Abstract
Climate change is affecting species worldwide, inducing range shifts, increasing mortality, and disrupting reproduction. For species that are hard to observe it is difficult to assess their vulnerability to climate change. Seabirds, which play important ecological roles, spend most of their lives at sea, with many species returning to land only to breed. They are among the most threatened avian groups, particularly due to introduced predators, fisheries bycatch, and climate change, this last factor primarily affecting them through altered prey availability. However, their ecology and life-history traits (i.e., long-lived, delayed maturity, low reproductive output) make it challenging to assess their vulnerability to climate change.
The Chatham petrel Pterodroma axillaris and Chatham Island tāiko P. magentae are gadfly petrels endemic to the Chatham Islands, New Zealand, and among the world’s rarest seabirds. Although they have been intensively managed on land since the 1990s and 1980s, respectively, their at-sea threats, particularly those related to climate change, remain poorly understood. Two key knowledge gaps limit our ability to assess threats: robust demographic estimates and detailed information on their movements during the breeding season. To fill these gaps and assess the species’ vulnerability to climate change, I tracked both species with GPS during breeding to characterise their at-sea movements and distribution, and to identify the factors driving these patterns. I then combined GPS tracking data with existing geolocator data of their non-breeding distribution and more than 30 years of long-term monitoring (burrow monitoring and capture-mark-recapture) to develop species-specific integrated population models (IPMs). These models enabled the first assessment of the effects of environmental covariates on the species’ key demographic rates while accounting for differences in management methods.
GPS tracking showed broadly similar ocean use by both species during breeding, with a preference for deep waters south of the Chatham Islands, between the subtropical (STF) and subantarctic fronts, with birds rarely recorded over continental shelves. However, the larger Chatham Island tāiko undertook longer trips, both in distance and duration. Many tracks were concentrated around the STF, which over the past two decades has become more productive but has also shifted farther from the Chatham Islands. Results from the IPMs showed that for the past three decades both species have maintained high juvenile and adult survival, breeding probability, and breeding success, resulting in steady population growth. Chatham petrel numbers increased from an estimated 423 adults in 1991 to 1,119 in 2024, while Chatham Island tāiko increased from 26 adults in 1995 to 187 in 2023. I found no strong evidence that environmental variability is currently affecting the demographic rates of either species, suggesting that their foraging strategies may presently buffer against environmental fluctuations. Changes in oceanographic conditions are not always detrimental, and Chatham petrels and Chatham Island tāiko may have benefited from the increased productivity of the STF despite its shift away from their breeding grounds. This study demonstrates how different datasets can be integrated in IPMs to reliably estimate demographic rates while simultaneously assessing extrinsic drivers such as management actions and environmental variability.