Penguin monitoring on Bird Island
Two species of penguin breed on Bird Island: macaroni penguins (Eudyptes chrysolophus) and gentoo penguins (Pygoscelis papua). These species have contrasting breeding and foraging strategies and ranging behaviour.
Gentoo penguins are inshore foragers that return to shore each night throughout the year, and so are indicators of local food availability in the shelf waters around South Georgia.
Macaroni penguins are more pelagic, migrating away from South Georgia during the winter, and during the prolonged incubation periods and pre-moult foraging trips they range hundreds of kilometres from their colonies.
Macaroni penguins are therefore indicators of food availability over a much wider area and range of habitats, and are able to buffer themselves against localised changes in prey abundance more readily than gentoo penguins. As such, many of the parameters monitored for these species show greater variation for gentoo than for macaroni penguins.
Adult arrival weights and laying phenology
Since 1988, around 50 macaroni penguins of each sex have been captured and weighed during the period of arrival back on the colony during early November. Males are generally heavier than females (average of 5.27 vs. 5.06kg), and they fluctuate in tandem through the time-series. This is likely to be due to variations in availability of krill during the late-winter and early-spring period, and may influence subsequent breeding population size and performance, as found for this species on Marion Island.
While macaroni penguins laying phenology is predictable, that of gentoo penguins is variable both within and across years in response to local foraging conditions. Hence, this is used as an index of early season food availability for this species rather than adult weight. The laying phenology estimate is based on the date at which 75% of nests within a study plot are laid is recorded for two colonies on Bird Island. The average laying date was 30 October, but this has varied by over a month among years.
Chick fledging weights
The mass of approximately 100 chicks have been collected just prior to fledging for both penguin species since 1989. Gentoo penguins averaged 5.4kg across all years of study and macaronis 3.28kg. The weights of the two species fluctuate through time in tandem, suggesting that local food availability has similar effects on chick condition of both species.
Breeding pairs and productivity
The number of breeding pairs of gentoo penguins and their productivity has been quantified by complete counts of all seven colonies on the island during the period when the maximum number of adults are incubating and just prior to chicks fledging, respectively. The number of pairs averaged 3,120 across years but has fluctuated markedly, with notable peaks and troughs over relatively short time periods. These are likely to be due to variation in the proportion of mature birds that attempt to breed, since the changes in numbers are too rapid to be explained by mortality or recruitment events.
Macaroni penguins occur in two large and one small colony on Bird Island, and an aerial survey in 2002 estimated 46,500 pairs. The population is too large to allow annual complete counts, and monitoring is achieved by complete counts of the smallest colony (since 1982) and a transect through the largest (since 1979). These show that numbers have declined at both colonies during the 1980s and 1990s, but since 2000 the decline has levelled off at the small colony and a partial recovery has occurred at the large colony. Studies of survival rates of birds marked with microchips at the small colony show that values of adult survival and breeding success are typical for crested penguins while that for recruitment is low, suggesting the latter might be responsible for the declines.
Modelling of population trends for both species shows a negative correlation with the oceanographic phenomenon known as the Southern Annular Mode, which affects sea temperatures around the Antarctic, and hence penguin food availability.