Black petrel population monitoring 2023/2024

Download the publications

POP2022-01 Key demographic parameters and population trends of tākoketai/black petrels (Procellaria parkinsoni) on Aotea/Great Barrier Island: 2023/24 (PDF, 8,439K)

POP2022-01 Black petrel population monitoring - at sea capures (PDF, 2,675K)

POP2022-01 Black petrel population monitoring - at sea capures addendum (PDF, 609)

Summary

Black petrel monitoring on Aotea /Great Barrier Island 

This report is part of the ongoing study of the tākoketai/black petrel (Procellaria parkinsoni) on Aotea /Great Barrier Island that began in the 1995/1996 breeding season.

During the 2023/24 breeding season 482 tākoketai study burrows were intensively monitored within the Mt Hobson/Hirakimata study area on Aotea.

There were 306 (63.5%) burrows occupied by breeding pairs, 58 (12%) occupied by non-breeding birds, and 118 (24.5%) were unoccupied at the time of check. Overall, 222 chicks were produced from the study burrows representing a fledgling success rate of 72.5%, but five chicks were found to be below weight and smaller in size during the May chick banding trip, and most of these chicks were not expected to survive to fledging. This would further reduce overall breeding success to 70.9%.

Nine census grids were monitored within the study area and accounted for 199 of the inspected study burrows. Of these, 120 were occupied by breeding pairs (60.3%) and 86 chicks were produced representing a fledging success rate of 71.7%. Again three of these chicks were in poor condition and were not expected to fledge, reducing breeding success to 69.2%.

A total of 700 adults and 223 fledgling chicks were captured during the 2023/24 field season with 274 adults banded this season (including 204 from study burrows). Of the 223 fledgling chicks banded during the 2023/24 field season, 212 were banded in study burrows and eleven chicks were banded on the surface or from random non-study burrows. An additional ten chicks from within the study burrows fledged prior to the banding trip in May.

There have been nocturnal surveys undertaken throughout the 29-year study period, but only over the past three seasons (2021/22 to 2023/24) has this effort been increased to 6-8 hour (between 9.15 pm to 5.15 am) searches each night. Over 940 nights of ad-hoc surveys undertaken between 1995/96 and 2020/21, 811 adults were recaptured, of which 365 were already banded and 108 were returned chicks. Over 41 nights of intensive surveys between 2021/22 and 2023/24, 427 adults were recaptured, of which 219 were already banded and 83 were returned chicks. At-sea surveys have also been conducted by WMIL over this same period with only 22 banded birds being caught and nine being recaptured chicks (Burgin 2024). The percentage of banded tākoketai caught out of all captures is higher for intensive night surveys (51.3%) than the ad-hoc surveys (45%) and much higher than the at-sea surveys (4.8%). This pattern is also the same for the percentage of returned chicks captured (intensive 19.4%, ad-hoc 13.3% vs at-sea 2%), for the number of banded tākoketai caught per survey (intensive n=5.3, ad-hoc n=2.1, at-sea n=1.5) and for returned chicks caught per survey (intensive 2.0 returned chicks/survey, ad-hoc 0.6 returned chicks/survey, at-sea 0.6 returned chicks/survey).

There have been a total of 461 returned chicks recaptured at the colony since they were banded prior to fledging. Of these, 126 returned chicks were identified during the 2023/24 breeding season; 30 of which were caught for the first time at the colony. Not all cohorts were represented this season as no re-captures of returned chicks were made from the 1995/96, 1996/97, 2001/01 and 2003/04 cohorts. Over the entire study, the majority of the 461 returned chicks were from the 2010 /11 cohort followed by the 2006/07 cohort. Understanding the factors affecting return rates of chicks within the 35-ha study site is vital. It is important to determine whether it is related to low juvenile survival and/or recruitment or if it is simply due to a lack of detection. Understanding juvenile survival and recruitment is necessary for accurate demographic modelling and for species risk assessment modelling. Therefore, it is recommended that effort to obtain this data is completed with urgency.

Additional monitoring of feral pig and other predator occurrence and impact on tākoketai at Cooper’s Castle was undertaken this season. Visual deterrents were established at two breeding burrows with trail cameras placed to record images and video. Footage confirmed rat presence only. There were no feral cat predation events and two rat predation events at the study colony on Hirakimata this season. Introduced species still pose a threat to the tākoketai population and it is imperative pest control measures continue.

WMIL recommends that:

• Intensive population monitoring using the study burrows on Aotea continues with three visits (i.e., at egg-laying (December); at chick hatching/chick guard in late January/early February and at chick fledging in late April/early May) per season to the colony to track population trends and determine impacts to the birds and colony.
• Multiple-night expeditions to focus on recruitment (i.e., nocturnal surveys to capture pre-breeders and returned chicks) to the Aotea study colony continue to determine juvenile survival and recapture probabilities.
• Sexing of all tākoketai caught during the recruitment expedition and in the study burrows is completed to determine any sex biases and survival differences between sexes at the colony and within the study burrows.
• A focused, consistent and repeatable mark/recapture session (e.g., a 2-hour capture period at known launch sites) is completed over a number of nights to capture as many banded and unbanded birds as possible. Data can then be used to provide another population estimate and compared to estimates obtained from at-sea captures and burrow monitoring.
• Transect surveys across the core tākoketai habitat (1000 ha around the summit) are undertaken to provide an updated population estimate for the core breeding area of Aotea.
• Satellite tracking of chicks to, and in, South American waters is undertaken to determine migration routes and foraging areas to estimate risk in these areas.
• The possibility of collaborative at-sea capture expeditions in Ecuador is investigated. Discussions between DOC and New Zealand Government with Ecuadorian Government and researchers will have to be conducted to enable this type of collaborative work. At-sea work in Ecuador could determine the level of juvenile tākoketai presence in this area and risk within this area, and this mark/recapture work could provide another population estimate to compare with the New Zealand data.
• Further investigation to determine whether particular areas of the colony are more at risk to rainfall events than others (e.g., burrows in flatter areas being more prone to flooding) as a preliminary assessment on climate resilience.
• In-depth modelling on the effect of age, age difference in pairs, and experience on breeding success is completed to understand this relationship in tākoketai.
• Analysis of, and comparison between, breeding success in public, and non-public, access areas is completed to determine whether human disturbance is a factor at the Aotea colony.
• Investigation into possible deterrence methods of all predators, but specifically feral pigs and feral cats, should be continued at Cooper’s Castle.

At sea captures 

This at-sea capture project continues work started in 2022 to look at survival and return rates of juvenile black petrels not visiting the main study areas. Capture, mark-recapture of black petrels in the Hauraki Gulf will also provide information from a random sample of birds away from the study colonies to help estimate current population size of this species.This report provides details of the methods used to capture black petrels, comments on what worked and what did not, and includes photographs illustrating the methods used. Recommendations for future capture projects include:  

• Continue to utilise the more versatile net gun method for captures, as this takes actual capturing away from the reliance of attracting birds in close to the main boat. Although, that will likely remain where most birds will be caught through the season.
• In terms of timing, we believe the capture season could be extended through to the end of April, although it is possible capture rates would drop as normally non-breeders depart ahead of the chick fledgling period which starts at beginning of May. Adults will still be around in late April. However, the overall project budget determines the numbers and duration of charters that can be undertaken.
• Seek out those events where black petrels are feeding in association with cetaceans. There are two reasons for this:
  1. It provides opportunities for catching birds when the chumming method is less likely to be successful.
  2. We believe this feeding association should be fully investigated. The occurrence of false killer and pilot whales is seasonal and there is a significant overlap with black petrel breeding. The first sightings of the false killer whales is usually in early December and the last ones in late April, sometimes in early May. Pilot whales can be seen all year round, however peak in December to May (J. Zaeschmar pers. comm.).
• Capturing black petrels further offshore (i.e., 200-500m bathymetry line) as well as from Repanga / Cuvier Island to the Far North would further increase the robustness of the overall opulation estimate. This would include areas outside Aotea / Great Barrier Island.
• There is significant potential to maximise data return from these captures. Currently we are taking standard morphometrics and checking brood patch status. Taking blood and/or feather samples from a sample of captured birds would enable additional analyses to complement this research. For example, stable isotope analysis of dietary shifts throughout the season. This would add minimal processing time (~2 minutes) and could be done during ‘slow’ capture periods. There are a wide array of analyses that can be done with small blood and feather samples, including population health assessments via energetic, physiological, and nutrition metrics in addition to the dietary usage noted above.
• Occasionally birds will regurgitate, with squid eyes and extremely smelly slurries most dominant. These samples could provide some dietary insight which could contribute to
  understanding seasonal prey targeting of black petrels throughout the breeding season. Samples could be either preserved in ethanol for visual identification where intact, or frozen, for further genetic identification and/or energetics analyses (e.g. bomb calorimetry). A better understanding of prey species of seabirds in northern Aotearoa New Zealand is required for informing ecosystem models and assessing the indirect threats posed by commercial fisheries.

Publication information

Bell, E.A.; Lamb, S. & Ray, S. 2024. Key demographic parameters and population trends of tākoketai/black petrels (Procellaria parkinsoni) on Aotea/Great Barrier Island: 2023/24. Unpublished Wildlife Management International Ltd. Technical Report to the Conservation Services Programme, Department of Conservation, Wellington. 67 p. 

Gaskin, C., Whitehead, E. 2024. Black petrel population monitoring: captures at sea. POP2022-01 final report provided by Northern New Zealand Seabird Trust to the Conservation Services Programme, Department of Conservation, Wellington. 19 p.