Differences in burrow site preferences between Chatham petrels (Pterodroma axillaris) and broad-billed prions (Pachyptila vittata) : investigating techniques to reduce the effects of burrow competition

The Chatham petrel (Pterodroma axillaris Salvin) is an endangered species endemic to the Chatham Islands. It is currently restricted to a population of less than 1000 individuals on South East Island. The key threat to breeding success is interference to chicks by broad-billed prions (Pachyptila vittata Forster), when they prospect for burrows for their oncoming breeding season. Management involves patrols around known Chatham petrel burrows and culling broad-billed prions found in the burrows. While relatively successful, these patrols disturb Chatham petrels, are labour and resource intensive, give only short term (hourly) protection and involves killing a native, protected species. This study investigated alternative methods of protecting the known population of Chatham petrel chicks. Three options were investigated: the possibility of exploitation of different habitat preferences whether microhabitat features around a Chatham petrel burrow attracted broad-billed prions, and the effectiveness of a burrow entrance flap that allows Chatham petrels to enter their own burrow but discourages broad-billed prions from entering. Alteration to breeding habitat has contributed to burrow competition. Habitat characteristics for both Chatham petrels and broad-billed prions were quantified and selection ratios compared. Both Chatham petrels and broad-billed prions selected characteristics indicative of mature forest. Chatham petrels are habitat specific and preferred habitat is now limited. Broad-billed prions are generalists and are not limited by habitat availability. All breeding Chatham petrel burrows have been replaced with artificial burrow chambers and tunnels that assist monitoring and reduce burrow collapse. This study investigated whether artificial boxes, logs and tracks attracted broad-billed prions to Chatham petrel burrows, increasing interference to chicks. Prospecting broad-billed prions were not attracted to Chatham petrel burrows. The presence of logs or tracks did not directly increase chick interference levels, but logs increased the number of broad-billed prions near a burrow. The burrow entrance flap exploited behavioural differences between the two species. Chatham petrels had a high incentive to push through a flap due to their investment in their burrow and chick, while prospecting broad-billed prions were influenced by the ease in entering potential burrows. This trial found 90% of Chatham petrels entered their burrows through the artificial flap. Flaps acted as barriers to most broad-billed prions, with only 22% entering the burrow through the flap compared to the control burrows. This study has provided several alternative methods for alleviating the effects of burrow competition between broad-billed prions and Chatham petrels. Habitat preferences should be used to guide searches for unknown Chatham petrel burrows and when establishing a second colony of Chatham petrels. Reducing the presence of logs decreases the number of broad-billed prions around Chatham petrel burrows. While not proven in this study this may potentially decrease interference. Artificial burrow entrance flaps have the potential to provide a low cost, low labour strategy for protecting Chatham petrel chicks.