These reports detail the seabird population monitoring and surveys conducted at Antipodes Island in 2022/23.
The Antipodean wandering albatross Diomedea antipodensis antipodensis has been in decline since a population crash in 2005-07. Declining numbers appear to have been largely driven by high female mortality, but low chick production—with fewer birds breeding and reduced breeding success—has compounded the problem. To tease out the causes of falling numbers of Antipodean wandering albatrosses and identify the effectiveness of potential solutions, research includes an annual visit to the breeding grounds on Antipodes Island. This report describes the results of the field programme in the 2022/23 breeding season, and the preliminary findings from tracking of juveniles since January 2022.
There are some signs that the rate of decline might be slowing. The number of Antipodean wandering albatrosses breeding has been roughly stable for the past four seasons, and female survival shows some suggestion of improving since 2014 (4-year rolling averages), although it is still highly variable year to year. Breeding success in 2022 at 72% approached the average pre-crash nesting success of 74%, although the mean 2006–2022 rate remains comparatively low at 62%. However, the actual number of chicks produced remains small, even in good breeding-success years, since numbers nesting remain low. Recruitment is starting to draw from the (much smaller) cohorts produced since the crash, so population numbers will soon no longer be supplemented by higher recruitment rates seen over the past decade.
The population has been approximately stable for the last four years. However, there is so far no evidence of any sustained improvement in Antipodean wandering albatross demography, as required for the population to recover, with tentative improvements recorded here merely slowing the decline.
Recommendations include ongoing mark-recapture monitoring of demographic and population-size trends; an island-wide population size estimate; and research into causes of declines. More-targeted ongoing engagement is also needed, internationally and domestically, to achieve better bycatch mitigation in line with ACAP best practice.
This study provides an updated estimate of the white-chinned petrel (Procellaria aequinoctialis) breeding population size on Antipodes Island. It also details the setup of a mark-recapture study suitable for estimating key vital rates and detecting population change, adult survival in particular. Lastly, the study documents blood and feather collection for a wider study on mercury contamination, and deployment and recovery of time-depth recorders for data on dive depth of white-chinned petrels.
Population size estimate. Burrow density is estimated from a representative sample of burrowed areas then corrected for burrow occupancy and extrapolated to the available area of nesting habitat to estimate the breeding population of white-chinned petrels. For an estimate as accurate and precise as possible we built on previous efforts in 2009–11 and 2021–22 (Thompson 2019; Elliott & Walker 2022). To estimate burrow density we used the distance sampling dataset from 2021–22 and expanded the sampling coverage across the whole island, adding 93 transects to a new total of 248 island-wide sampling locations. Distance sampling enabled burrow density estimates that explicitly account for burrow detectability. Occupancy was assessed by inspecting 293 burrows just after laying, calculating rates and corrections using the approach developed for the 2009–11 study (burrow numbers corrected for entrances that are not in fact burrows, and for other species using white-chinned petrel burrows). The area used by white-chinned petrels, with two habitat types distinguishable, was drawn from comprehensive habitat mapping 2021–22. Antipodes Island had an estimated 26,400 (95% CI: 22,200–31,600) white-chinned petrel pairs breeding in Dec 2022 during early incubation. Burrow detectability was different in the two habitat types and occupancy rates differed, so for accuracy the estimate used burrow density, area and occupancy specific to each habitat type. These refinements to 2009–11 and 2021–22 methods result in a population size estimate here that is smaller but more accurate and precise.
Demographic study setup. Population change is more readily detected via intensive study of birds in a representative study population, so we established a mark-recapture study to estimate vital rates, survival in particular. Marked burrows in two study areas contain 169 banded white-chinned petrels. For accurate, precise survival estimates this marked population needs building further, along with recaptures at existing marked burrows for a minimum of three years.
Recommendations include an efficient and effective long-term monitoring strategy which could combine annual intensive monitoring effort in a representative study population, as set up here, supplemented by occasional whole-island population size estimates (5–10-year intervals). Ongoing mark-recapture will enable robust trend estimation over time, with whole-island estimates providing occasional more-general overview of breeding numbers.