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POP2023-02 Population survey of Southern Buller’s albatross on the Solander Islands (PDF, 3,016K)
Summary
Population studies of southern Buller’s albatrosses at Tini Heke / The Snares Islands and Hautere / Solander Islands
This report presents a summary of the results of demographic studies at three study colonies of southern Buller’s albatrosses Thalassarche bulleri bulleri breeding at Tini Heke / The Snares from 6 to 16 April 2024. A brief visit to southern Buller’s albatross colonies on Solander Island on 9 March 2024 is also described.
Demographic studies at the three study colonies on The Snares’ North East Island have been undertaken annually 1992–2024, with the exception of 2018 and 2021. Estimates of the numbers of breeding pairs, made by recording the contents of each nest mound, decreased substantially in all three study colonies compared to 2023, with numbers in the Mollymawk Bay study colony being the lowest recorded during the current study. With the assumption that the combined total number of breeding pairs in the three study colonies was representative of North East Island as a whole, then the breeding population probably peaked in 2005–2006 and has since undergone marked annual variations, with decreases in the past two years. If the decreases of 27.3% to 34.8% in the three study colonies are reflected in the population as a whole, then this will be a major concern for the conservation status of the species.
A total of 315 birds that had been banded previously in the study colonies as breeding adults of unknown age were recaptured. A further 26 breeding birds were banded in the study colonies - these are presumed to be first-time breeders. Although the most recent estimate of annual survival of birds banded as breeders (0.93 ± 0.03) was similar to the previous year’s estimate of 0.94 ± 0.01, the last four estimates over the period 2018 to 2023 have varied between 0.84 and 0.94. During the period 1992–2004 all chicks that survived to near-fledging in the study colonies were banded and their survival to return to the study colonies in subsequent years has been monitored. This year 92 of these birds were recaptured, with birds from cohorts banded from 1994 and 2002 recorded as breeding for the first time. This demonstrates the long-term monitoring required to obtain reliable estimates of survival of such known-age birds. In addition, five birds that had been banded as near-fledging in the study colonies during Sep 2013 and Sep 2014 were also recaptured for the first time.
At Solander, 20 trackers were deployed on breeding Buller’s albatrosses to follow year-round at sea distributions. At the Snares Time Depth Recorders (TDRs), Global Location Sensing (GLS) light-based geolocators and IgotU Global Positioning System (GPS) data loggers were deployed on 13 breeding Buller’s albatrosses to investigate diving behaviour and at sea distribution patterns. These deployments were short-term, and 12 of 13 devices were recovered during the trip. Eight Druid satellite transmitting tags paired with eight GLS were also fitted to breeding Buller’s albatrosses and these were securely attached for long term deployments to inform year-round at sea distributions.
In 2020 50 GLS tags were attached to the metal leg bands of breeding birds in the Mollymawk Bay study colony at The Snares; of these, 31 were retrieved in 2022, a further three during the 2023 field season, and one this year. A further 26 GLS tags were deployed at Mollymawk Bay study colony in the visit reported here.
Twelve replacement trail cameras were deployed to upgrade those previously installed in 2022 at breeding colonies on The Snares and set to record one photograph every hour during daylight for a further year.
Population survey of Southern Buller’s albatross on the Solander Islands
An aerial photographic survey of Great and Little Solander islands was carried out on 9 March 2024, mid-way through the Southern Buller's Albatross incubation period. Overall, 6771 individuals were counted in non-overlapping zones drawn on the images: 6215 (92 %) on Great Solander, 556 (8 %) on Little Solander. Of these, 4581 individuals were associated with occupied nests, 3845 (84 %) with a sitting bird alone (no assumptions being made about whether or not the nest contained an egg); the remaining 736 individuals (16 %) were in pairs, with one bird sitting. This means a further 368 occupied nests, giving an initial total of 4213 occupied nests, considered to be the minimum number.
Among other birds seen clearly, 583 were standing by empty nests (referred to here as occupied sites), 75 % as single birds. Loafers (145) comprised the balance of the definable birds. The status of a further 1462 individuals (22 % of the total birds counted) could not be determined directly. Assuming that their status was in the same proportions to those of the clearly observed birds, 1160 of these initially indeterminate individuals would be sitting on nests, giving an overall total of 5373 occupied nests, considered the maximum number.
Compared with the last survey in 2016, which combined aerial survey and counts of sitting birds from vantage points, the minimum estimate would loosely imply 25 % fewer birds at nests. But with the more likely number of occupied nests being c. 5373, the falloff since 2016 might only be about 4 %.
Surveys of 54 occupied nests along seven short transects found only 62.3 % on average had eggs (33 nests). The rest (21 nests, 37.7 %) had birds sitting on empty nests (Sagar et al. 2024). The status of these latter birds is unclear. They could be pre-breeders returning to the islands prior to nesting for the first time; recent failed breeders that have not yet abandoned their nest; or breeders from previous years taking a break from breeding for some reason. Given the timing of the survey, about halfway through the incubation period, it is unlikely that any of these birds were yet to lay eggs. The proportion of birds sitting on empty nests had only previously been reported in the 2016 survey. If the 2024 ground survey is considered broadly representative of the population, then 27% fewer birds were breeding in 2024.
The number of birds associated with empty nests, both from the aerial survey counts and from the ground survey, together with the numerous empty nests and nest sites seen on the aerial photographs, suggests that many birds may not have bred in 2024.
More needs to be known about the nature of birds sitting on empty nests. If some of them are skipping breeding, and if this is occasionally widespread in the population, caution may be needed in inferring demographic trends only from changes in the number of apparently nesting pairs between two surveys.
The rugged nature of Great Solander Island and dense tree canopy cover in places on both islands means that some nesting birds may go undetected during aerial photographic surveys. Short-duration ground surveys are similarly limited. As a population monitoring tool, future aerial photographic surveys may be better focused on comparing counts from images taken of several clearly demarcated, more-open areas on the two islands, interspersed at longer intervals with longer-duration, more intensive, combined ground and aerial censuses of both islands, as done in the past.
Publication information
Sagar, P., Rexer-Huber, K., Thompson, D., Parker, G. 2024. Population studies of southern Buller’s albatrosses at Tini Heke / The Snares Islands and Hautere / Solander Islands. Final report to the Conservation Services Programme, Department of Conservation. Parker Conservation, Dunedin. 13 p.
Frost, P,. Baker, B.G., Fischer, J., and Sagar, P. 2024. Population survey of Southern Buller’s Albatross Thalassarche bulleri bulleri on the Solander Islands | Hautere, March 2024. Final report to the Conservation Services Programme, Department of Conservation. 25 p.