Burrow competition between broad-billed prions (Pachyptila vittata) and the endangered Chatham petrel (Pterodroma axillaris)

Abstract

The endangered Chatham petrel (Pterodroma axillaris) competes for nesting burrows with the locally abundant broad-billed prion (Pachyptila vittata) at its only breeding site on South East Island, Chatham Islands. Prions are the most serious threat to Chatham petrels and if left unmanaged cause over half of all breeding failures. This report discusses burrow occupancy and prospecting habits of the prions, habitat preferences of both species, and the development of a burrow entrance flap that discourages prions from entering Chatham petrel burrows. Most prions banded in study burrows were never recaptured. Of those recaptured, most were either in, or within 5 m of the burrow they were banded in. A few were found in study burrows up to 100 m from their banding point. Up to six different prions regularly visited study burrows within a non-breeding season, and up to four within a breeding season. During their non-breeding season prions spent 12% of their time prospecting. Prospecting was most common between 0230 hours and first light. A few prions investigated up to six burrow entrances during a 25-minute observation period. No correlation was found between nights with letter or no prospecting and weather or lunar patterns. Habitat characteristics for both Chatham petrels and prions were quantified. Chatham petrels exhibited greater habitat specificity and their preferred habitat is now limited. Prions were generalists and were not limited by habitat availability. The burrow entrance flap exploited behavioural differences between the two species. The petrels with a chick inside the burrow had a high incentive to push through a rubber flap, whereas prospecting prions were deterred by the flap. 90% of Chatham petrels entered their burrows through the flap. Only 22% of prions that entered control burrows entered burrows while a flap was in place.