The Mystery of Ōi

MICHAEL FOX (University of Auckland)

Have you ever watched the night sky, waiting for something extraordinary to appear? Every April, as darkness falls and the wind picks up, ōi (grey-faced petrels) begin their journey home to Aotearoa New Zealand's coastal headlands. These seabirds are rarely seen or heard by most people, but on those special evenings, you might just catch a glimpse of their darting and diving silhouettes or hear their distinctive “o-hoe” during the dusk chorus as they reunite with their partners to begin the breeding season.

Ōi are a remarkable example of what New Zealanders can achieve through conservation. Like many of our native species, they face challenges from introduced predators—rats, stoats, and cats—which pose a significant risk to their long breeding cycle and burrow-nesting habits. But the story of ōi is one of resilience and recovery.

A special species in a changing landscape

Ōi are among the few burrowing seabird species still making a comeback in mainland colonies, whereas most have been relegated to predator-free islands. Historically, these birds thrived on islands and coastal headlands across the country. Today, small but growing colonies have emerged in places like Whangārei, the Waitākere Ranges, Mount Maunganui, and Taranaki. This resurgence isn’t happening by accident. It’s the result of everyday New Zealanders taking action—trapping predators and creating safe spaces for these birds to thrive. Their story of recovery is flying under the radar but deserves to be celebrated as a quiet conservation success.

A PhD in conservation: exploring ōi restoration

Inspired by their resilience, I dedicated three and a half years to studying ōi restoration during my PhD. My goal? To find out if suppressing predators was enough to support their recovery and to uncover what conditions were key to their survival. Three times a week, from July to December, I visited a dozen colonies across the west coast—from the Kaipara Harbour, through the Waitākere Ranges, down to Taranaki. Armed with a burrowscope—essentially a live-feed camera attached to a harakeke branch—I carefully inspected burrows, some stretching metres underground, to check for eggs or chicks. Each fortnight, I returned to track their progress.

Predator suppression: is it enough?

These colonies were all under different predator control regimes, so I wanted to know, under what level of suppression do ōi thrive? I paired my burrow observations with data from 30 trail cameras monitoring predator activity to create density-impact functions. While complete eradication of predators is ideal, my research found that suppressing predator numbers below a critical threshold—especially during early chick rearing (August–October)—dramatically improved breeding success. By November, chicks were seemingly large enough to resist most predation, even though predator numbers start increasing in summer, meaning targeted efforts during those early months are key.

Using my research, I recommend predator threshold targets for this period: fewer than 12 ship rat detections, 2 Norway rat detections, and 0.5 stoat detections per 1,000 camera trap hours. Achieving these targets during early chick rearing can result in nest success rates exceeding 60% on the west coast of Aotearoa. Looking ahead, my next project aims to translate these thresholds into actionable predator suppression strategies. This includes identifying the optimal number and placement of traps, the frequency of trap checks, and the balance between traps and bait stations required to meet these targets effectively.

Unlocking dispersal and genetic diversity

Another aspect of my research explored how ōi disperse and establish new colonies—an essential process for their long-term survival and restoration. Like many seabirds, ōi are known for their strong nest site fidelity, returning to the same burrow year after year. This behaviour, known as philopatry, can make recolonisation of new or restored habitats challenging, especially when colonies have been wiped out by predators.

Using genetic data and banding records, I uncovered evidence that while dispersal events are rare, they occur frequently enough to maintain genetic diversity across populations. An important finding for managing their restoration efforts—even small numbers of ōi can play a role in recolonising habitats—if conditions are right. By suppressing predator numbers and improving habitat quality, we can create environments that attract these natural dispersers. In doing so, it’s possible to passively restore colonies that have been lost, allowing ōi to reclaim their historic range. For smaller, isolated colonies, their philopatric behaviour becomes an advantage: once birds find suitable habitat, they are likely to return year after year, laying the foundation for long-term colony persistence.

Community conservation: a shared effort

Aotearoa’s conservation success relies heavily on dedicated community volunteers, and ōi recovery is no exception. Community-led predator control in the Waitākere Ranges and across Aotearoa has already benefited many bird species. To understand local perspectives on ōi restoration, I conducted an online questionnaire, revealing that while most residents knew little about the species, attitudes towards their recovery were overwhelmingly positive. Support for predator control was strong (93–95%), particularly for managing invasive species like rats, stoats, and feral cats. However, concerns were raised about restoration initiatives that could lead to stricter pet regulations. These findings highlight the need to balance ecological goals with social feasibility.  Ōi recovery in the Waitākere Ranges exemplifies the importance of collaboration between communities and conservation practitioners. By aligning restoration strategies with community values and scaling up predator control efforts, we can ensure the long-term viability of ōi and strengthen public support for future conservation initiatives.

Be part of the ōi success story

The recovery of burrowing seabirds on mainland Aotearoa is an unexpected but inspiring result of growing community conservation efforts. If you’re reading this, there’s something you can do to help: set up a trap in your backyard or local park, and head out on a windy autumn evening to experience the magic of the dusk chorus. You might just witness the next chapter in the remarkable story of ōi.

Acknowledgements

I extend my gratitude to the University of Auckland and my supervisory team — James Russell, Brendon Dunphy, and Todd Landers — for their invaluable support and guidance. I am also grateful to the mana whenua, Te Kawerau ā Maki, Ngāti Whātua o Kaipara, and Ngāti Mutunga, for allowing me to work within their rohe.

A special thanks to the community groups and individuals dedicated to predator trapping in the Waitākere Ranges and beyond, whose efforts have made significant contributions to conservation.