Inter-annual variations in breeding participation at four Western Australian colonies of the Wedge-tailed shearwater Puffinus pacificus

Abstract

Wedge-tailed Shearwaters Puffinus pacificus breeding at colonies on three islands in the North West Shelf Region of Western Australia showed marked inter-annual variations in the number of burrows excavated at the start of the season, the percentage of these burrows in which eggs were ultimately laid and in breeding success. The number of burrows excavated and the proportion in which eggs were ultimately laid were not correlated indicating that different factors influence these components of breeding participation. Breeding success was affected in some years by local factors, including cyclone tracks and predation, but the greatest factor influencing the number of young reared each season was the number of breeding attempts (participation rate). The percentage of burrows in which eggs were laid varied in similar ways at the three colonies on different islands on the North West Shelf. Because these colonies were separated by up to 140 km, broad-scale, regional oceanographic/trophic factors were implicated. During the 1997 El Niño event the percentage of burrows in which eggs were laid dropped dramatically at all the North West Shelf colonies and at a major Wedge-tailed Shearwater colony on Pelsaert Island in the Houtman Abrolhos Group, over 1000 km farther south. The number of burrows excavated each year within the entire Varanus Island colony has been monitored consistently since 1987. There was a significant correlation between the three-year running mean in active burrow numbers and the three-year aggregate, annual Southern Oscillation Index values. This cumulative lag affect in the number of birds excavating burrows at the start of each season suggests that this parameter may be a useful index of the size of the breeding population. Conversely, the percentage of burrows in which eggs are laid, seems to be a measure of prevailing oceanographic/trophic conditions.