Behavioural ecology of lonnbergi skuas in relation to environment on the chatham islands, new zealand

Abstract

A population of lonnbergi skuas (family Stercorariidae) was studied over December and January 1974-75 on Rangatira Island, Chatham group (44°22'S,176°1TW). The main aim was to compare the behavioural ecology of these birds with that of maccormicki skuas of Antarctica, and to assess the moderating effect of environment on habits. The skuas held extensive territories along the coast and in inland areas clear of forest and bracken. Five of the 11 study territories were occupied by trios of adult birds. A major effect of the vegetation was to reduce contact between neighbours on the ground at territorial boundaries, and most territorial defence was by aerial display and attack. Eggs were laid from late September, and chicks began flying in late December and January. This breeding schedule is about 2 months earlier than for lonnbergi at Signy Island or for skuas in Antarctica, but falls on a trend line of latitude against egg-laying when all populations are considered. Seventeen chicks were reared on the 11 territories, a breeding success of 1.55 chicks per nest. This high success rate was attributable largely to success in rearing both chicks when two were hatched. This is seldom achieved in more extreme climates, and is apparently determined by the intensity of sibling competitiveness. It is hypothesised that this intensity is graded, and aggression by the older chick of the pair is provoked more easily by food shortage as environmental conditions become increasingly severe. The most important prey items recorded were adult petrels, especially broad-billed prion (Pachyptila vittata vittata) and white-faced storm petrel (Pelagodroma marina maoriana) captured on the ground at night. The skuas were mostly inactive during the day, and were not seen to fish or to chase or harass any other bird on or about the island. Southern blue penguins (Eudyptula minor minor), though very common on the island, were not attacked by the skuas. Skuas were not seen to attempt to dig out petrel or penguin nesting burrows. Of the differences in ecology recorded, only the presence of three adults on territories appears to be characteristic of lonnbergi; it has not been recorded for maccormicki or hamiltoni, and is either unknown or very rare in skua. © 1978 Taylor & Francis Group, LLC.