Burrow occupancy and related behaviour of broad-billed prions (Pachyptila vittata) on Rangatira (South East) Island, Chatham Islands, New Zealand

Abstract

Broad-billed prions (Pachyptila vittata) return to Rangatira (South East) Island. Chatham Islands, New Zealand, in early February after their post-nuptial moult, and begin egg laying in late August. During this non-breeding season, prions prospect (search) for burrows and mates for the following breeding season. This stage in the broad-billed prion life cycle has seldom been studied. The purpose of this thesis was to examine both breeding and nonbreeding season burrow occupancy patterns, and non-breeding season prospecting behaviour and colony attendance patterns of broad-billed prions.

Seventy-seven artificial burrows were monitored nightly for a total of 4.5 months between 1996 - 1998, and any prions found in these burrows were banded. Most of these birds were never recaptured. Of those recaptured, most were either in, or within five metres of the burrow they were originally banded in. A small number were found in study burrows up to 100 metres from the original banding point. Up to six different prions regularly visited study burrows within a non-breeding season, and up to four prions visited study burrows within a breeding season. Tunnel entrances of 14 artificial burrows were blocked in the broad-billed prion non-breeding season and over the following breeding season to test whether broad-billed prions with a known breeding history could be discouraged from returning to their previously bred in burrows. Blockading in the breeding season reduced use of these burrows in the following non-breeding season.

Broad-billed prion prospecting behaviour was studied in the 1997 and 1998 non-breeding seasons. Results suggest that on Rangatira Island broad-billed prions spend 12% or their time prospecting. Prospecting was most common between 02:30 hrs and dawn. A small number of individuals, observed for 25 minutes each, investigated burrow entrances up to six times, some birds at a different burrow entrance each of these times. No correlation was found between nights with little or no prospecting behaviour and weather or lunar patterns.

The numbers of broad-billed prions on Rangatira Island fluctuated greatly between nights. Prions were counted along a fixed transect on 245 non-breeding season nights between 1996 and 1999, and these counts were correlated with 12 weather and lunar variables. High numbers of broad-billed prions ashore correlated with bright nights leading up a full moon.

There is intra- and inter-specific competition for breeding burrows between broad-billed prions and other small burrowing sea birds, including the endangered Chatham petrel (Pterodroma axillaris), on Rangatira Island. This competition is most intense in the broadbilled prion non-breeding season. Current management of broad-billed prions is only effective on a nightly basis. This thesis provides information that can be used to help alleviate broad-billed prion pressure on Rangatira Island in the medium and long term.