Burrow flaps a species saver

Chatham petrels are an endangered seabird species, endemic to New Zealand’s Chatham Islands. In 1990 Chatham petrels were classed as critically endangered, restricted to a single breeding population of 200-400 birds on Rangatira Island, and declining rapidly thanks to unsustainably low breeding success. Over 20 years of conservation effort has returned this species from the brink of extinction.

The Chatham Islands (Image credit Newsbie Pix)
The Chatham Islands (Image credit Newsbie Pix)

Broad-billed prions are primarily responsible for the petrel’s low breeding success. Broad-billed prions are native to New Zealand and are the largest of the prions, distinguished by the very wide bill and high forehead. Classified as a relict species, prions are deemed to be at risk due to an approximately 90% reduction in their historical breeding range.  However due to their great overall population, over 15 million, they are deemed as least concern by the IUCN (International Union for Conservation of Nature).

Both Chatham petrels and broad-billed prions are burrow nesters and, due to their similar size, compete for the same nesting burrows. Burrow competition has dramatically increased due to the introduction of mammalian predators to New Zealand, significantly reducing the number of viable breeding locations for both species. Differences in the timing of their breeding seasons causes prions to be prospecting for suitable nesting burrows while petrels are trying to raise chicks. Prions will go to extremes in order to obtain prime burrow real estate, commonly evicting or killing petrel chicks occupying the burrows. Petrel chicks are defenceless against such an invasion, with their parents away for two to three nights at a time finding food. Prions were responsible for 70% of failed petrel breeding attempts before conservation initiatives were instigated.

The initial petrel management programme required conservation staff to essentially act as chick body guards. Staff checked petrel nesting burrows six times a night, removing and eliminating any invading prions (prion culling). This strategy was relatively successful, resulting in significantly increased fledging rates, but it definitely had its drawbacks. It was an unsustainable, short term solution as no measures were taken to prevent prion re-invasion. This strategy was very costly not only in monetary terms, due to the labour intensive nature, but also with respect to the culling of a protected, native species, and significant disturbance to the petrel nests.

Burrow blockades are the other major strategy utilised to reduce the impact of prions. This involved the covering of burrow entrances by mesh gates in the non-breeding season, while the petrels were absent from Rangatira Island, and their removal on the birds return. Blockades exclude prions from forming associations with the burrows and allowed petrels to return to unoccupied burrows ready for the breeding season. This strategy reduced the number of prions  building associations with suitable petrel nesting burrows, but was ineffective in deterring prospecting prions from interfering with established petrel nesting burrows during the breeding season.

Broad-Billed Prion Chick (image credit Leon Berard)
Broad-Billed Prion Chick (image credit Leon Berard)

Wendy Sullivan and Kerry-Jayne Wilson developed a new strategy to protect the Chatham petrels from burrow competition with broad-billed prions. They proposed the use of burrow entrance flaps to deter prions from entering already occupied nest burrows. The theory was that petrels were invested in their nesting attempt, due to the significant amounts of time and energy expended, and would have more incentive to push through burrow flaps to access the burrow. Whereas prions, who were only prospecting suitable breeding burrows and not yet invested, would not have the same incentive to push past the obstacle presented by the flap.

Two different flap designs were trialled, one of neoprene and one of tire tubing. Both designs covered the burrow entry but presented a slightly different obstacle for the birds to overcome. The neoprene design was found to be slightly more successful in deterring prion nest entry; although both flap designs were found to significantly deter prion burrow prospecting without negatively impacting the petrels. Decreases in prion entrance frequency of 73% and 80%, with tire and neoprene flaps respectively, were found in burrows with flaps. This success lead to the inclusion of burrow flaps into the management plan for petrels.

Burrow flap development has since been described as a “major breakthrough in petrel recovery” in a review of Chatham petrel recovery. The flaps had negligible negative consequences on the petrels breeding behaviour, productivity or chick health, while significantly increasing petrel breeding success. Flap development allowed for the reduction of monitoring intensity and cessation of prion culling, both saving resources and protecting another native species. The use of burrow flaps was a major contributor to the increase in petrel population from around 300 to over 1400 birds in a twenty year period.

The increase and stabilization of the petrel population allowed for the translocations of 200 petrel chicks to both the Ellen Elizabeth Preece Conservation Covenant on Pitt Island, and Sweetwater Conservation Covenant on the main Chatham Island. Both translocations were successful with chicks returning to breed as adults. There are now more than 17 breeding pairs established on Pitt Island and the first breeding pairs have returned to the main Chatham Island.

The development and incorporation of burrow flaps into the conservation strategy for petrels has drastically increased petrel breeding success and therefore population growth; which in turn has allowed for successful translocations and the establishment of new colonies. Thus, burrow flaps were instrumental in the classification change for the Chatham petrels from critically endangered in 1990 to their current status of endangered. It is amazing how much can be achieved by simple materials and a bit of innovation. Hopefully with continued management and monitoring we can see this status further reduced in the future.

The author Courtney Hamblin is a postgraduate student at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

Central references:

Sullivan, W; Wilson, K. 2001. Use of burrow entrance flaps to minimise interference to Chatham petrel (Pterodroma axillaris) chicks by broad-billed prions (Pachyptila vittata). New Zealand Journal of Ecology 25(2):71-75.

Gummer, H; Taylor, G; Wilson, K; Rayner, M. J. 2015. Recovery of the endangered Chatham petrel (Pterodroma axillaris): A review of conservation management techniques from 1990 to 2010. Global Ecology and Conservation 3:310-323.

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