We estimate population size in terms of the number of occupied nests for the Adélie penguin metapopulation in western Mac. Robertson Land, East Antarctica in 2009/10 and 2019/20. We also assessed demographic data from a single breeding site in the central part of this area (Béchervaise Island: 67°35'S, 62°49'E) including reproductive success, resight data, and fledgling mass from 1991/92 to 2019/20. We collated environmental covariates of potential drivers in this area over the same time period from sources described below. These are presented in the file “Time series demography and environmental covariates.xls”.

Environmental covariates:

Sea-ice concentration: Summer sea-ice concentration (SIC) was obtained for the area bounded by longitudes 60 - 65°E, to the south by the Antarctic coastline and the north by latitude 66.75°S. This approximately 250 km stretch of coastline incorporates the location of all Adélie penguin breeding sites across the metapopulation. The area defines the most northerly limit of fast-ice during chick rearing and encompasses the longitudinal range of the birds’ summer foraging activities. The sea-ice contained within this ‘near-shore’ region is predominantly composed of fast-ice (ice that is attached to land but covers seawater). Summer SIC was calculated as an average over the three-week period 25th December to 15th January when adults are guarding chicks for each breeding season.

Winter SIC was determined in the following three areas of the penguins’ winter migratory route as defined previously. Each area was defined between specific longitudes and from 50°S south to the Antarctic coastline. The sea-ice contained within this area is composed of fast-ice near the coastline and pack-ice (all sea-ice that is not fast-ice) beyond the fast-ice edge. Two sectors defined the outward journey as they travelled westward towards their winter foraging grounds (50 - 65°E during March, and 30 - 50°E during April), a winter area (15 - 30°E during May-Jul) was considered as the sea ice became more extensive with both 15-100% SIC and 15-80% SIC which is considered more in line with suitable winter foraging ice conditions. The final area was associated with their eastwards journey towards the colony (30 - 50°E during Aug-Sep). For each area and time period, an average SIC was determined for each year in each of these areas.

SIC values reflect the total area (km2) covered in sea-ice between either 15-100% or 15-80% SIC in each year and time period using 25x25km pixels. Sea-ice data were obtained from the National Snow and Ice Data Center (NSIDC) (Cavalieri et al. 1996) using Raadtools (Sumner 2017).

Broad-scale climatic indices and local weather conditions: We determined the weather conditions during periods reflecting the end of the austral summer when the penguins were leaving their colonies (Feb-Mar) and the inter-breeding winter period (Apr-Sep). The Southern Oscillation Index (SOI) and the Southern Annular Mode (SAM) were included as broad indicators of climatic conditions, and local weather conditions included air and windchill temperatures. SOI was obtained from the Australian Bureau of Meteorology (www.bom.gov.au) and SAM from the NOAA Climate Prediction Centre ( http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/aao/aao_index.html).

Mawson Station local weather: Local weather data recorded at Mawson Station were obtained from the Australian Bureau of Meteorology. We considered two covariates: windchill and air temperatures both reported in °C. Windchill temperatures were determined from the ambient air temperature, wind speed and the relative humidity: AT= Ta +0.33e-0.7ws-4.0, where Ta is the dry bulb temperature (°C), e is the water vapour pressure (hPa), and ws is the windspeed (ms-1) at 10 m elevation. Water vapour pressure was determined from: (see the actual equation in the download file - "Emmerson_AADC Metadata Records_GCB_2022.docx" - unable to be reproduced here), where rh is the relative humidity (%). This formula follows the Australian Bureau of Meteorology calculation (www.bom.gov.au/info/thermal_stress/).

Seabird population parameters:

Pre-fledging mass "adjusted.fledge.mass.5_Feb": We determined pre-fledging mass (g) of chicks on the 5th February by either measuring their mass on that date, or by standardising to that date from measurements made between the 3rd and 14th February.

Total chick productivity "tot.chick.prod.past.5.yrs": Cumulative five-year total chick productivity (total chicks) was calculated for each year using total counts across Béchervaise Island from the preceding five years. This represents the cumulative pool of pre-breeders on the basis that Adélie penguins typically recruit into the breeding population between the ages of one and five years.

Breeding success "bs.3.yr.ave": at Béchervaise Island was measured as the number of chicks crèched (end-January) in relation to the number of nests occupied at the start of incubation (late November and beginning of December). Units of measurement are chicks per occupied nest. Nest and chick counts were obtained annually from on-ground island-wide surveys. Because reproductive performance fluctuates dramatically across years, we calculated three-year rolling averages centred on the year of interest.

Resight data: Age of first return to the colony or recruitment into the breeding population “Age first.nesting.all.6.years” were based on resights of birds in their natal colony. Marked birds were resighted via colony-wide detection from a tag reader when they were on nests. Files for each year contain data from resighting with hand-held tag readers across Béchervaise Island including date of resight and the tag number with each file named as “2003_04 resights.xls” for the resights in 2003/04 split-season for example. For resight data outside the years available in this data repository, please contact Data Custodians.

Population growth rates:

Circum-Antarctic population growth rates: To allow a circum-Antarctic comparison of this populations growth rate with other sites or regions, we performed a literature review of published data or growth rates for estimating a consistent metric of growth rate. Data from this search are included in this dataset along with estimates of population growth rate in this study in file “Circum-Antarctic estimates of population growth rates for Adelie penguins Figure 2.pdf”.

Occupied nest counts Mac. Robertson Land: Adult counts were adjusted for phenology-related variable attendance and potential methodology bias to a standard metric (the number of occupied nests at the beginning of the incubation period). The adjustment process is described in detail in Southwell et al. (2013) and propagates the uncertainties from accounting for these biases through to the final estimate of occupied nests. Data include 1000 bootstrap estimates of occupied nests from this procedure for the Mac. Robertson Land area to standardise raw counts to the metric of occupied nests labelled as “O.N.bootstrap.estimates.2009_10” for 2009/10 and 2019/20 which we summarised with the median and 95 percentile limits. Please see manuscript for further details on the standardisation process. Data presented in file “W Mac. Robertson Land Adelie penguin population estimates.xls”.

Any data use from this repository in any publication, report or presentation, should include the following acknowledgement in each data file based on the following “Data from Béchervaise Island or Mac. Robertson Land were derived from Australian Antarctic Science projects 2205, 2552, 4088, 4086 and 4518. All procedures were approved through Australian Antarctic Division animal ethics and ATEP approvals.”

Please refer to the Seabird Conservation Team Data Sharing Policy for use, acknowledgement and availability of data prior to downloading data.