and Incubation
Gentoo Penguins generally lay 2 eggs within a period of 2.5-4 days. There are minor variations on size depending on the sub-populations studied, but generally the eggs are about 6.9 cm long and 5.8 cm in diameter, with the first laid eggs being about 3% larger than the second (Stonehouse 1970. Ibis 112, p.52-57). A mean number of yolk rings of 14.3 is found, suggesting that the yolk develops over a period of 2 weeks. The Gentoo has a relatively short incubation period and high hatchling mass (72% of initial egg mass, compared to 51% in King Penguin). This means that little energy is wasted. Oxygen consumption by the embryo increases exponentially as it develops (Adams 1992. Comp. Biochem. Physiol 101A(3), p.497-503).
Incubation usually lasts about 37 days for the first egg and 35.5 days for the second, with the first egg generally hatching first. A mean incubation period of only 35 days was reported in one study from S. Georgia (Williams 1990. J. Zool. 222(2), p.247-258).
The first laid egg is partially incubated until arrival of the second. Both eggs are then incubated equally. Incubation temperatures gradually rise with mean temperatures of 30’C reached after 7 days and temperatures of about 36’C recorded during the second half of the incubation period (Burger and Williams 1979. Auk 96(1), p.100-105). Hatching success can be up to about 90%, but is often significantly lower.
Incubation Duties / Nest Relief
Males and females alternate in performing incubation duties and shifts are similar in length (usually from 2-3 days). Especially during the incubation period, when a bird returns from foraging, well coordinated nest relief is important to ensure that eggs are not exposed to predation as it replaces its mate on the nest. Gentoo penguins perform a nest-relief ceremony (NRC), significantly different from that observed in the other Pygoscelid penguins. The ceremony consists of a combination of “donkey call” (a form of Loud Mutual Display (LMD)) and “Bow-Gape-Hiss” (BGH) displays. In the Gentoo LMD, the birds stretch skywards and utter a series of donkey-like braying noises. Unlike in Adelie and Chinstrap penguins, the head is not swung from side to side during the process. In the BGH display, the bird bows forwards, opens its bill and hisses. Other Pygoscelids do not perform this display. The nest bird usually instigates the LMD which is performed on average 2 times (up to 8 times), whilst the returning bird most often instigates BGH displaying (average 3 times, max. 13). If both birds join in a rapid series of BGH displays, nest relief is usually quickest, yet average times for relief are about 2.4 minutes from arrival of the returning bird.
If a parent fails to return, the nesting bird will eventually abandon the nest, resulting in loss of eggs or chicks. At Crozet, this was observed to occur after a mean of 5 days of absence of the mate (Bost and Jouventin 1991. Ibis 133, p.14-25), although the exact time probably largely relates to the condition of the bird on the nest. Nest abandonment can cause significant breeding failure, for example 35% of eggs were lost during early breeding in 1984-85 at Crozet.
Brood / Guard Phase
Chicks hatch with a protoptyl down which allows efficient heat transfer from parent to the initially poikilothermic chick. This starts to be replaced within days by the mesoptile down which progressively increases in thickness with 8, 10, 19 and 27 mm being measured after 10, 15, 25 and 40 days, respectively (Taylor 1986. Auk 103, p.160-168). The increasing thickness, together with metabolic changes, leads to thermoemancipation of the chick by the end of the brood period. By about 10 days the chicks can generate sufficient heat, but only after over 15 days is the insulating capacity of their down sufficient to prevent excessive heat dissipation under dry conditions (Taylor 1985. Envir. Physiol. 155(5), p.615-628). The insulation of the chicks down is reduced by 50% when wet, although it does remain watertight (Kooyman et al., 1976. Comp. Biochem. Physiol. 54A, p.75-80).
Hatch mass is usually around 95 g and increases essentially linearly, with the rate of increase depending on food availability. The two chicks compete for food and at northern breeding sites in particular, where breeding success is generally low, the larger chick rapidly outgrows the smaller, eventually leading to its starvation as it is increasingly unable to compete for food. At other sites, the successful raising of two chicks is more common. During the 1986-2000 period, breeding success of up to 1.51 chicks/pair has been reported in the Falklands (Clausen and Pütz 2002. Mar. Freshw. Ecosyst. 12, p.51-61), with a low of 0.41. Similarly, at S. Georgia, 0-1.6 chicks/pair were reported over a 23 year period (Croxall et al., 1999. Mar. Ecol. Prog. Ser. 177, p.115-131). These large fluctuations partially account for the large interannual fluctuations in Gentoo populations frequently observed in population censa. Occasional strong year classes lead to rapid population increases and then, after a period of more than 10 years, a relatively rapid drop as these year classes reach senescence.
Large colonies with experienced breeders are considered to be most productive if conditions allow.
Provisioning of chicks is alternately performed by the parents with the length of foraging trips and amount of food provided depending on site and food availability. For example, at the Falklands, the long-term average from 1986-1999 of mass of prey landed was 464 g, with a max. annual mean of 1278 g (Pütz et al., 2001. Polar Biol. 24, p.793-807). Trip lengths were not established in this study but a study on breeding Gentoos at the Volunteer Beach colony revealed mean trip lengths of over 17 hours during brood, with many trips lasting more than 1 day (Otley et al., 2004. Mar. Ornithol. 32, p.167-171). At S. Georgia, Gentoos landed 880 g/day with daily trips of 10 h on average in 1985 (Croxall et al., 1988. Condor 90, p.157-167), and similarly 868 g/day in 1986. However, in 1994, only 82 g were landed on daily foraging trips and almost complete breeding failure resulted (Croxall et al., 1999. Mar. Ecol. Prog. Ser. 177, p.115-131). Less data is available for antarctic sites, yet at King George Island, S. Shetlands, feeding trips during both incubation and brood only lasted a mean of about 12 h (Trivelpiece et al., 1987. Ecol. 68(2), p.351-361) and earlier data showed mean stomach contents of 432 g at this site in 1977 (Volkman et al., 1980. Condor 82, p.373-378). At Crozet, feeding trips had a mean duration of 2.5, 1.2 and 1.4 days during incubation, guard and creche, respectively, for early breeders, with slightly longer periods for late breeders (Bost and Jouventin 1991. Ibis 133, p.14-25). Food quantities were not determined. Mean foraging trip length durations for guard phase birds have been reported as 0.6 days at Marion (Adams and Wilson 1987. Polar Biol. 7, p.51-56) and 1.6 days at Kerguelen.
Rapid growth is important, since after the end of the brood / guard phase, small chicks are highly susceptible to predation by Skuas (Bost and Jouventin 1991. Ibis 133, p.14-25). Mass usually reaches about 2 kg just after 20 days and continues to increase to about 6 kg after about 60 days, after which it tends to stabilize.
Growth is not equally distributed, with feet and flippers being disproportionately large at birth and reaching 86 and 66% of adult size, respectively, at creching.
The time at which chicks join creches is highly variable and can be from 20-37 days (mean 29) on Crozet. Similar figures are reported from other sites.
During the creche phase growth continues and competition for food, if two chicks remain, can sometimes be observed in the form of a “food chase”. Here, the parent calls the chicks and then runs away from them with the fastest bird catching up most quickly and being fed more often. Whilst the parents are responsible for provisioning, there has been one fascinating observation of a 7 week old chick feeding a smaller sibling at Paloma Beach in the Falklands (Bingham 1997. Mar. Ornithol. 25, p.70).
During the creche, the penguins obtain their juvenile plumage. This moult has been described in detail for Gentoos on Macquarie Island (Reilly and Kerle 1981. Notornis 28, p.189-202). The moult was considered to proceed gradually from an age of about 25 till 72 days. Other publications place the moult differently (e.g. from 40-85 days), yet this may reflect natural fluctuations and also the fact that the first stages of the moult are difficult to see. Shortly after the moult, chicks are ready to fledge, although they may in rare cases remain on land for over a month after the moult.
Acoustic Parent-Chick Recognition
During the creche phase, the chicks no longer remain at the nest but remain in its vicinity. Adult-chick recognition now relies at least partially on acoustic signals, especially as the chicks venture further from their nests as the creche phase progresses. When arriving at the nesting site, adults emit calls ranging in length from 0.7-1.9 secs (mean 1.16). The calls are made up of a single series of sound components known as syllables which are relatively evenly spaced in the Gentoo (Jouventin and Aubin 2002. Animal Behav. 64, p.747-757). In Gentoo Penguins, recognition is solely based on the harmonic content of the call. The Adelie Penguin call was compared and is fundamentally similar, meaning that Pygoscelid penguins have a relatively simple acoustic recognition system. In contrast, Eudyptid penguins (e.g. Macaroni and Rockhopper) have a double vocal signature, where a distinctive pattern of the syllables additionally contributes to recognition. This is in turn less complex than the double vocal signature of the Aptenodytes penguins (i.e. Emperor, King), in which two simultaneous series of harmonically related bands are combined to create distinctive beats due to band interaction. The higher complexity of the call in non-nesting penguin species is accounted for by the fact that the acoustic signal does not only have to distinguish between birds at a particular location (i.e. nest and immediate surroundings) within a colony, but has to be able to discriminate essentially all birds in a colony without a supplementing positional signal. Why the Pygoscelid call is simpler than the Eudyptid call is less obvious.
The Gentoo call has an intensity of about 83 db at 1 m in front of the bill and based on playback experiments it was found that a single syllable is usually sufficient for recognition. Even a reversed syllable or a synthetic syllable with a single frequency could elicit recognition (a natural syllable has its crescendo at this frequency but builds up and tails off at either end). Frequency modulation experiments showed that the frequency is recognized if its frequency is within 25 Hz of the natural parental call.
The call needs to be distinguishable in the noisy environment of the colony. In Adelie penguins it was found that colony noise and call were at the same level at a distance of 7 m from the nest. Adelie chicks were able to recognize the call at this distance and only failed to do so at distances where the call was at least 3 db weaker than the background. It is assumed that Gentoos have similar capabilities, although Gentoo colonies are less dense than those of other Pygoscelids, meaning that the system may not need to be so highly developed. When a chick recognizes the parental call, it generally raises its head, calls and then rapidly scurries towards the parent, with the nest probably serving as an additional visual cue for recognition at the beginning of the creche phase. Parents recognize own chicks acoustically and peck other chicks that approach them in the hope of being fed.
Fledging / Extended Parental Care
Gentoo Penguins are the only penguin species for which “extended parental care” has been observed after chicks have fledged. This behaviour appears to be common across the whole breeding range since it was reported at Crozet (Bost and Jouventin 1991. Ibis 133, p.14-25) and has been studied in detail at the S. Shetland Islands in the 2005 breeding season (Polito and Trivelpiece 2008. Mar. Biol. 154, p.231-240).
At the Shetlands, chicks were first observed making trips at sea (fledging) at around 70 days of age, yet only left the colony (dispersal) after about 82 days. Fledglings of the other Pygoscelid species tend to fledge in large groups for their first trip and do not return until later in the year.
Mean pre-dispersal trip lengths were 4 hours, with initial trips usually within a 1-2 hour range and final trips before dispersal being up to 7 hours in duration. Parental birds or siblings did not appear to accompany the fledglings on these trips although the fledglings were observed joining other adults or fledglings as they departed. To what extent the fledglings learn from adult birds, if at all, was not established. Parental birds were however observed sporadically feeding their chicks, although since they do not necessarily meet at their nest sites during this period the extent to which this was occurring could not be established accurately.
At Crozet, mean fledging was after 90 days (range 81-99), but chicks were fed for up to 103 days (range 81-124). Chicks were observed following their parents to sea, but not returning with them. Feeding of chicks was regularly observed in the evening.
Post-fledging feeding is a form of extended parental care unique to the Gentoo among penguins, but not seabirds in general. It is hypothesized that this behaviour may account for higher fledgling survival of Gentoos compared to other Pygoscelids. This may play a role in the relative long-term stability of Gentoo populations in the antarctic peninsula region.
During the breeding season, adult birds experience significant weight loss between courtship and creching of the chicks. This must be regained by intensive foraging over a period of about a month in preparation for the annual moult. The moult has been described in detail for Gentoos at Macquarie Island (Reilly and Kerle 1981. Notornis 28, p.189-202). Gentoos nearing moult were heavy and clumsy and the plumage was dull and brown due to extensive wear. Several days before moult, tail feathers started to be lost and the flippers were swollen and prone to bleeding. The moult was considered to have started when the body feathers started to lift and tail was lost. For the next 21 days, old feathers were progressively lost as new feathers pushed up from underneath, with the new tail slowly starting to emerge after about 15 days. During the moult, the penguins lost about 3.3% of mass per day due to energetic demands of basal metabolism and production of the new plumage.
The feathers of penguins consist of stiff contour feathers which give the plumage its structure and underlying downy afterfeathers which provide thermal insulation. Unlike in most other birds they are not organized in tracts, but are instead spread evenly over the skin at a density of about 35(+/-5) feathers per square cm (Dawson et al., 1999. J. Theor. Biol. 199, p.291-295). Each down feather has a principle shaft (ramnus) from which about 50 smaller structures (barbules) extend perpendicularly. From each of these, about 1250 tiny barbules extend in a spiral pattern. The feathers insulation properties interestingly increase with increasing wind speed, possibly due to pressing together of the outer feathers forming a virtually impenetrable layer. A similar effect may be occurring in water and could account for the extremely low drag of the plumage and possibly an improved insulation with increased trapped air when the bird is motile (Taylor 1986. Auk 103, p.160-168). The penguin can itself also regulate the extent of erection of the feathers and presses these close against the body to form an about 0.5cm thick layer when in water. The feathers of penguins make up only 3-4% of total mass, much less than in flying birds.
The high insulating properties have drawn interest from the textile industry (Du et al., 2007. J. Theor. Biol. 248, p.727-735).
Gentoo Penguins are generally considered to be a relatively calm and timid species. Nevertheless, chicks and juveniles are relatively inquisitive and may readily approach humans.
As other Penguins, Gentoo penguins spend significant amounts of time on land preening in order to maintain their plumage and to remove parasites when breeding at more moderate locations.
The preening serves to arrange the feathers and to spread around the oily substance secreted from the uropygial gland at the base of the tail to maintain the water-repellent properties of the down. The insulation properties of a washed pelt lacking the oil are reduced 2-fold in water at atmospheric pressure (Kooyman et al., 1976. Comp. Biochem. Physiol. 54A, p.75-80).
Allopreening is not generally observed in Gentoos or the other Pygoscelid penguins.
Gentoo penguins can frequently be observed making donations of stones to their partners, presumably as a means of maintaining the pair bond. It is possible that this behaviour functionally partially substitutes for the allopreening observed in many other penguins species. Non-breeding Gentoos can also be observed moving rocks around or practising building nests.
Gentoo penguins usually sleep in a prone position although they may also be observed resting in an upright position for long periods of time. The most common display observed is the ecstatic display which is performed with the flippers next to the body, in contrast to the same display in several other penguin species. The accompanying braying sound is much like that of a donkey, accounting for the german name "Eselspinguin", meaning donkey-penguin.
The images below document an incident in which a Gentoo Penguin in the Port Lockroy area near the antarctic peninsula porpoised into a zodiac full of tourists in order to escape from a hunting pod of Orcas. This unprecedented behaviour followed several minutes in which the penguin was chased in the vicinity of the zodiac by at least 3 Orcas. The penguin not only porpoised into the zodiac once, but after a brief return to the ocean returned a second time after establishing that the Orcas were still in the vicinity. It is possible that the penguin (thereafter named "Lucky") was partially habituated to humans as tourist visits to the area are frequent, yet the zodiac must have been uncharted territory for it.
Threats to Gentoo Penguins include predation, disease and environmental factors that affect e.g. food availability.
Eggs and small chicks are subject to significant predation by avian predators such as Skuas (Catharacta spp.) or Giant Petrels (Macronectes spp.). Whilst parents attempt to protect them, the predators walk or fly around the colony waiting for opportunities to pounce when parents are off-guard. Skuas may also hunt cooperatively. For example, one bird walks around the nest picking at the edge of the nest bowl and making the parent continually swivel around on the nest to keep facing towards it. This persistent harassment may result in nest abandonment or the penguin may become slightly unbalanced during the procedure and expose the egg, at which point the second Skua may pounce on it from behind (Schulz 2004. Notornis 51, p.167). Skuas can be distinguished into territorial birds and non-territorial intruders. Territories generally cover hundreds of nests and the owners have almost exclusive primary access to prey patches on their territory. The Skuas eat soft tissues first (i.e. gut and stomach) and generally abandon the prey when about 50% of it has been consumed (Hahn et al., 2005. Proc. Roy. Soc. B. 272, p.1449-1454).
Adult Gentoos or larger chicks and juveniles are no longer threatened by avian predators, yet a number of larger predators threaten them at sea. Leopard seals are major predators of penguins, which may make up a large proportion of their prey (Ainley et al., 2005. Antarct. Sci. 17, p.335-340). Fur Seals and Weddell Seals have also been reported to prey on Penguins although this seems to be relatively rare. Nevertheless, individual seals of these species may at least temporarily specialize on penguins as was observed at Port Lockroy, where one Weddell Seal captured and killed several Gentoos near the shoreline (Cobley and Bell 1998. Mar. Mamm. Sci. 14(4), p.881-883).
Fur Seal populations have expanded rapidly in recent years, and whilst predation of penguins seems insignificant, the large populations are likely to negatively affect penguin numbers due to competition for food. For example, at the S. Shetland Islands, penguins, Fur Seals and commercial fisheries all compete for the same limited food source, antarctic krill (Croll and Tershy 1998. Pol. Biol. 19, p.365-374). Further, each competitor is active in similar areas and largely at the same time. The penguins are probably the weakest competitors in this group since they have the most restricted foraging range.
Climatic changes and natural interannual variation are however the most significant determinants of krill availability. Warmer temperatures correlate to reduced krill availability, since krill are dependent on sea-ice around the antarctic peninsula for breeding. Rising temperatures have significantly reduced the average extent of sea-ice in recent decades (Miller and Trivelpiece 2007. Polar Biol. 30, p.1615-1623 and references therein). Seasons with extremely low krill biomass result in almost complete breeding failure (e.g. Croxall et al., 1999. Mar. Ecol. Prog. Ser. 177, p.115-131). The climatic changes are thought to be largely responsible for the drop in numbers at most of the subantarctic sites. However, in the antarctic peninsula region, Gentoo penguins appear to be doing relatively well and the range appears to be gradually expanding southwards as more areas become snow-free during the breeding season. Snow cover is considered detrimental to Gentoo breeding success (Cobley and Shears 1999. Polar Biol. 21, p.355-360) and these penguins will generally not establish nests on snow-covered ground.
At Kerguelen, Fur Seals and Gentoo Penguins compete for Icefish, which was further commercially depleted by fisheries until these were temporarily suspended in 1995. These factors are implicated in the drop in Gentoo Penguin numbers at this site (Lescroel and Bost 2006. Antarct. Sci. 18(2), 171-174).
Studies at New Island during the incubation period showed that during the breeding season Gentoos remain at a mean distance of about 5 km from the colony, with a maximum distance of 25 km recorded. This suggests that a marine reserve could provide some protection against the local effect of fisheries (Boersma et al., 2002. Aqua. Conserv.: Mar. Freshw. Ecosyst. 12, p.63-74). However, general depletion of fishing stocks cannot be counteracted by such measures and fishery restrictions can only be enforced effectively at few sites.
The greatest single cause of mortality in Gentoo Penguins ever recorded was the Harmful Algal Bloom (HAB) in Falkland Island waters in the summer of 2002/2003. Algal toxins taken up by the penguins caused fatal Paralytic Shellfish Poisoning. This resulted in the loss of an estimated 100,000 Gentoos, if one assumes that the drop in numbers between the 2000 and 2005 population census is attributable thereto (Huin 2006. Falkland Isl. Peng. Census 2005/06).
The impact of disease and parasites on wild penguin populations has only been subjected to limited study, and only few cases of mass mortality have been reported. Penguins may be affected by ectoparasites such as ticks, fleas, lice or mosquitos, by endoparasites such as nematodes, cestodes or coccidians, or by a variety of bacteria and viruses.
Avian Pox Virus (APV) epidemics have been reported at several colonies on the Falkland Islands, and at one of these at least 27% of birds present at the colony were found to be dead, with many others significantly weakened (Munro. Falklands Conservation Report, 2006). Almost all birds within affected colonies appeared to have been infected, whilst nearby colonies were often apparently virus-free. APV may be transferred by mosquitos, yet these are presently absent from the Falklands. If climatic changes facilitate the establishment of mosquitos on the Falklands, the risk of intercolony transmission is likely to increase. Further, studies have shown that penguins living in warmer habitats have higher immunoglobulin levels, probably accounting for higher levels of exposure to parasites and pathogens (Barbosa et al., 2007. Pol. Biol. 30, p.219-225).
Coccidian parasites of various types have been found in 60% of Gentoo Penguins assessed on King George Island, South Shetlands, and coccidiosis has been suggested to be a limiting factor on the population at the site (Golemansky 2008. Acta Zool. Bulg. 60(1), p.71-75).
The effect of human disturbance has been assessed at a number of sites. Studies of Gentoo Penguins at Macquarie Island showed that those nesting near the research station were habituated and thus less perturbed by human approaches than those far from the station, yet in both cases no negative effect of human approaches on breeding success could be measured (Holmes et al., 2006. Polar Biol. 29, p.399-412). In contrast, the presence of Elephant Seals harems or other penguins did have a negative effect on breeding. The longest-term studies to date were performed by comparing visited and unvisited colonies in the Port Lockroy area, which is one of the most frequently visited antarctic sites (Trathan et al., 2008. Biol. Conserv. 141, p.3019-3028). These studies found that over a 12 year period nest numbers decreased slightly on average in the visited colonies, in contrast to slightly rising numbers in unvisited nearby colonies. Whilst breeding success of nesting birds was similar at both, it was concluded that fewer birds appear to choose to nest at the frequently visited sights. Breeding success of Gentoo Penguins at Volunteer Point on the Falklands, the main Gentoo site visited by land-based tourists, has not been noticeably affected by tourism (Otley 2005. Mar. Ornithol. 33, p.181-187). The theory that human disturbance could distract the penguins sufficiently to allow increased predation by Skuas can also be discounted (Crosbie 1999. Mar. Ornithol. 27, p.195-197). Nevertheless, in comparison to King and Royal Penguins, Gentoo penguins at Macquarie appear to be more easily unsettled by human approach based on their behavioural responses thereto (Holmes 2006. J. Wildl. Manag. 71(8), p.2575-2582). Hence, Gentoos need to be treated with extreme care, especially when on the nest or during the moult.
Where To See:
There are a number of sites at which Gentoo Penguins can easily be observed in the wild. Further, a number of Zoo collections include Gentoo Penguins. The most commonly visited Gentoo breeding sites are those on and near the Antarctic Peninsula, which has experienced a boom in ship-based tourism in recent years. Gentoos can be seen at a number of landing sites such as Hannah Point, Port Lockroy, Mickelson Harbour or Nico Harbour. Further Gentoo sites can be found at South Georgia and the Falklands, which are both included in some of the longer cruise options. For Australasians, Gentoo Penguins are most easily accessed by cruises which visit Macquarie Island (e.g. Heritage Expeditions), where small numbers of Gentoos can be seen. Non-cruise options are limited to the colonies of Gentoo Penguins found on the Falkland Islands, although these are not suitable for mainstream tourism.