Introduction

One could be forgiven for thinking that the bustling masses of penguins at the colony represents an almost unending supply of food for the skuas. However, even the most cursory examination of the breeding groups shows that this apparent abundance is largely illusory. Except for a month or so from mid December skuas find that the food available here is limited. Skuas cannot kill adult penguins so that the food resource is made up entirely of eggs and chicks. This chapter documents the numbers of eggs and chicks on the Northern Colony in each of the five years and then uses these figures with the mass of eggs and chicks to estimate the total biomass on the colony during the year. At a later stage the biomass will be partitioned among the pairs of skuas holding colony territories.

Estimating numbers and biomass for the penguin colony

The calculation of the numbers of eggs and chicks present throughout the season on the colony, of the total biomass and of the amount actually available to skuas was made each year from three sets of data: from total counts of penguin nests in early December; from records of the changing egg and chick numbers through the season in a series of study breeding groups; and from the mass of chicks in representative groups. The colony-wide count of nests can be made with great accuracy, virtually without error, but there is substantial variability in nest contents through the season and in chick mass on the colony on any one date which must be incorporated into the estimates.