Population regulation in Little Penguins (Eudyptula minor): the role of intraspecific competition for nesting sites and food during breeding

Although studies of seabirds have had a fundamental influence on development of theories of population regulation, the role and importance of potential factors regulating seabird populations remain unresolved. Food supply is commonly thought to be a significant limiting factor regulating seabird populations but difficulty in measuring availability of prey has meant that direct evidence is usually lacking. Here we examine the role that intraspecific competition for breeding sites and food during the breeding season plays in determining the size of populations of Little Penguins (Eudyptula minor) in south-eastern Australia. The relation between population sizes and available breeding area suggest that numbers of Little Penguins may be limited on smaller islands by area but clearly are not on larger islands. Little Penguins have very short foraging ranges and thus are likely to experience density-dependent food shortages when breeding. We found support for Ashmole's ‘halo effect’ in that there was an inverse relationship between colony size and mean mass of chicks near fledging. However, more recent variations of this model (e.g. the ‘hungry horde’, or ‘hinterland’) do not explain the observed distribution and numbers of penguins breeding in Victoria. We conclude that both factors may be involved in the regulation of Little Penguin populations locally, and propose a model in which the area available for nesting is combined with intraspecific competition for food during breeding as co-determinants of population size.