Burrow competition between Chatham petrels and broad-billed prions : the effectiveness of burrow blockading as a management strategy

Abstract

The Chatham petrel (Pterodroma axillaris) is an endangered seabird endemic to the Chatham Islands, New Zealand. The breeding range is now restricted to Rangatira Island. Burrow competition between Chatham petrels and broadbilled prions (Pachyptila vittata), has a negative effect on Chatham petrel productivity. Burrow blockading is a management strategy developed by the Department of Conservation in response to burrow competition. Chatham petrel burrows are blockaded (by a gate set over the entrance) between July and October each year to prevent broad-billed prions from breeding in them. Blockades are removed shortly before Chatham petrels return to commence breeding. To determine the effectiveness of burrow blockading, Chatham petrel burrows were monitored daily over two consecutive breeding seasons (November 1995 to May 1996 and November 1996 to June 1997) to record the frequency and result(s) of broad-billed prion interference. This study found burrow blockading to be ineffective in deterring broad-billed prions from interfering with Chatham petrel breeding burrows, but effective in ensuring burrows are available to returning Chatham petrels. Levels of interference over both seasons were high, with 87.3% of all known Chatham petrel breeding burrows affected. Broad-billed prions are likely to be responsible for 68.9% of known Chatham petrel breeding failures. Although broad-billed prions are known to oust, injure and kill Chatham petrel chicks, the majority of interferences (78.8%) caused no physical harm. Broad-billed prions caused at least four adult Chatham petrels to desert during incubation. Most broad-billed prions (where sex was confirmed) that interfered with Chatham petrel burrows were male (64%) and had no recorded prior association with the burrow.